Thermostats

I would recommend that as you proceed

through this power point presentation that

you refer to your textbook, Unit 14, for some

great visuals and additional information.

There used to be two major players when it came

to the ID (identification code) used on thermostats.

Those two players were: ITT (International

Telephone and Telegraph) and Honeywell. You

may still see some ‘old’ thermostats with the ITT

ID code on them, but Honeywell’s ID code is what

you will see today.

There may also be some manufacturer specific ID

code as well. This presentation will briefly mention

the ITT code, but will focus on Honeywell’s ID

code.

Honeywell ID code ITT ID code Meaning

R

W

Y

G

V

H

C

G or F

Voltage/Power

Heating

Cooling

G or Fan

The terminals shown above are basic and there

are more than these four. Additional terminal

letters and their ID meaning can found via the

internet and/or manufacturer’s literature.

The ‘R’ or ‘V’ ID is the terminal on the thermostat

that is connected to the ‘P’ power side of the low

voltage transformer. The color of the wire is

unofficially ‘red.’

The ‘W’ or ‘H’ ID is the terminal on the thermostat

that is connected to the heating equipment. The

color of the wire is unofficially ‘white.’

The ‘Y’ or ‘C’ ID is the terminal on the thermostat

that is connected to the cooling equipment. The

color of the wire is unofficially ‘yellow.’

The ‘G’ or ‘F’ ID is the terminal on the thermostat

that is connected to the fan relay which controls

the fan speed for heating and cooling. The color of

the wire is unofficially ‘green.’

Moving onto the application of thermostats.

HEATING ONLY thermostat

This would be used where air conditioning would

not be required. The heating only thermostat has

a base and a mounting plate for installation.

Thermostat Equipment

R R

WW

RED Wire

WHITE Wire

HEATING ONLY

This circuit would require only two wires to function.

The ‘older’ electromechanical thermostats had an

‘anticipator.’ There is a good discussion of this in

your textbook.

What is important for you to know about the

electromechanical thermostat heating anticipator?

1. is wired in series

2. if it should fail open the heating equipment will not

work

3. it is adjustable and needs to be adjusted based on the

actual load current in the heating circuit. The amp.

(current) value for the circuit is performed with an

ammeter and a 10 loop coil. The 10 loop is wired in

series with the W (heating) circuit.

The electronic thermostats do not use an

anticipator like the older electromechanical

thermostats. BUT they do need to be adjusted.

This is not done with the 10 loop method, but one

must check with the manufacturer of the

thermostat and follow their directions.

COOLING ONLY thermostat

This would be used where heating would not be

required. The cooling only thermostat has a base

and a subbase for installation.

The ‘older’ electromechanical thermostats had an

‘anticipator.’ There is a good discussion of this in

your textbook.

What is important for you to know about the

electromechanical thermostat cooling anticipator?

1. is wired in parallel with the thermostat contacts

2. if it should fail open the cooling equipment will

continue to work

3. it is NOT adjustable

4. occupant would probably never know it is

defective.

The electronic thermostats do not use an

anticipator like the older electromechanical

thermostats. BUT some of them may require

adjustment. Check with the manufacturer of the

thermostat and follow their directions.

Thermostat Equipment

R R

YY

RED Wire

BLUE Wire

COOLING ONLY

This circuit would require three wires to function. The unofficial color code for the wires could be what is shown,or different. Sometimes the RED wire is a BLACK wire.

G GGREEN Wire

COOLING ONLY thermostat

The cooling only thermostat would have a subbase with the

following two switches on it. A system switch to set the

thermostat for CLG – OFF and a fan switch with the

functions: AUTO – ON.

COOLING ONLY thermostat

The system switch has two positions designated to turn the

air conditioner on (CLG.) or off (OFF).

COOLING ONLY thermostat

The fan switch has two positions designated to turn the

fan on (ON) or auto (AUTO). Please refer to your textbook

for some visuals on this switch. The fan ON position will turn

the fan on continuously for the speed required for cooling.

The AUTO position will cycle the fan when the air conditioner

is operating.

COOLING ONLY thermostat

In both situations, this fan switch found on the thermostat will

only operate the fan on the speed for air conditioning. So do

not confuse this fan switch with the fan switch on a furnace

that operates the fan on the speed for heating.

HEATING and COOLING

thermostat

The older electromechanical heating and cooling thermostats

required a base and a subbase. The subbase is where the

system and fan switch. The electronic thermostats are

usually a one piece assembly and do not have a subbase.

There are electronic touch screen programmable thermostats

that do not have a subbase either. All functions of the

thermostat are programmed at the time of installation.

Thermostat Equipment

R R

WW

RED Wire

YELLOW Wire

HEATING and COOLING

This circuit would require four wires to function. The unofficial color code for the wires could be what is shown.

Y YGREEN Wire

G G

WHITE Wire

Here is an example of an electronic thermostat withits cover removed to expose the terminals.

System Switch

Fan Switch

Terminal Identification

The next thermostat circuit is going to show an

‘isolated’ heating and cooling circuit. This means

that there are two low voltage sources. One

source would be for the heating system and the

other source would be for the cooling system. The

sources MUST not be put together, so they are

‘isolated’ at the thermostat.

Thermostat

AC Equipment

RH R

RRC

RED Wire

YELLOW Wire

This circuit would require five wires to function. The unofficial color code for the wires could be what is shown.

Y YGREEN Wire

G G

WHITE Wire

Heating Equipment

W W

BLACK Wire

Did you notice that on the previous slide the

thermostat had 2 ‘R’ terminals? One labeled R or

RH for heating and the other one labeled RC for

cooling. The thermostat may have both terminals

with a ‘jumper’ connecting the two. If the ‘isolation’

is not required, the ‘jumper’ would be left alone. If

the system requires ‘isolation,’ the jumper would

have to be removed.

Terminal Identification

R and RC with jumper installed from factory.

Examples of isolation:

1. Gas or Oil fired boiler with air conditioning.

2. Heating and cooling system with heating

system using DC volts and air conditioner

using AC volts.

There are heating systems with stages of heating

and this is represented on the thermostat and

equipment as:

W1 - first stage

W2 - second stage

There are cooling systems with stages of cooling

and this is represented on the thermostat and

equipment as:

Y1 - first stage

Y2 - second stage

A review of transformers:

1. Step down – output voltage lower than input

2. Step up – output voltage greater than input

3. Rating is: VA and Primary and Secondary E

4.Primary is connected to the line.

5.Secondary is connected to the load.

This concludes this review of thermostats.