forced hot air furnaces troubleshooting and repair

FORCED HOT AIRFURNACES

TROUBLESHOOTINGAND REPAIR

Roger Vizi

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Library of Congress Cataloging-in-Publication Data

Vizi, Roger.Forced hot air furnaces : troubleshooting and repair / Roger Vizi.

p. cm.Includes index.ISBN 0-07-134171-41. Hot-air heatingEquipment and suppliesMaintenance and

repair. I. Title.TH7601.V57 1999697'.3dc21 99-18119

CIP

Copyright 1999 by The McGraw-Hill Companies, Inc. All rights reserved. Printed in theUnited States of America. Except as permitted under the United States Copyright Act of1976, no part of this publication may be reproduced or distributed in any form or by anymeans, or stored in a data base or retrieval system, without the prior written permission ofthe publisher.

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I would like to take the time to thank several people and organizationswho assisted me in the preparation of this book. First, to my wife,Nadine, who allowed me the time to write, and encouraged me in thisendeavor. To my father, Edward Vizi, for his encouragement. To myfather-in-law, Robert Speers, for the testing of the original version ofthis book.

The following companies and organizations contributed to this book: Lennox Industries, The Honeywell Company, The U.S. Departmentof Energy, Energy Efficiency and Renewable Energy Clearinghouse, Mr. Michael Lamb (1-800-237-2957). Illustrations by Wade Owens.

A C K N O W L E D G M E N T S

x i

Other Books in McGraw-Hills Complete Construction SeriesBianchina Room AdditionsCarrow Energy SystemsGerhart Home Automation and WiringPowers Heating HandbookWoodson Radiant Heating Systems: Retrofit and Installation

Dodge Cost Guides SeriesAll from McGraw-Hill and Marshall & Swift

Unit Cost BookRepair and Remodel Cost BookElectrical Cost Book

ABOUT THE AUTHOR

Roger Vizi has over 15 years experience in maintaining andrepairing home heating systems and high-pressure steamboilers. He is currently senior buyer for SPM, an Oregon-based company that specializes in the manufacturer of plastic-injection molded parts. He is also the author of TheHomeowners Guide to Basic Gas Furnace Repair.

Acknowledgments xi

Chapter 1 Introduction 1

Chapter 2 Listening and Observing 3

Chapter 3 Components of a Gas Forced Air Heating System 5

Thermostat 5

Gas Valve 8

Pilot Assembly 9

Fan and Limit Control 10

Blower Assembly 11

Heat Exchanger 12

Gas Regulator 14

Hot Air Ducting 14

Cold Air Ducting 14

Chapter 4 Electric Circuits 17

Chapter 5 Operation of a Gas Forced Air Heating System 23

Chapter 6 Tuning Up a Gas Forced Air Heating System 33

The Pilot Assembly 36

The Blower Assembly 45

C O N T E N T S

v

v i CONTENTS

Chapter 7 Troubleshooting a Gas Forced Air Heating System 61

Pilot Lights, But Main Valve Not Open (Standing Pilot System) 61

Electronic Ignition 73

There Is No Spark or Pilot Light, and the System Will Not Work 75

Electronic Ignition 75

Spark Is Present, But the Pilot Will Not Light (Electronic Ignition Only) 77

Blower Will Not Come On 78

Blower Will Not Shut Off 82

Chapter 8 Introduction to Humidifiers 99

Chapter 9 Installation and Maintenance of Humidifiers 103

Installation 105

Maintenance 114

Chapter 10 Is an Electronic Air Cleaner Right for You? 119

Chapter 11 Installation and Maintenance of Electronic Air Cleaners 123

Installation 123

Maintenance 128

Other Operation 129

Chapter 12 Introduction to Oil Forced Air Heating Systems 133

Chapter 13 Electric Circuits for Oil Forced Air Heating Systems 137

Chapter 14 Controls for an Oil Forced Air Heating System 143

CONTENTS v i i

Chapter 15 Protecting Oil Tanks in the Winter 155

Heat Tape 155

Insulation 156

Chapter 16 Operation of an Oil Forced Air Heating System 159

Sequence of Operation 160

Chapter 17 Tuning Up an Oil Forced Air Heating System 167

Oil Tank 168

Burner Assembly 171

Blower Assembly 181

Chapter 18 Troubleshooting an Oil Forced Air Heating System 197

No Heat 198

Fuel Looks Clear and Has a Good Flow 202

Oil Has a Milky Look to It 204

Ignition Transformer 208

Electrodes 210

Burner Starts and Fires, but Locks Out on Safety 213

Burner Starts, Fires, but Loses Flame and Locks Out on Safety 214

Burner Starts and Fires, but Short Cycles 216

Burner Will Not Shut Off 220

Chapter 19 Is Electric Forced Air Heat Right for You? 225

Energy Savings Measures 226

Conclusion 228

Chapter 20 Controls for an Electric Forced Air Heating System 229

Thermostat 230

v i i i CONTENTS

Control Box 230

Door Interlock Switch 231

Terminal Strip 231

Transformer 231

Circuit Breakers 232

Autotransformer 232

Transformer Fuses 232

Blower Relay 232

Blower Motor 232

460-V Motor Winding 234

Blower Motor Capacitor 234

Electric Heat Components 234

Chapter 21 Circuits for an Electric Forced Air Heating System 251

Low-Voltage Circuit 251

208/230-V Circuits 252

460- and 575-V Circuits 252

Chapter 22 Operation and Maintenance of Electric Forced Air Heating Systems 263

Single-Phase 208/230-V Sequence of Operation 264

Single-Phase 208/230-V Sequence of Operation with Second-Stage Heat 264

Single-Phase 208/230-V Sequence of Operation with Three-Stage Heat 265

Maintaining an Electric Forced Air Heating System 266

Chapter 23 Troubleshooting Electric Forced Air Heating Systems 269

No Heat 269

Not Enough Heat 275

Conclusion 277

CONTENTS i x

Chapter 24 Is a Heat Pump Right for You? 279

Electric Forced Air Heat 279

Wood Heat 280

Hot-Water Baseboard Heat 280

Forced Air Heating Systems 280

Heat Pumps 280

Chapter 25 How Does a Heat Pump Work? 283

Ground Source Heat Pumps 285

Water Source Heat Pumps 287

Chapter 26 Introduction to Heat Pumps 289

COP 289

EEP 290

HSPF 290

SEER 291

Defrost 291

Supplemental Heat 292

Balance Point 292

Btus 293

Sizing the Unit 293

Heating and Cooling Load Calculations 294

Ways to Improve Your Heat Gain Ratio 296

Chapter 27 Operation and Maintenance of Heat Pumps 303

Chapter 28 Troubleshooting Heat Pumps 309

The Unit Will Not Operate 310

Liquid Pressure too Low 311

Liquid Pressure too High 315

Pressures are Normal, but There Is Still Not Enough Heat/Cooling 316

x CONTENTS

The Unit Will Not Defrost 318

The Unit Will Not Stop Defrosting 319

Chapter 29 Conclusion 323

Appendix A: Parts Inventory for a Well-Stocked Repair Truck 325

Appendix B: Questions to Ask Before Going on a Service Call 329

Appendix C: Oil Nozzle Selection 331

Appendix D: Sample Summer Tune-Up Checklist 333

Appendix E: Trouble Call Sheet Sample 335

Index 337

As a heating professional, you will be called on to service many typesof home heating systems. This book will cover the most commontypes of home heating systems in use today: natural and propane gas,oil, electric, and heat pumps. These systems come in many styles andare manufactured by several companies, but the basic operation andrepair are the same.

While I have geared this book for the person who is either in theheating profession now or would like to become a heating profes-sional, I also have made every attempt to make this material as easy tounderstand as possible for the average homeowner. While the averagehomeowner may never attempt to replace the heat exchanger in his orher own unit, as an example, he or she will have the knowledge todecide when this type of repair is needed.

Wherever I felt that more information on a subject was neededbeyond the discussion included herein, I have included diagrams anddrawings to better illustrate the point.

As a heating system professional, you must have a complete under-standing of the different types of add-ons that are available for homeheating systems as well. I have included sections on installation andmaintenance of the two most common types of humidifiers on the mar-ket, when you should install one, which one is right for the application,

I n t r oduc t i on

1

1

C H A P T E R

2 CHAPTER ONE

and how to properly maintain these units. I also explain the benefit tosome homeowners of installing an electronic air cleaner.

Each section of this book will cover the basic controls of the heat-ing system and the operation of the system and will end with a dis-cussion of troubleshooting and repair of the system.

It is important for the heating professional to follow a systematicapproach to diagnosing problems when they occur. By understandingthe different components of the heating system and how they worktogether, you can isolate the problem to a particular circuit and elimi-nate the circuits that do not have a bearing on the problem.

You will learn, for example, that if a client calls you and states thatthe burners will not light but the blower runs all the time that thesetwo conditions have no bearing on each other. Both these problems arecontrolled by different circuits, and the problems are not related. How-ever, by knowing the proper troubleshooting techniques, you shouldbegin to diagnose the problem in your mind so that you have an ideaof what to look for when you arrive at the clients home.

Chapter 2 of this book will go into more detail on the subject of lis-tening and observing. These are two of the most important traits that agood heating professional can have, since the client is your best sourceof information on the problem.

The working world of the heating professional is a rewarding andchallenging one. You will encounter many different situations thatyou must have the knowledge and experience to handle. While thisbook does not attempt to cover all the situations that you may be calledon to handle, it will explain the techniques used to troubleshoot andrepair the most common problems. By applying these same techniquesto your particular challenge, you should be able to come up with asolution quickly and efficiently. This knowledge and confidence willbe apparent to your clients, and they will call on you again becauseyou have shown them that you are truly a professional at your trade.

One of the most important parts of the job of heating professional isknowing what to look for when you receive a call from a home-owner that his or her gas heating system, as an example, is in need ofrepair. You must learn how to listen to the homeowner, because he orshe is your most valuable source of information.

The homeowner may tell you, I do not have any heat in thehouse. It is your job to be able to ask the proper questions that willlead you to the correct diagnosis of the problem.

When you receive this type of call, you should ask the followingquestions:

1. Is the thermostat turned up?

2. Did you check the fuses?

3. Is the pilot light lit?

4. Did you hear any noise when the heating system was running?

5. Were any repairs made in the last several months?

L i s t en i ng andObserv i ng

2

3

C H A P T E R

4 CHAPTER TWO

6. What type of heating system do you have? If this is an oil orpropane system, ask this question:

7. When was the last time fuel was delivered?

All these questions will help you to diagnose the problem beforeyou leave for the call (see Appendix B). Even though the client hasanswered all these questions and has assured you that all these itemshave been checked, it is always good practice to double-check thisentire list once you arrive at the home. Some homeowners are notaware of the location of all the fuses or breakers that are attached to thecircuits, for example, and you may find that this is the answer to theproblem. If you do not double-check these items, you may find your-self looking for a ghost.

In the chapters on troubleshooting, I will cover many of the tech-niques that are required to properly diagnose the symptoms and to cor-rect the problems of a heating system that is not operating properly. Asyou begin to use these techniques, you will find that it will becomemuch easier to solve these problems in a timely manner. The moretroubleshooting calls that you go on, the more you will understandwhy it is so important to listen to the homeowner, ask the proper ques-tions, and observe what is taking place with the heating system so thatyou can quickly diagnose the problem and make the needed repairs.

You should always take a systematic approach to troubleshootingany heating problem. Always check for the simple solution to theproblem first before proceeding. By using this approach, you willsolve more problems quickly and will not waste time chasing yourtail because you are looking at a symptom and not the problem.

Before we can begin to understand how to repair common gas fur-nace problems, we must first get an understanding of what makesup a home gas heating system and how the components worktogether. The typical home gas heating system is simply a series ofswitches and circuits that work together to heat the home. As webegin to explain each component, it would be helpful to locate eachcomponent described so that when a home heating repair is neededon a gas heating system, you are already familiar with the location ofthese components. Figure 3.1 shows a typical gas forced air heatingsystem.

ThermostatThe thermostat is the device used to regulate the temperature in thehome. It is the switch that tells the home gas heating system

Componen t s o f aGas F orced A i rHea t i ng Sys t em

3

5

C H A P T E R

6 CHAPTER THREE

yy

GLASS FIBER GASKET

FRESH AIRINTAKE FITTING

BURNER BOXASSEMBLY

PATCHPLATE

FLAME SIGHTGLASS

BURNER BOXCOVER

GAS VALVE ANDMANIFOLD

COMBUSTION AIRPROVE (PRESSURE)

SWITCH

BURNERACCESSPANEL

BLOWERACCESS

DOOR

COMBUSTION AIRBLOWER

COMBUSTION AIRORIFICE

COLD HEADER(COLLECTOR)

BOX

CONTROL BOX COVERDOOR INTERLOCK SWITCH

CONTROL VOLTAGECIRCUIT BREAKER

CONTROL TRANSFORMERCONDENSER COIL

PRIMARY LIMIT(ALTERNATE STYLES)

SUPPLY AIRBLOWER

FLUETRANSITION

CABINET

TOP CAPFLUE COLLAR

WARMHEADER

(COLLECTOR) BOX

PATCH PLATE WITHBARBED FITTING

AND FLAMEROLL-OUT SWITCH

DuralokPlusHEAT EXCHANGER

ASSEMBLY

F I G U R E 3 . 1

Exploded view of gas forced air heating system. (Courtesy of Lennox Industries, Inc.)

COMPONENTS OF A GAS FORCED AIR HEATING SYSTEM 7

to begin the heating cycle. This devicecomes in several styles and typically isinstalled on an interior wall of the home.Some of these styles include round,square, and rectangular. All these ther-mostats operate in the same basic man-ner. Figures 3.2 to 3.4 show thesethermostats.

When the dial or lever is moved to theright and is pointing to a temperature thatis higher than the temperature in thehome, the circuit is closed, and a signal issent to the gas valve to begin the heatingcycle. Some of these thermostats have oneor two levers that are manual controls.One of these levers will say, Heat OffCool, and the other will say, Fan OnOff. It is important to know about thelocation of these levers and what they con-trol. On occasion you may have to diag-nose a heating problem that is related toone of these switches being placed in thewrong position.

Another part of the thermostat iscalled the heat anticipator. This is asmall coil that is heated. It is used toanticipate when the temperature in thehome is reaching the set point on thethermostat. Figure 3.5 shows a heat antic-ipator on a thermostat. When the temper-ature is getting close to the set point, thethermostat will shut off the call for heatto the gas valve to allow the blower toremove the balance of heat in the systemso that the actual temperature in the room is correct. If this device isnot set properly, the heating system will not be synchronized, and

F I G U R E 3 . 2

Round-type thermostat. (Courtesy of Honeywell,Inc.)

F I G U R E 3 . 3

Round thermostat with heat/cool/fan control.(Courtesy of Honeywell, Inc.)

8 CHAPTER THREE

the burner may turn on and off erratically. It is scaled in amps, anda good starting point is 0.4 to 0.5.

Gas ValveThe gas valve is the device that controls the natural gas flow (gas sup-plied by the gas company in your area) or propane gas flow (held in atank outside the home) to the home heating system when the thermo-stat calls for heat. This device typically is located behind the front

F I G U R E 3 . 4

Various styles of thermostats. (Courtesy of Honeywell, Inc.)

COMPONENTS OF A GAS FORCED AIR HEATING SYSTEM 9

panel of the gas furnace. This locationcould be either in the upper section on anupflow-type furnace or in the lower sec-tion on a downflow-type furnace. Figures3.6 through 3.8 show combination, contin-uous pilot, and electronic gas valves.

There are many other possible loca-tions for this device depending on thetype of gas furnace you have. In mostcases, this device can be located by fol-lowing the large gas line to the furnace.The gas valve will be either round orsquare and will have wires attached to itas well as typically two smaller linescoming from it. One may be silver incolor and the other gold. Most will havea dial on the top that reads on, off, andpilot. The silver and gold lines lead to thepilot assembly.

Pilot AssemblyThe pilot assembly consists of the pilotlight and the thermocouple on older unitsand a pilot, spark ignition, and sensor onnewer units. Both these types of unitstypically will be attached to the gas valveand will be located by the main burner(s).There may be an access panel coveringthis unit that must be removed or lifted togain access to the pilot assembly. In someinstances, especially on older gas fur-naces, this may not be the case. In anolder gas furnace, the pilot gas line maybe attached to the main supply line bymeans of a small brass petcock valve, andthe thermocouple may be attached to a

F I G U R E 3 . 5

Thermostat with heat anticipator.

F I G U R E 3 . 6

Combination gas valve. (Courtesy of Honeywell, Inc.)

1 0 CHAPTER THREE

silver box attached to the inside of the gasfurnace behind the access door. Figures3.9 and 3.10 show these types of pilotassemblies.

The main purpose of the pilot assem-bly is to maintain a steady pilot flame toignite the burner(s) in the gas heating sys-tem when there is a call for heat. The jobof the thermocouple is to sense that thepilot light is burning prior to the gas valveopening to supply the main gas supply tothe burner(s). If the thermocouple doesnot sense that the pilot flame burning, itwill shut off the supply of gas to the pilotand not allow the main valve to open. It isone of several safety devices on yourhome gas furnace. Figure 3.11 shows athermocouple.

By keeping this pilot lit during the non-heating-season months, it will not allowmoisture to form causing rust that canreduce the efficiency and life of yourhome heating system.

Fan and Limit ControlThis is a device that controls when andhow long the blower on your furnace willremain on to force heat into your home.(This is where the term forced air gas heatcomes from). Figure 3.12 shows a fan andlimit control.

This device typically is located on thefront of the furnace above the burner(s) or may be located on the hot air plenum(the sheet metal that is attached to the

F I G U R E 3 . 7

Continuous pilot dual automatic gas valve. (Courtesyof Honeywell, Inc.)

F I G U R E 3 . 8

Universal electronic ignition gas valve. (Courtesy ofHoneywell, Inc.)

COMPONENTS OF A GAS FORCED AIR HEATING SYSTEM 1 1

heating section of the furnace, betterknown as the supply side). This devicealso serves as another form of safetydevice; if the blower does not come onduring a heating cycle, this device willshut off the flow of gas to the burners tohelp eliminate the chance of overheatingyour furnace.

The most common types of these con-trols are as follows:

1. Dial type. In this type, there is a dial onthe front of the fan control that allowsyou to set the on and off temperature ofthe blower. This type of fan controlusually will have a manual blowerswitch attached so that the homeownercan run the blower manually in thesummer if needed.

2. Rectangular type. This is a bimetallictype of fan control that has slidelevers that are used to control the on and off settings of the blower. Thisstyle of control will control theblowers manual setting by moving the lever all the way to the left.

3. Clicson type. This type uses a disk thathas preset on and off settings. You cannot set the manual control of the blowerwith this type.

Blower AssemblyThe blower assembly is the device thatblows the warm air into your home dur-ing the heating cycle. This device is

F I G U R E 3 . 9

Electronic pilot assembly. (Courtesy of LennoxIndustries, Inc.)

F I G U R E 3 . 1 0

Standing pilot with thermocouple.

1 2 CHAPTER THREE

made up of the blower housing and amotor. The motor will be connected tothe blower housing in one of two ways.The motor may be attached by means of a belt and pulley; this is known as beltdrive (Fig. 3.13). Alternatively, theblower may be attached directly to the motor shaft; this is known as directdrive (Fig. 3.14). In either case, the basicfunction is still the same.

The belt-drive method requires moremaintenance than the direct-drive methodand will be explained further in the sectionentitled, Summer Tune-up. The blower iscontrolled during the heating cycle by thefan control.

Heat ExchangerThe heat exchanger is the unit that theburners are attached to (Fig. 3.15). Whenthe burners come on to start the heatingcycle, the heat exchanger performs twofunctions:

1. It conducts the heat transfer from the burners, to cause the fan con-trol to start the blower, forcing warm air into the home.

2. It is the channel for the removal of carbon monoxide to the chimney.

This second item is extremely important to know because a heatexchanger that is damaged or not in proper operating condition mayallow carbon monoxide to enter the home. Carbon monoxide is a color-less, odorless, tasteless gas that can cause the homeowner and his or herfamily to become very sick, and in high enough concentrations, deathcan occur. How to examine this vital part of the heating system will becovered in Chapter 5, Tuning Up a Gas Forced Air Heating System.

F I G U R E 3 . 1 1

Thermocouple. (Courtesy of Honeywell, Inc.)


50100 150

200

50 100

MOVE FAN CONTROL LEVER TO ITSLOWEST SETTING TO PUT BLOWER INTOCONTINUOUS OPERATIONTO RETURN BLOWER TO INTERMITTENTOR AUTOMATIC OPERATION MOVE FANCONTROL LEVER TO 90

TYPE IMOVE FAN CONTROL LEVERS TO THEIRLOWEST SETTINGS TO PUT BLOWER INTOCONTINUOUS OPERATIONTO RETURN BLOWER TO INTERMITTENTOR AUTOMATIC OPERATION MOVE FANCONTROL LEVERS TO APPROXIMATELY115 "ON" AND 90 "OFF"

DO NOTMOVELIMIT

CONTROLLEVER

TYPE III

TYPE IIMOVE FAN CONTROL LEVERS TO THEIRLOWEST SETTINGS TO PUT BLOWER INTOCONTINUOUS OPERATIONTO RETURN BLOWER TO INTERMITTENTOR AUTOMATIC OPERATION MOVE FANCONTROL LEVERS TO APPROXIMATELY115 "ON" AND 90 "OFF"

F I G U R E 3 . 1 2

Different types of fan/limit controls. (Courtesy of Lennox Industries, Inc.)

1 4 CHAPTER THREE

Gas RegulatorsA gas regulator is the device that controlsthe amount and pressure of the gas that isused for combustion in the gas heating sys-tem. In the case of liquid propane (LP) gas(also known as propane gas), the regulatoris in the line between the tank and the gasvalve. With propane, there will be no regu-lator on the gas valve. In the case or nat-ural gas, the regulator will be located onthe gas valve. This is one quick way for theheating professional to determine whichtype of fuel is being used.

Hot Air DuctingThis is also known as the supply-sideducting. This is the ducting that comesfrom the hot air side of the heating systemand supplies the warm air to the home.This ducting may be round or square andcan be the flex type of metal. Figure 3.16shows different types of ducting. The out-lets for this warm air are registers thatshould be located along the exterior wallsof the home. They can be used to regulatethe amount of heat that is provided to eachroom of the home. A later chapter willdescribe how to balance the heating sys-tem by regulating these registers.

Cold Air DuctingThis is also known as return ducting. Thisis the metal ducting that returns the coldair from the home to the blower side of

F I G U R E 3 . 1 3

F I G U R E 3 . 1 4

Belt-drive blower unit.

Exploded view of a direct-drive blower assembly.(Courtesy of Lennox Industries, Inc.)


the heating system. The registers for thisducting should be located along the inte-rior walls of the home. It is important tonote that you cannot warm a home if youcannot properly remove the cold air fromthe home. One cold air return is worthtwo warm air registers in a home. If youdo not see enough cold air return registersin a home, you should talk to the home-owner to see if there are cold spots inthe home in the winter. If the answer isyes, you should recommend adding morecold air returns.

F I G U R E 3 . 1 5

Heat exchanger. (Courtesy of Lennox Industries,Inc.)

1 6 CHAPTER THREE

Metal duct

Fiberglassduct

board

Flex-duct

F I G U R E 3 . 1 6

Types of ducting.

All the heating systems covered in this book use electricity for power.Some of the units use this power to operate the electric circuits,others use it to generate heat, and heat pumps use it to move heat fromone place to another. As a heating professional, it is important for youto understand how these circuits operate so that you are betterequipped to troubleshoot these circuits.

I will start out with an explanation of the circuits used in the gasforced air heating system. As I mentioned earlier, a heating systemuses a series of switches to control the operation of the system. Theseswitches use either low voltage (24 V ac) or a higher voltage (110 V ac)to operate.

Low-voltage circuits primarily operate the thermostat and some ofthe gas-valve circuits. A stepdown transformer is used to convert the110 to 24 V ac. In some installations, the thermostat will be connectedto the 110 V ac circuit. You can tell this by looking at the size of thewire that is used. On a 24-V circuit, the wire is very thin and normallywill be two colors, red and black. The wire will be single solid copperwire. On the 110-V circuit, the wire will be much larger in diameter,and the colors may be black and white. Of course, you should alwayscheck the voltage with a voltmeter to make sure of the circuit withwhich you are working.

E l ec t r i c C i rcu i t s

4

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C H A P T E R

1 8 CHAPTER FOUR

Lets first examine the low-voltage circuit of a gas heating system.The stepdown transformer typically will be located inside thewiring cabinet. Figure 4.1 shows a typical 24-V transformer. Thiscabinet is located in the blower section of the heating system. Someother locations for this transformer are in the front of a low boyheating system or, on older systems, on the outside of the cabinet

or mounted somewhere close to the heat-ing system.

There are two wires that run from theback of the transformer, one white wireand one black wire. The black wire is con-sidered the hot wire, and the white wireis the ground wire. The transformer willbe wired into the constant 110-V powersupply that is being fed in from the SPST(single pole single throw) switch (theswitch on the outside of the heating sys-tem that has a switch and fuse, Fig. 4.2) orfrom the breaker or fuse panel (Fig. 4.3).With the transformer mounted and wiredproperly, you will be able to get a readingof 24 V from the two screws on the front ofthe transformer. Using a voltmeter set for24 V ac, place the black lead on one of thescrews and the other lead on a groundingsource (bare metal, etc.). You should get areading. If not, try moving the black leadto the other screw. The screw that pro-duces the 24-V reading is the hot side; theother is the ground.

As the 110-V power enters the trans-former, it goes through a stepdown coil,which converts the power to 24 V ac. It isvery important for you as a heating techni-cian to understand how this power con-version works.

The thermostat is wired in the 24-V cir-cuit to act as the main switch that allows

Blower Relay

Transformer

F I G U R E 4 . 1

A 24-V transformer and blower relay. (Courtesy ofLennox Industries, Inc.)

F I G U R E 4 . 2

SPST switch.

ELECTRIC CIRCUITS 1 9

the gas valve to open when the thermostat is turned up and is callingfor heat. The hot wire from the transformer is run to one side of thethermostat. Another wire is run from the other terminal on the ther-mostat the one terminal on the low-voltage side of the gas valve. Theother wire is connected between the other terminal on the low-voltageside of the gas valve and then back to the other terminal on the trans-former. Because the hot side of the transformer voltage is broken atthe thermostat, when the thermostat is turned up and is calling forheat, this closes the circuit and allows the voltage to flow to the gasvalve. This, in turn, energizes a coil that opens the flow of main sup-ply gas to the heating system, starting the heating cycle.

On the high-voltage side (110 V ac), the power is supplied directlyfrom the main fuse box or breaker panel. This power normally will be

F I G U R E 4 . 3

Fuse panel.

Wiring diagram for spark ignition system. (Courtesy of Lennox Industries, Inc.)

E G26-1 and -2 models.1. When disconnect is closed, 120V is routed through door interlock

switch (S51) to feed the line voltage side of the blower control(A3) and the Transformer T1 primary. Door interlock sxitch mustbe closed for A3 and T1 to receive voltage.

2. T1 supplies 24VAC to terminal 24VAC on A3. In turn, terminalR of A3 supplies 24VAC to terminal RC of the indoor ther-mostat (not shown).

3. When there is a call for heat, W1 of the thermostat energizes Wof the furnace control with 24VAC.

4. CAB of the blower control evergizes the combustion air blower(B6). When the combustion air blower nears full speed, com-bustion air prove switch (S18) closes.

5. When S18 closes, assuming primary limit (S10) is closed, theignition control opens the pilot valve and begins spark.

6. When flame is sensed, spark stops and main valve opens to lightmain burners.

7. After 45 seconds, blower control (A3) energizes the indoorblower.

8. When heat demand is satisfied, W1 of the thermostat de-ener-gizes W of the furnace control and the furnace control immedi-ately de-energizes the gas valve. The combustion air blowerimmediately stops. Also, the indoor blower runs for a desig-nated period (90330 seconds) as set by jumper on blowercontrol.

21

F I G U R E 4 . 4

2 2 CHAPTER FOUR

connected to a switch with a fuse located on the outside of the heat-ing unit.

The blower circuit is then connected tothese wires. The black wire is the hot wire,and the white wire is the ground. Therealso may be a bare copper wire that is alsoa grounding wire. You must make sure thatyou always connect the same color wires

together. Never connect the white (ground) wire to the black (hot) wireor you will blow a fuse or worse.

In this circuit, the fan control is acting as the switch. It is connectedinto the circuit so that when the burners are on and the fan controlreaches the desired on set point, the circuit is closed, and power issent to the blower to operate. This same circuit is used to send powerto the high-voltage side of the gas valve. In the event that the blowerdoes not come on for any reason, the circuit will open and will closethe flow of main gas supply to the burners. This is a safety device thatis built in so that the unit will not overheat. Figure 3.12 shows howthis circuit operates.

By understanding how these circuits operate, you will be able tobetter isolate the problem when you are called on to troubleshoot a gasheating system. By knowing that the low-voltage circuit only operatesthe thermostat and low-voltage side of the gas valve, as an example,you can eliminate the circuit if the problem involves the blower notoperating. This knowledge will save valuable troubleshooting time.

Figure 4.4 shows a typical wiring diagram for a spark ignition gasforced air heating system.

Black = hot wireWhite = grounding wireBare copper = grounding wireAlways connect same color wires together

QUICK>>>T IP

There are three types of gas forced air heating systems that you willencounter as a heating professional. These three types are:1. Standing pilot systems. In this type of system, the thermocouple is

used as a safety device to detect that the pilot is burning prior to thegas valve opening to supply main-line gas to the burners. Figure 5.1shows a thermocouple.

2. Thermopile generator system. The thermopile generator uses the heatfrom the pilot to operate the gas valve. No outside power source isneeded in this type of unit. Figure 5.2 shows a thermopile generator.

3. Electronic spark ignition system. When the thermostat calls forheat, an electronic circuit is closed, causing a high-energy spark tostart. Once this spark is detected, the pilot side of the gas valveopens to light the pilot. A sensor is used to detect the pilot andsends a signal to the gas valve to open, allowing main-line gas to

Opera t i on o f aGas F orced A i rHea t i ng Sys t em

5

2 3

C H A P T E R

2 4 CHAPTER FIVE

flow to the burners, starting the heating cycle. Figure 5.3 shows thistype of system.

There are other types of systems on the market as well, but they allwork in one of these three basic ways.

In a standing pilot system, the gas forced air heating system oper-ates in the following manner:

1. The thermostat is turned up or begins to call for heat.

2. The circuit is closed at the thermostat, and this allows electricity toflow to the gas valve.

3. The gas valve checks to make sure that the pilot is lit by means ofthe thermocouple. If the thermocouple senses the pilot, the gasvalve opens, allowing main-line gas to flow, and the pilot ignitesthe burners.

F I G U R E 5 . 1

Thermocouple. (Courtesy of Honeywell, Inc.)

F I G U R E 5 . 2

Thermopile generator. (Courtesy of Honeywell, Inc.)

OPERATION OF A GAS FORCED AIR HEATING SYSTEM 2 5

4. Once the temperature reaches the on set point of the fan control, theblower begins to run, forcing warm air into the home.

5. This operation continues until the thermostat reaches its setpoint, and the circuit is opened, closing the main gas supply tothe burners.

6. The blower continues to operate until the lower limit on the fancontrol is reached. This opens the circuit, and the blower stops.

A gas forced air heating system that is equipped with electronicignition is slightly different and operates as follows:

1. The thermostat is turned up or calls for heat. This closes the circuitand sends a signal to the pressure switch to begin the 15-s purge cycle.

2. The system then begins the 20-s igniter warm-up cycle.

F I G U R E 5 . 3

Components of a spark ignition system. (Courtesy of Honeywell, Inc.)

2 6 CHAPTER FIVE

3. The igniter begins a 4-s trial for ignition.

4. The burners ignite, and the flame sensor checks for flame.

5. The blower delay sequence begins.

6. The blower begins the forced air cycle.

7. Once the thermostat is satisfied, the combustion blower continues5 s postpurge.

8. The main blower runs until the low limit is reached and thenshuts down.

No matter which type of gas heating system you are called to work on,the basic operation is the same. Figure 5.4 shows the sequence of oper-ation of a spark ignition system.

All the newer home heating systems are designed for maximum effi-ciency and fuel savings. To accomplish this, many manufacturers haveincorporated more efficient heat exchangers, burners, ignition systems,controls, etc. One such device is the automatic vent damper. Thisdevice is installed in the flue and is used to close off the flue pipe whenthe heating cycle is complete to allow the unit to be more efficient.

When the unit calls for heat, a signal is sent to the damper to open.Once the damper opens, the normal cycle begins. This type of devicecan be used on any type of gas forced air heating system. Figure 5.5shows such a damper.

I will now explain how all this comes together to produce heat forthe home. In a forced air heating system, fossil fuel is burned to pro-duce the heat. Air enters the forced air heating system to provide oxy-gen to the burners, and the combustion products are vented tooutdoors in the combustion airstream, usually through the chimney(Fig. 5.6). The combustion airstream moves because the combustionair products are lighter than the cold air. Movement is sometimes alsoassisted by a combustion air intake fan.

Another airstream moves from the return air grill, through the returnair ducts and filter, to the blower, which pushes the air past the heatexchanger (Fig. 5.7). The circulated airstream and the combustionairstream are separated at the heat exchanger and not allowed to mix, asseen in Fig. 5.8. The circulated air is heated as it passes the heatexchanger. From there, the heated air passes through the supply ducts

1710

8 2

4 5

6

4

93

L1DOOR INTERLOCK

SWITCH INDOORBLOWER

RELAY

FANCONTROL

BLACK

RED

BLOWERMOTOR

YELLOWBROWNBLUE

WH

ITE

CAPACITOR

N

QS MOTOR WIRING SHOWN

TRANSFO

RMER

BLUE24V

120V SENSINGPRO

BEYELLO

W

ELECTRODE

PROVEN PILOTVALVE LOCKOUTTIMER & IGNITER

G G G

INTERNALGROUND

PILOT

MAIN

GASVALVE

1 2 3

4 5 6

FAN CO

NTROL H

EATER (IF USED)

LIMIT

CONTRO

LOPTIONAL

AUTOMATICVENT

DAMPERMOTOR

AUXILIARYLIM

IT CO

NTROL

G1208-137O

NLY

R R1

W1Y1

W Y G

THERMOSTAT

COOL AUTO ONHEAT

COM

PRESSOR

CONTACTO

R

INDO

OR

BLOW

ER RELAYW

HITE

BROW

N

1 - Line potential feeds through the door interlock (if used).The blower access panel must be in place to energizemachine.

2 - Transformer provides 24 volt control circuit.3 - On a heating demand the thermostat heating bulb

makes4 - The control circuit feeds from "W" leg through limit

control(s) to initiate pilot operation.5 - After the pilot flame has proven, the main valve is ener-

gized. Main burners are ignited.

6 - As the main valve is energized, the fan control heater (ifused) is also activated.

7 - After a short period, the heater provides sufficient heatto close the fan control contacts.

8 - This then energizes the blower motor on low speed.9 - As the heating demand is satisfied, the thermostat heat-

ing bulb breaks. This de-energizes the ignition control,gas valve and fan control heater.

0 - The blower motor continues running until the furnacetemperature drops below fan control set point.

1

1

2 3

ORANGE

F I G U R E 5 . 4Sequence of operation. (Courtesy of Lennox Industries, Inc.)

2 8 CHAPTER FIVE

and past dampers, which are used to bal-ance the airflow in the home, and thenthrough supply diffusers and into eachroom of the home.

At the heart of the forced air heatingsystem is the heat exchanger, as seen inFig. 5.9, which does not allow the combus-tion gases to mix with the circulating air-flow. It does, however, allow for thetransfer of heat from the combustion gasesto the circulating airstream by means ofheating the metal heat exchanger.

If the heat exchanger becomes corroded,cracked, or has holes (making this examina-tion will be covered in the next chapters onsummer tune up), carbon monoxide andother dangerous and sometimes lethal com-bustion products will be allowed to enterthe home. If this is the case, the heatexchanger of the forced air heating systemmust be replaced.

All forced air heating systems must bevented in some manner to allow the com-bustion gases to be exhausted to the out-side. All fuel-burning systems lose someheat through the flue. The more efficientthe forced air heating system is, the coolerwill be the combustion gases that must bevented. On some of the more modern andhighly efficient systems, the flue can bemade of PVC pipe and can be ventedthrough the wall of the home, and thus nochimney is required.

The temperature of the combustiongases on older forced air heating systemsoften exceed 400F. These hot gases rise

F I G U R E 5 . 5

Automatic vent damper. (Courtesy of Honeywell, Inc.)

F I G U R E 5 . 6

Chimney vent system.


through the chimney very quickly, keepingit warm enough to not allow moisture fromthe combustion gases to condense. Recentfederal legislation requires new forced airheating systems to have efficiency ratingsof 78 percent or higher, which means thatthey may produce flue gases with tempera-tures of 200F or less. A chimney thatworked well with a lower-efficiency forcedair heating system may begin to show signsof staining or material breakdown when ahigh-efficiency forced air heating system isinstalled as a result of the lower flue gastemperatures. This lower flue gas tempera-ture will allow the combustion gases tocondense in the chimney.

As an example, an induced-draft forced air gas heating system is rated at about 80 per-cent efficient. An electrically powered fan blows the combustion products through theinside of the heat exchanger and into the flue. Because the gases are cool and do not heatthe flue as much, there is a greater chance for condensation to form. Another example isthe condensing forced air gas heating system that has an efficiency rating of 90 percent.In this type of forced air heating system, the vented combustion gases are so cool thatthey condense even before they leave the heating system. These gases are rarely dis-charged through a chimney. Instead, they are discharged through PVC pipe that ismounted through the wall of the home.

Now that we have learned the controls, electrical system, and operation of the homegas forced air heating system and the different types of gas forced air heating systems, itis time to move on and put this knowledge to use by performing a summer tune-up on a

F I G U R E 5 . 7

Heat exchanger.

If you are going toreplace an older, less efficient forced air heating system for a client with onethat is rated 78 percent or higher, thechimney must be replaced or upgraded ifnecessary.

QUICK>>>T IP

3 0 CHAPTER FIVE

F I G U R E 5 . 8

Combustion airstream.


F I G U R E 5 . 9

Heat exchanger airflow.

gas heating system. You should understand all these systems beforemoving on to the next chapter. If there was something that you did notunderstand, you should go back over the chapter again before movingon. If you are confident that you have an understanding of the preced-ing chapters, then lets move on and learn how to properly perform asummer tune-up on this type of system.

Now that we have examined the basic components of a home gasforced air heating system, it is time to learn how to perform a basicsummer tune-up that could save the homeowner hundreds of dollarsin unnecessary repairs that can be avoidedby some preventative maintenance. Forthe homeowner who wishes to have abasic summer tune-up performed, the costof the supplies needed to do the job shouldnot be more than $30.

The first thing to remember whenattempting any repairs on a home gasforced air heating system is safety. Youwill be in close contact with fire, gas, lowvoltage (24 V), and high voltage (110 V ac).

Tun i ng Up aGas F orced A i rHea t i ng Sys t em

6

3 3

C H A P T E R

1. Flat blade andPhilips screw drivers

2. A set of open-end wrenches

3. Oil can with SAE30 oil

4. Flashlight ordrop light

5. Thermostatwrench

6. Allen wrench set7. Voltmeter8. Millivolt tester

T 00LSSUMMER TUNE-UP

3 4 CHAPTER SIX

You must always be aware of these dangers. The most common causeof injury while working on these systems is not using common sense.You should never smoke around a gas heating system at any time. Youalso must make sure that the power is turned off to this device beforeattempting any repairs. With this said, lets begin the summer tune-upof your home gas forced air heating system.

The first thing that you must do is locate the power source to the furnace. This typically will be 110 V ac power, the same powerthat runs to the outlets and lights in the home. There usually will be two separate switches to disconnect the power to the furnace.The first one will be located in the fuse box or breaker box that supplies power to the home (Fig. 6.1). Once you locate this panel,open the front cover and look to see if any of the fuses or breakers is marked for the furnace. If there is one marked for the furnace,either remove the fuse or turn off the breaker. If there are no mark-ings, you will have to locate the fuse or breaker that controls thepower for the furnace. To accomplish this, the first thing you mustdo is have the furnace call for power. This can be done in one ofthree ways:

1. Locate the fan control, as described in Chap. 5, and see if there is abutton on the outside of the cover marked fan. If there is, pull thisbutton out, and the fan should start to run. Figure 6.2 shows a fancontrol with this button.

2. If the fan control cannot be used to start the blower manually, onsome thermostats there is a lever on the side of the unit that saysfan on or off. Turn this to the on position, and the fan should startto run. Figure 6.3 shows this type of thermostat.

3. Turn the thermostat up until the setting is higher that the tempera-ture in the home so as to start the heating cycle.

Once either the fan or furnace is running, go back to the fuse orbreaker panel and start either removing fuses (one at a time) or turningoff breakers until the furnace shuts off or the blower stops running.When you remove a fuse or turn off a breaker and the furnace or fandoes not stop, replace the fuse or reset the breaker and try the next one.Once you locate the fuse or breaker that controls the power to the fur-nace, mark this location on the inside of the panel for future use. This

TUNING UP A GAS FORCED AIR HEATING SYSTEM 3 5

will be a valuable tool in the chapter on troubleshooting the heatingsystem (Chap. 7).

The second switch for the furnace should be located either on ornear the unit. It will be a silver box with a switch and fuse or a graybox with a lever that you pull down. In this case, you must pull thelever down before you can open the cover to examine the fuse. In rarecases, there will be no second switch for the furnace, and the onlyplace to turn off the power to the furnace will be at the fuse panel orbreaker box. It is very important that you conduct a thorough searchfor this second switch because it will save valuable time if the home-owner should need to check the fuse on the furnace in the event thatthere is no heat one day and the cause is a bad fuse. This will be a very

F I G U R E 6 . 1

Fuse panel.

3 6 CHAPTER SIX

expensive fuse if the homeowner needs to call a service professionalto service the furnace in the night only to find out that there was a sec-ond fuse that he or she did not see. One woman who called me forservice late one night found this out the hard way, since that fuse costher over $200. She said that she was going to have it bronzed!

Now that the power is disconnected, be sure to either push the but-ton back in on the fan control, turn the thermostat down, or return theblower switch on the thermostat to the auto position at this time.

The Pilot AssemblyYou are now ready to begin the summer tune-up on the gas furnace. Itdoes not matter whether you have natural gas or propane, the stepswill be the same. We will begin with what I believe to be the most

F I G U R E 6 . 3Thermostat with manual fan control. (Courtesy of Hon-eywell, Inc.)

F I G U R E 6 . 2

Fan control with manual fan switch. (Courtesy of Hon-eywell, Inc.)


important safety device on the home gas furnacethe pilot assembly.Please read this section carefully and completely before conductingthe safety check on the pilot assembly!

From the preceding chapter, locate the pilot assembly on the fur-nace. It will be located with the burner(s). It may be behind a coveror panel, so you will either have to remove this panel or open theaccess panel. Figure 6.4 gives an example of an electronic pilotassembly. You will notice that the assembly consists of a pilot light(flame) and a thermocouple. This thermocouple is the safety devicethat sends a signal to the gas valve to either continue supplying gasto the pilot because it can sense that the pilot is lit by means of theheat or to shut off the supply of gas to the pilot because no heat is sensed. Without the pilot light being lit, the gas valve will notopen so as to allow dangerous gas to fill the home. This is why I feelthat the thermocouple is the most impor-tant safety device on the furnace. As youcan see, if the thermocouple is not inproper operating condition, it will notallow the furnace to operate, and youwill have no heat.

As I mentioned earlier, the thermo-couple operates by sensing the heat fromthe pilot. This heat is converted into mil-livolts (a small electric current). It takes aminimum of 20 mV to keep the pilot sec-tion of the gas valve open to supply gas to the pilot light. As I said, this is theminimum amount needed. The pilot maystill operate at less that 20 mV, but thiswould be a borderline situation, and thechance for a failure of the thermocouplein the winter would be high.

If you own a millivolt tester, you cantest the thermocouple by performing thefollowing steps. (Note: If you have a pairof alligator-type clips for your tester, thiswould be a big help. Otherwise, this stepcould be tricky.)

F I G U R E 6 . 4

Pilot assembly. (Courtesy of Lennox Industries, Inc.)

Make it a point to alwayscheck the operation of the thermocouplewhen performing a summer tune-up.

QUICK>>>T IP

3 8 CHAPTER SIX

1. Turn the knob on the gas valve to the pilot position. Press down andhold the button. If you release this knob, the pilot will go out, and youwill have to relight it. On an older-style furnace, where the pilot lineis connected directly to the main gas line, disregard this step.

2. Disconnect the thermocouple from the gas valve. (You do nothave to worry about gas escaping from the gas valve because thethermocouple is not connected to the gas.) The thermocoupleline is the gold-colored tube. On some older-style heating sys-tems, the thermocouple will be connected to a box with a redreset button on it.

3. Set your meter to millivolts. Make sure that it will read a minimumof 30 mV.

4. Connect the negative lead to a ground (gas line, bare metal, etc.).

5. Connect the positive lead to the end of the thermocouple lead thatyou removed from the gas valve (Fig. 6.5). Provided that the pilot isstill lit and that you have a good ground, you should be getting areading. If the reading is above 22 mV, you have a good thermocou-ple reading. If the reading is below 22 mV, you should replace thethermocouple as described later in this section.

If you do not own a meter, there is the manual approach. While thisapproach is not as accurate as the meter test, the results will be thesame if the test is done properly. To check your thermocouple manu-ally, perform the following steps:

1. Remove the access panel or lift the accessdoor that covers the pilot assembly.

2. Leaving the knob on the top of the gasvalve in the on position (the positionit should be in now), blow out the pilotlight. You will have to listen closely tohear the gas running to the pilot.

You are also listening to hear the gas valveclose. If the gas valve closes in 0 to 15 s,you will need to replace the thermocouple.

F I G U R E 6 . 5

Thermocouple check.


If the gas valve closes in 15 to 25 s, you have a marginal thermocouple.If this step takes 25 to 60 s, you have a good thermocouple.

Most people think that you would want the gas valve to close veryquickly to show a good thermocouple, but this is not the case. If ittakes a relatively long time for the gas valve to close, this indicates thatthe thermocouple is getting plenty of heat from the pilot light. This isalso a good indication that the pilot light is burning properly, with agood flame.

Before we discuss the steps to replace the thermocouple, lets takea moment to discuss the pilot light. In the test that we just performed,if the gas valve dropped out in a short amount of time, the reasonmight be traced back to a poor pilot light. How does the pilot lightlook? The flame should be dark blue in color. A poor pilot flamecauses many cases of failure. As you will recall from our previous dis-cussion on the operation of the pilot assembly, without proper heatfrom the pilot light, the thermocouple cannot keep the gas valve openbecause not enough current is generated by the pilot. Figure 6.6 illus-trates what the pilot light should look like.

The reason for this discussion is that if you are going to remove thepilot assembly to replace the thermocouple, you might as well cleanthe pilot at the same time. Many cases for failure can be traced to thepilot not being cleaned at the time a thermocouple was replaced. Thiswould be like replacing the engine in your car when the true cause ofthe problem was a faulty transmission. It will not operate properly.

To remove the pilot assembly, performthe following steps. Figure 6.7 shows thepilot assembly attached to the gas valveand burners.

1. Turn the knob on the top of the gasvalve to off. If the pilot line is notconnected to the gas valve, turn thevalve off. In either case, the pilot lightshould go out. If not, check to see wherethe pilot gas supply is coming from, andshut that off. (Warning: Do not performthe next step until the pilot light is out.

F I G U R E 6 . 6Makeup of the pilot flame.

4 0 CHAPTER SIX

This will cause gas to escape and couldpotentially cause an explosion!)

2. Disconnect the pilot line from the gasvalve or the valve to which it is connected.

3. Disconnect the thermocouple from thegas valve or control device.

4. Before disconnecting the pilot assem-bly, take a look at how it is connectedto the burner. Is the bracket in front ofthe burner or behind it? Taking a fewseconds to make this determinationnow will save you time when youreplace it. Also, take a look at the waythe pilot gas line runs. Since this is softaluminum tubing, it will bend veryeasily. It is fine to adjust this tubing,but care must be taken not to crimp it.If the tubing becomes crimped, youwill have to replace it.

5. Now remove the screws that hold thepilot assembly in place, and remove thepilot assembly from the furnace. Thismay be a little tricky, since you will

have to maneuver it under the burner and gas manifold. Take yourtime, and do not force it out.

Now that you have the pilot assembly in hand, you will need todetermine the length of the thermocouple lead. The most commonlengths are 24 and 36 in. It is always better to get a little longer leadthan one that is too short.

Remove the thermocouple by either unscrewing it from the pilotassembly or pulling it from the pilot assembly if there is no nut toremove. When you purchase a new thermocouple, it should come witha universal mount kit so that it will work in either case. To replace thethermocouple, find the items in the kit that will be needed to adapt thethermocouple to the pilot assembly as it was when you removed it. Ifyou are planning to clean the pilot light at the same time (this is recom-

F I G U R E 6 . 7

Burner assembly with pilot assembly.


mended as long as you have the assemblyout), do not replace the thermocouple untilafter you put the pilot section back together.You will need the extra room for access tothe nuts that hold the pilot gas supply lineand orifice to the pilot assembly. If you arenot planning to replace the thermocouplebecause you have determined that thecause of the failure is a dirty pilot, you will need to remove the thermo-couple to allow enough room to remove the lines and orifice to the pilot.You should consider replacing the thermocouple at this time, however,since it is inexpensive and could save you time later.

If you had to unscrewthe thermocouple from the pilot assembly, use the adapter with threadsattached to it. If the thermocouple waspulled out, use the adapter for the bayonet-type assembly.

QUICK>>>T IP

TO CLEAN A PILOT LIGHT:

1. Remove the thermocouple.2. Place one wrench on the orifice (this is located closest to the

pilot). Place one wrench on the nut that screws the pilot gas lineinto the orifice.

3. Turn the nut to remove the gas line from the orifice.4. Hold the pilot assembly with vice-grip pliers while removing the

orifice from the pilot assembly. (Caution: The orifice is made ofvery soft brass. Care must be taken not to crush the end of this ori-fice or the pilot gas line will not screw back into place. If this hap-pens, you must replace the orifice.)

5. Blow through the orifice to remove any dirt from the holes. Checkthat they are clear by looking through the orifice while looking at alight source. If the hole or holes look clear, the orifice is clean.(Warning: Do not attempt to clean the holes by placing anything inthem. This will only enlarge the holes and make the pilot unsafe tooperate. These are precision-sized holes and should never be altered.)

6. Holding the pilot assembly, blow through, from the bottom, toremove any dust or dirt from the pilot section. You have nowcleaned the pilot assembly.

7. Assemble the pilot assembly by reversing the preceding steps. Whenfinished, you should have the thermocouple securely in place andthe orifice and pilot gas line in place.

T R O U B L E-S H O O T I N G

4 2 CHAPTER SIX

Before attaching the pilot assembly to the burner, you should takethe time now to see if there is any debris on the burners themselves.This will act as an insulator and will reduce the efficiency of yourheating system. If there is debris on the burners (e.g., rust flakes, soot,etc.), you will want to remove the burners to properly clean them. Toclean the burners, perform the following steps. Figure 6.8 shows howto disassemble the burners.

1. Turn off the main gas supply to the gas valve. This valve should belocated on the main gas line leading to the gas valve.

2. Using a pipe wrench or slip-joint pliers,loosen the union fitting on the main gasline close to the gas valve.

3. Loosen or remove the screws that holdthe gas manifold to the sheet metal bythe burners.

4. Lift the manifold up to release it fromthe screws that you just loosened. Insome cases, you may need to removepart or all of the screws to free themanifold.

F I G U R E 6 . 8

Burner removal. (Courtesy of Lennox Industries, Inc.)

SIMPLIFIED BURNERREMOVAL:

1. Remove cover by loosening bottomscrews and removing cover frontscrews.

2. Remove pilot tube, spark wire andsensor wire. Remove gas valve and manifold assembly.

3. Remove burner assembly.

T R O U B L E-S H O O T I N G


You should now be able to remove the burners from the heatexchanger. It is important to remove and clean only one burner at atime because they each have their own location within the heatexchanger. Please note the silver metal ring at the end of the burnerwhere the orifice from the manifold is located. This ring controls theamount of air that is allowed into the burner during the operation.Care should be taken not to move the location of these rings, sincethey have been set at this point. When you begin the heating cycleafter the summer tune-up is complete, you will examine the way theflame of each burner looks, and you may need to adjust these regis-ters at that time.

Each burner should be cleaned with either a brush or a vacuumcleaner to remove any debris and dirt that is on them. Once you havecleaned the burner, you need to blow air through the burner to removeany dust that is inside them. Simply blow into the round end of theburner to blow the dust out. When no more dust blows out of theburner, replace it in the location from which it came, making sure thatthe tab at the rear of the burner is in contact with the slot in the heatexchanger. You can check this by twisting the burner once it is inplace. If you are able to twist the burner, it is not in the proper location.Adjust the burner as needed to lock it intoplace. Now repeat the operation for theremaining burners.

Once all the burners have been cleanedand installed, lift the manifold back intoposition, making sure that the orifice onthe manifold is inside the hole in the endof the burner. Replace or tighten thescrews that hold the manifold in place,and examine the location of the orifice tothe burner one more time to make sure thateach orifice is resting inside the hole in theend of the burner. Connect the union backinto place and tighten. Whenever youremove any main gas lines, you mustcheck them for leaks before starting theheating system.

SAFE WAYS TO CHECKFOR LEAKS IN MAIN GAS LINES

1. Use a gas sniffer to determine if thereis any gas leaking from the connection.If a leak is found, tighten the connec-tion until no more gas is detected. Thisis a very easy way to check for leaks.

2. Make a thick mixture of detergent andwater. Simply brush the solution ontothe connection. If there is a leak, theleak will cause the soap to form bubbles. If a leak if found, wipe the solution from the connection, tightenit, and test again. Repeat this opera-tion until the leak is fixed.

QUICK>>>T IP

4 4 CHAPTER SIX

Now attach the pilot assembly to theburner with the screws that you removed.Next, install the pilot gas line in theproper location on the gas valve or line,and then attach the thermocouple lead inthe proper place.

Turn the knob on top of the gas valve to pilot. You will need along match or lighter to light the pilot light. When you have one ofthese items ready, hold it next to the pilot assembly, and press the but-ton down to light the pilot light. Continue to hold the button down for30 to 60 s. Release the button and turn the knob to on. If the pilotlight does not remain on after 60 s, check all your connections; look atthe pilot flame to make sure that it is clean and that it is in good con-tact with the thermocouple. If not, turn the gas valve off and repeat thepreceding steps until the pilot remains on. If everything looks goodand the pilot still will not remain on, remove the thermocouple fromthe gas valve and look to see if there is anything inside the hole thatyou screw the thermocouple into. I have seen a situation in which theend of the thermocouple that was replaced broke off and remainedinside. This will cause the new thermocouple to not make contactwith the valve, and this will keep the gas valve from operating.

If you still cannot keep the pilot lit, you may have a problem withthe gas valve, and it will have to be replaced. If the thermocouple isconnected to a box with a reset button on it, you will have to replacethis item.

In some units that have not been serviced in some time, this is quitecommon. The pilot remains on all the time, and a problem is notdetected until you begin your work. You may have a hard time explain-ing to the homeowner why you are replacing an expensive gas valvewhen everything was working fine before you arrived. My answer to thisis: Mrs. Jones, it is good that we found this problem before the heatingseason starts, because now you will not have to worry this winter.

If everything is working properly, you can now move on to the nextpart of the heating system that must be servicedthe blower. This isthe same procedure that you will have to go through if you have a fail-ure in the thermocouple in the winter, and as you have seen, it is muchmore enjoyable to do this in the summer than when your client dis-covers that he or she has no heat in the winter.

Use a magnetic screw-driver to hold the screw while you positionthe pilot assembly in place.

QUICK>>>T IP


The Blower AssemblyNow that you have serviced the heating section of the furnace, letsturn our attention to the blower section. As I mentioned at the begin-ning of this book, there two basic types of blower units: (1) directdrive, where the blower cage is attached directly to the motor shaft,and (2) belt drive, where the blower cage is connected to the motor bya belt and pulley system. Both these types of systems perform the samefunctionto blow warm air into your home during the heating cycle.

Locate the blower section of the heating system. If you have deter-mined that you are dealing with an upflow system, the blower will belocated in the compartment below the burner unit. If you have a down-flow system, the blower will be located in the compartment above theburner unit. If you have a low boy system, the blower will be locatedin the rear of the system. The first thing that must be done before begin-ning to service the blower unit is to make sure the power is disconnectedto the heating system. Once the power has been disconnected, removethe door(s) to gain access to the blower assembly.

Direct-Drive Unit

If you are dealing with a direct-drive unit,look at the end of the motor that is protrud-ing out from the blower housing (Fig. 6.9).Now look at the top of this motor and see ifthere is a small hole at that location. If thereis, then you will want to add 2 to 3 drops ofoil in this hole to lubricate the bearings. Insome cases, there also may be a hole on theopposite end of the motor that is inside theblower cage. If there is, you will need tolubricate this area as well. If you cannotlocate any small oil ports on your motor,then you have a sealed motor unit, and nolubrication is necessary.

You should now take the time to exam-ine the blower (squirrel cage) as well. Ifthere is a buildup of dirt on the fins of thecage, you need to clean them out. A

F I G U R E 6 . 9

Direct-drive blower motor. (Courtesy of Lennox Indus-tries, Inc.)

4 6 CHAPTER SIX

buildup of dirt on the fins will cause a reduction in the amount of air-flow that the blower can provide. You can use a screwdriver to scrapethe dirt off the fins starting from the rear and pulling it toward you.Remove this dirt with either a vacuum cleaner or some device so thatthe fins are clean. You also will want to make sure the blower cabinetis free from dirt and dust. It will not do much good to clean the blowerfins and leave dirt in the blower cabinet that will be pulled into theblower the next time the blower is started.

The next item to examine is the filter. In the downflow type of heat-ing system, the most common type of filter is the mat filter. In this typeof filter, a metal cage holds the filter material. This material typicallycomes in a roll that is cut to fit the cage. Some heating companies havethis material cut to size and prepackaged for the homeowner whowants to replace his or her own filters. The heating professional,though, probably will have a roll of this material on his or her truck.

Remove the filter cage from the blower cabinet. Slide the bar ateach end of the filter cage, and separate the two sides. Remove the oldfilter material and measure the length so that you have the proper size.

Select the width of filter material that youwill need, and cut the new filter to size.You will note that the filter material has alight color on one side and a darker coloron the other side. You also should notethat one side of the filter material is coatedwith an oily substance. This is the sidethat faces up into the cold air duct. This isthe side of the material that will collect thedust and dirt that is pulled into the blowercabinet.

Once you have the filter material cut tosize, lay it on the cage and assemble thecage unit. Now install this cage back intothe blower cabinet. Replace the door(s) onthe blower cabinet.

The other type of filter is the box filter.Figure 6.10 shows this type of filter. Thebox filter may be located in a slot in thecold air return or in the top of the blower

F I G U R E 6 . 1 0

Box filter.


compartment. It is made up of a cardboard frame and filter material.On one side of the filter material there will be wire mesh. This isthere to protect the blower from the filter material being pulled intothe blower. This is the side that faces the blower. There is an arrowon the side of the filter that shows the direction of the airflow. Thefilter should be installed with the arrow pointing toward the blower.There are several styles of this filter on the market. Some of these fil-ters do not have wire mesh covering the filter material. You do notwant to use this type of filter because there is a chance that the filtermaterial can be pulled into the blower. This will cause damage to theblower and could mean that you will have to replace the motorbefore it is time.

This unit is now serviced and is ready for use.

Belt-Drive UnitsIf you have a belt-drive unit, this will require slightly more maintenancethan a direct-drive unit. Most older heating systems use this type of sys-tem (Fig. 6.11). On an upflow unit, the blower cabinet will be locatedunder the burner assembly. On a low boy type of heating system, theblower cabinet will be located in the rear of the unit.

First, locate the motor unit. This typically will be located on topand to the rear of the blower unit. You will find an oil hole located ateach end of the motor. You will need toadd 2 or 3 drops of oil into each of theseholes. In some cases, where the motor hasbeen replaced with a more modern motor,there will be no oil ports. In this case, youhave a sealed bearing unit, and no lubrica-tion is required. Next, remove the beltfrom the pulleys. To do this, pull the beltover one of the pulleys, and rotate the pul-ley clockwise to remove the belt. Caremust be taken not to get your fingerscaught between the belt and the pulley.Turn the belt inside out, and examine it. Ifthere are signs of splitting or cracking, thebelt will need to be replaced. Turn the beltright-side out, and look for the size of the

1/4 TO 1/2Deflection

F I G U R E 6 . 1 1

Belt-drive motor.

4 8 CHAPTER SIX

belt. This will be printed on the outer cas-ing of the belt. If you cannot read thisinformation, take the belt to an auto partsor hardware store, and they should be ableto measure this for you so that you get theproper size replacement belt. If you are at aservice call location where it is not conve-nient to leave to find an auto parts or hard-ware store, find a belt in your service truckthat is slightly smaller than the one thatyou removed. It can be assumed that thebelt has been on the unit for some time andthat it has stretched. It is more important

to get the proper width of belt to the pulley that is on the unit, sinceyou can adjust for length if it is required. You also should look aroundthe heating unit to see if the last person to service the unit left an oldbelt there. This will give you an idea as to the proper size as well.

Next, examine the blower unit. In some cases, oil ports will belocated on the bearings that the blower shaft goes through. If such portsexist, add oil to them. If there are no oil ports, check to see if your blowerhas grease cups. These would be located on the upper portion of thebearings. If there are grease cups, remove the cup(s) to see if there isenough grease in them. Most of these types of lubricating systemsrequire a special grease that you may need to purchase from your localheating company. The heating professional should always have at leastone tube of this grease on the service truck. Fill the cups, and replacethem on the bearings. Turn them down until you feel a resistance (itbecomes harder to turn them). The homeowner should turn these greasecups one-quarter to one-half turn every other month during the heatingseason to allow for proper lubrication to the bearings. Failure to do thismay cause the bearings to go dry. This will cause a metal-to-metal situ-ation that will wear out the blower shaft.

If the heating system has not been serviced for a long period oftime, this is one item that you should check as part of the tune-upprocess. You should ask the homeowner if the blower is noisy whenthe unit is running. If he or she says yes, this will give you a goodindication if there is excessive wear on the blower shaft.

You can check for this condition in one of two ways:

Always leave the partsthat you replace with the homeowner.This will serve two important functions:(1) to show the homeowner that you didin fact replace the parts, and (2) as areminder so that the next time you service the unit, you can remember whatparts have been replaced. Always writethe date of replacement on the cartonsor packages that the parts came in andleave them next to the heating unit.

QUICK>>>T IP


1. With the belt removed, pull up and down on the blower shaft pul-ley. If there is excessive play (a good bearing and shaft should notmove at all), you have a worn-out bearing.

2. Replace the belt on the motor and blower, and turn the fan switchto manual to start the blower. Listen to the sound of the blower. Ifyou hear a squealing noise or a rumble, you have a bad wear situa-tion. You also can look at the way the pulley turns. If you detect awobble in the pulley, you have wear.

You need to inform the homeowner of this finding before you pro-ceed any farther. You must get the homeowners permission to make thisexpensive repair. In most cases, this will be a time and material job, anddepending on how far you are from your shop (you will need to disas-semble the unit and go to the shop to repair it), you could be looking at2 to 3 hours of labor. Explain to the homeowner that the situation willonly get worse and that he or she could be looking at this unit failing inthe winter, and then there will be no heat. Also explain that this is thereason you come to the home in the summer to tune up the heating sys-tem so that the chances of having a failure in the winter are reduced.

Once you have the homeowners per-mission to do the repair, you will need todisassemble the unit and remove it fromthe blower cabinet.

Once you get the blower removed fromthe cabinet, examine the blower fins forsigns of dirt buildup. If there is a buildupon the fins, prior to taking the unit to theshop, stop at a local manual car wash andspray down the blower unit. The high-pressure spray works great to remove theold buildup on these types of units. Wherethe buildup is very heavy, some of thesecar washes have a setting for degreasers,and this works well.

Once this is complete and the blowerunit is at the shop and on the bench, youwill need to remove the locking collarsthat hold the shaft in place. Normally, you

DISASSEMBLING THEUNIT FROM THE BLOWER CABINET

1. Check to make sure that the power tothe unit is turned off.

2. Remove the locking straps that holdthe motor to the blower unit.

3. Remove the motor from the motorbracket, and place it on the bottom ofthe blower cabinet. Make sure that youdo not allow the motor to hang fromthe wires or conduit that is connectedto the motor. Use some kind of brace ifneeded to support the motor.

4. Remove the bolts that hold the blowerunit to the heating unit.

5. Remove the blower unit from theblower cabinet.

QUICK>>>T IP

5 0 CHAPTER SIX

will find two of these collars located on the opposite end of the blowerfrom the pulley. One should be on the outside and one on the inside ofthe rear bearing. On some units, these devices will be incorporatedinto the blower cage, one at either end. In this case, simply loosen thesetscrews, but do not remove them. Loosen and remove the outsidecollar, and loosen the inside collar. This will then allow the blowershaft to be removed. If the bearings have been neglected for a longperiod of time, the shaft may be frozen to the bearings and will notslide out of the unit. In this case, lubricate the entire shaft with motoroil. You will then need to use something to drive the shaft out of thebearings.

Never strike the end of the shaft with a hammer or other drivingdevice. This may cause the end of the shaft to flare. The best approach isto use a piece of shaft material that is either the same size or slightlysmaller to drive the shaft out of the bearings. Once you have the shaft freefrom the rear bearing, you can attempt to pull it out the remainder of the way by twisting the shaft and turning the pulley back and forth whilepulling at the same time. If this works, pull the shaft free from the blowerunit. If the shaft still will not come free, you will need to continue withthe driving method until the shaft is free from the blower unit.

Once the shaft is free, you will need to remove the pulley from theshaft, since you will need to use this pulley on the new shaft. Loosenthe setscrew that holds the pulley on the shaft, and remove it from theshaft. If the pulley will not come off, place motor oil between the endof the shaft and the pulley, and use a wheel puller to remove the pul-ley. Place the jaws of the puller around the pulley, and screw the jack-ing bolt down to make contact with the end of the shaft. Tighten thejacking bolt with a wrench until the pulley comes free. Care must betaken if the pulley is aluminum so that you do not bend it. If it doesbend, you will need to replace the pulley as well.

Next, remove the bearings from the blower unit, and find replace-ments that are the same number. It does not matter if the replacementbearings have grease fittings or oil fittings; they will both work equallywell. I prefer grease fittings over oil fittings because it is much easierto get the homeowner to turn the grease fittings down than to have himor her put oil in the oil fittings.

Once you have the new bearings installed, you will need to make anew shaft. You will need to take three measurements of the shaft:


1. Measure the length of the shaft.

2. Measure the diameter of the shaft.

3. Measure the length of the flat spot on the shaft that is used totighten the pulley.

Once you have all three of these measurements, cut the shaft tolength, and grind the flat spot on the shaft. Lightly oil the shaft, andslide it into the first bearing and through the blower cage, and insertthe first locking collar. Next, slide the end of the shaft through the rearbearing, and attach the other locking collar. Tighten the locking collarsonly finger tight, since you will have to make the final adjustmentsonce the blower is located back in the blower cabinet. In the case of setscrews that are part of the blower cage, use this same procedure, andonly tighten them finger tight as well. Slide the pulley onto the shaft,making sure that the set screw is lined up with the flat spot on theshaft. Tighten the set screw finger tight as well.

Once the blower is reinstalled in the blower cabinet, replace themotor on the motor bracket, and secure in place. You will need to mea-sure the distance from the outside of the blower housing to the insideof the motor pulley. This measurement will need to be the same as thedistance from the blower housing to the inside of the blower pulley.This distance is critical so that the belt will travel in a straight linefrom the blower motor to the blower itself. If the belt does not travel ina straight line, the belt can jump off either pulley or cause excessivewear in the new bearings that you just installed.

Once you have the alignment correct, tighten all set screws on theblower shaft and the pulley. Also double-check to make sure that themounting bolts for the blower unit and motor mounts are tight. Nowcheck the belt tension, and make sure that you have the proper 14- to12-in deflection. Make the necessary adjustments to the motor adjust-ment jacking screw to achieve this deflection.

Turn on the power to the heating unit, and manually start the bloweronly by means of the blower switch or by adjusting the fan control tostart the blower. Watch how the belt turns on the blower pulley. If theredoes not appear to be a straight line of travel between the motor pulleyand the blower pulley, turn off the power and adjust as necessary; thenrecheck. When the alignment is correct, this repair is complete.

5 2 CHAPTER SIX

You will now want to examine and/or replace the filter. The filterwill be located either inside the blower compartment or inside thecold air duct leading to the blower compartment. Filters come in sev-eral types depending on the type of heating system that you have. Itwill either be a box filter (see Fig. 6.10), in which the filter material isenclosed in a cardboard box and has a mesh cover on the side thatfaces the blower, or a mat filter, in which the material will be enclosedin a metal frame. Filters come in several different sizes as well.

Locate the filter in the heating unit and remove it. If you have a boxfilter, the size of the filter will be imprinted on the outside of the filter.Replace this filter with the same size filter. Make sure that you look forthe arrow on the side of the filter that will show you which way the fil-ter is to be installed. Typically, the way to remember this is that theside with the metal mesh that covers the filter material will face theblower compartment. This is designed so that the filter material willnot be sucked into the blower during the heating cycle. If you have amat filter that is mounted in a removable frame, you will need to mea-sure the amount of filter material you will need. This can be done bymeasuring the existing material that is mounted in the frame. Writethis information down for future use. Cut the proper size filter fromthe roll, and install it in the frame, making sure to put the coated sideout. Place the other side of the frame on the filter, and lock it intoplace. Reinstall the filter frame into the blower cabinet.

The summer tune-up of the gas forced air heating system is nowcomplete. All that remains is to replace any doors and access panelsthat you removed during this operation and turn the power back on byresetting all breakers or replacing the fuse(s). The final step is to checkthe operation of the heating unit by running it through a completeheating cycle and checking the safeties.

Before you turn on the power to the gas heating unit, you will needto disconnect the power to the blower unit. The reason that this isdone is so that you can check for the proper operation of the high limitsafety (Fig. 6.12). This is the high limit that is set on the fan controland will shut off the power to the gas valve and blower if the blowerdoes not come on during the heating cycle.

If you have a direct-drive unit, open the access panel to the wiringcabinet (make sure that the power it turned off first), and locate theblack (hot) wires. Look at the wires, and you should find a wire


marked for the blower. Remove the wire nut that holds all the wirestogether, and remove the wire for the blower. Replace the wire nut onthe remaining wires, and make sure that the black wire for the bloweris not touching any metal objects. You also must make sure that you donot touch this wire while the power is turned on.

If you have a belt-drive blower, you will not have to disconnect anywires. Simply remove the belt to simulate this same condition.

Before you turn the power back on to the unit, you need to checkthe thermostat to make sure that it is operating properly. Remove the

F I G U R E 6 . 1 2

Limit switch. (Courtesy of Lennox Industries, Inc.)

5 4 CHAPTER SIX

front cover from the thermostat, andcheck for any lint or dirt, removing any ifnecessary (Fig. 6.13). Next, turn the ther-mostat up slowly until either you see themercury bulb drop to the right to indicatethat the thermostat is calling for heat oryou hear the bimetallic points cometogether indicating the same thing. Nowlook at the pointer on the top of the ther-mostat that shows where the reading is atthe point that the heating cycle shouldstart. Now compare this to the reading onthe lower part of the thermostat (the ther-mometer reading that shows the actualtemperature in the home). These readingsshould be the same. If they are not, youneed to adjust the thermostat so that theyare the same.

If you have a thermostat wrench, place it on the nut behind the coilthat controls either the mercury bulb or the points. Deter

forced hot air furnaces troubleshooting and repair

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