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Heating & Air Conditioning Duct & Flexduct Routing & Support Specifications

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Page 1: Heating & Air Conditioning Duct & Flexduct Routing & Support Specifications

3/30/13 Heating & Air Conditioning Duct & Flexduct Routing & Support Specifications

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Heating & AirConditioning Duct &Flexduct Routing &Support SpecificationsInspectAPedia® - ShareThis

Heating & Air Conditioning Duct Routing & Support Guide

Air Conditioning (or Heating) Duct Defects: HVAC ducts installed in

contact with the ground

Sharp bends in ductwork

Excessive or Unnecessary Air Duct Lengths Increase Cooling or Heating Cost

Pinched or crimped ductwork, air ducts in concrete floors

Flexduct support requirements & specifications

Other duct routing & support mistakes to avoid

Flex Duct Connection & Splicing Specifications

Questions & answers about proper HVAC duct routing and support

References

Click to Show or Hide Related Topics

Air duct routing & support mistakes to avoid: this HVAC duct design and repair article describes proper (andimproper) Heating & Air Conditioning Duct Routing & Support such as duct routing and support details toprevent or fix loose, sagging, crimped, bent or otherwise defective heating or cooling ductwork. Crimped,squashed, or sagging ductwork restricts cooling or heating airflow into a building. Air ducts routed in aconcrete floor slab (see SLAB DUCTWORK) also invite many problems including air quality issues andcollapsed ductwork that reduces airflow and increases system operating cost.

InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers nor with topics or services discussed at this website.

Green links show where you are. © Copyright 2013 InspectAPedia.com, All Rights Reserved. Author Daniel Friedman.

HVAC Air Duct Routing & Support Errors & Guide totroubleshooting

Air conditioning duct system defects include a remarkably wide range of errors, from failure to supply coolair or failure to even circulate air in the building, to health hazards such as use of asbestos material in oron duct work, to very dangerous conditions such as drawing heating equipment combustion gases intothe building cooling (or heating) air.

This chapter describes errors to avoid when installing ductwork in buildings. Avoiding these errors willresult in improved air flow, lower system operating cost, and may help in avoiding problems of mold orother contaminants in the heating or cooling duct system of a building.

The master document, of which this is a chapter, describes the inspection of residential air conditioningsystems (A/C systems) to inform home buyers, owners, and home inspectors of common cooling systemdefects and repairs.

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DATA TAGS on AIR CONDITIONERSDEFINITION of Heating & Cooling TermsDEHUMIDIFICATION PROBLEMSDEW POINT CALCULATION for WALLSDEW POINT TABLE - CONDENSATION POINT GUIDEDIAGNOSE & FIX AIR CONDITIONER / HEAT PUMPDIRECT VENTS / SIDE WALL VENTSDIRECTORY of OIL TANK EXPERTSDRAFT HOODS - gas firedDRAFT MEASUREMENT, CHIMNEYS & FLUESDRAFT REGULATORS, DAMPERS, BOOSTERSDUCT SYSTEM & DUCT DEFECTSDUST, HVAC CONTAMINATION STUDYDUST SAMPLING PROCEDURE

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GALVANIC SCALE & METAL CORROSIONGAS BURNER Flame & Noise DefectsGAS FIRED WATER HEATERSGAS PIPING, VALVES, CONTROLSGAUGES ON HEATING EQUIPMENT

Sketches adapted from GA recommended construction code & from: [13][14][15][16]

HVAC Ducts in ground contact

When heating and cooling ducts are placed in ground contact, such asin the crawl space shown in the photo, not only is the contact going toconduct heat or cooling away from the duct before the air reaches itsdestination, but further, there is a significant risk of waterleaks/condensation in the duct in damp or flooding areas (risking amold or health concern) or rodent entry.

Heating or cooling air ducts should be supported away from groundcontact such as in crawl spaces.

Sharp Bends in Air Conditioning or Heating Duct Work Reduce Airflow

Sharp bends in ductwork restrict airflow (and violate

ASHRAE or SMACNA guidelines for duct installations).Restrictions in airflow through duct systems increasethe heating or cooling system operating cost andreduce the comfort of building occupants.

This photograph shows flex-duct in an attic making atoo-tight 180 degree turn, crimping and restrictingairflow in the duct system.

Bends in flex duct should not be acute and should notbe less than one duct diameter in radius - Sources: [13]

[14][15][16]

Excessive or Unnecessary Duct Lengths Increase Cooling or Heating Cost

Excessive length of ducts is often found where flex-duct is installed by an amateur. If you see a length offlex-duct snaking across an area with multipleunnecessary twists and turns, the combination of lengthand unnecessary bends reduces airflow, with the costsjust cited above.

This photograph shows unnecessary lengths of small-diameter flex duct left by the installer. The smalldiameter of these ducts also tells us that we're lookingat a high-velocity air conditioning system that uses acombination of small-diameter ducts and higher airvelocity to deliver cooling air to the conditioned space.

Flex duct runs should be as short and direct as possible without at the same time causing

inappropriately sharp bends or kinks.- Sources: [13][14][15][16]

HVAC air duct kinks & sharp bends restrict airflow

Flex duct runs should be as short and direct as possible without at the same time causing

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GAUGE, REFRIGERATION PRESSURE TESTGEOTHERMAL HEATING SYSTEMS

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inappropriately sharp bends or

kinks.- Sources: [13][14][15][16]

Illustration adapted from [13].

The smoothness of the duct interior(flex duct is more resistant to airflowthan a solid duct surface) is affectedby the degree of flexduct extension.ACCA Manual D includes a frictionchart that takes this into consideration(Appendix 2, chart 7).

Incidentally, when installing a length offlexduct, you should not just leaveextra duct length "squashed" and in

the run. Duct calculations assume that you have removed excess length of flexduct and that the installed length of flex-ducthas been "fully extended".Why? For duct interior smoothness and better airflow. Fully extended flex duct will significantlyredue the friction losses in the sysem.

Flexible Ductwork Support Specifications

Pinched or Crimped Air Ducts Reduce Heating or

Cooling Airflow

Other common duct routing errors include sharp bendsin duct work, mismatched sizes of duct work amongsections, flex duct which has become crimped orpinched to restrict air flow such as in the photo at left,and of course ducts which have become disconnected.

We discuss and illustrate disconnected heating or airconditioning duct defects at DUCT CONNECTIONS.

We show the interior of crimped or squashed flexduct atDUCT DAMAGE, MECHANICAL.

The photo at left violates several of the flex-duct installation guidelines we cite just below:

Flexible duct shall be supported at manufacturer’s recommended intervals, but at no greater

distance than 5 feet (1524 mm). Maximum permissible sag is ½ inch per foot of spacingbetween supports.

Hanger or saddle material in contact with the flexible duct should be of sufficient width(minimum 1 ½” (38 mm)) to prevent any restriction of the internal diameter of the duct when the

weight of the supported section rests on the hanger. Individual ducts should be separately

supported.

Flexible ducts may rest on ceiling joists ortruss supports. Maximum spacing between

supports should not exceed the maximum

spacing per manufacturer’s installationinstructions.

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SAFETY HAZARDS & INSPECTIONSSAFETY HAZARDS GUIDESAFETY, HEATING INSPECTIONSAFETY RECALLS, Chimneys, Vents, HeatersSEER RATINGS & OTHER DEFINITIONSSOOT on OIL FIRED HEATING EQUIPMENTSPILL SWITCHES - Flue Gas DetectionSPLIT SYSTEM AIR CONDITIONERS & HEAT PUMPSSTACK RELAY SWITCHESSTAIN DIAGNOSIS on BUILDING INTERIORSSTEAM HEATING SYSTEMSSWAMP COOLERSSYSTEM OPERATION

TANKLESS COILSThermal Expansion Cracking of BrickTHERMAL EXPANSION of HOT WATERTHERMAL EXPANSION of MATERIALSTHERMAL IMAGING, THERMOGRAPHYTHERMAL IMAGING MOLD SCANSTHERMAL MASS in BUILDINGSTHERMAL TRACKING & HEAT LOSSTHERMOSTATS, HEATING / COOLINGTHERMOSTATS, WATER HEATERTHERMOSTATIC EXPANSION VALVESTransite Pipe Chimneys & Flues

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More Information

Support the duct between a metal connection and a bend

by allowing the duct to extend straight for a few inches

before making the bend.

Vertically installed duct should be stabilized by support straps at a

maximum of 6 feet (1829 mm) on center.

Also, for cool air distribution from a blower assembly located below the areaserved, the height of ductwork determines how much cold air the blower hasto push "up" - don't use flex-duct for vertical air supply risers past more thantwo floors in a building.

- Sources: [13][14][15][16]

HVAC Ducts in Concrete Floor Slabs are trouble

Our photo (below left) illustrates how you may spot an air duct routed through the building floor slab. In thiscase the furnace return air plenum was also located below the concrete floor. The air duct system in thisbuilding had been subject to periodic flooding, as illustrated in our second photo (below right).

A description of the health and functional problems that may be traced to air ducts that were routed in aconcrete floor slab as well as our advice on how to properly abandon and seal in-slab air ducts are found atDUCT in CONCRETE FLOOR.

There we describe concerns with ductwork run in floor slabs in the article above, including risks of air ductcollapse that interferes with air flow through the system, water leaks into the in-slab duct system (not aproblem unique to transite ducts), and rodent or insect infestations or even mold contamination. Odorcomplaints may be traced to the duct system because of these problems (DUCT & AIR HANDLER ODORS).

Below, reader-contributed photographs of problems in spiral metal ductwork routed in a concrete floor slabillustrate rust, flooding history (below left) and ductwork collapse (below right).

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We discuss and illustrate disconnected heating or air conditioning duct defects at DUCT CONNECTIONS. Wealso show the interior of crimped or squashed flexduct at DUCT DAMAGE, MECHANICAL.

Hopefully needless to say,

flex duct should never be buried underground nor set into concrete slabs. [13]

SLAB DUCTWORK - catalogs the functional and environmental problems found when HVAC air ducts are routedin or below floor slabs

Flex Duct Connection & Splicing Specifications

All connections, joints and splices should be made in

accordance with the manufacturer’s installation instructions.

All tapes, mastics and non-metallic fasteners (plastic clamps)

used for field installation of flexible ducts should be listed and

labeled to UL 181B, Closure Systems for Use With Flexible AirDucts and Air Connectors. Non- metallic fasteners are limited to

6 inch w.g. (water gauge) maximum positive pressure.

Sheet metal collars to which flexible ducts are attached should

be a minimum of 2 inches (50.8 mm) in length and should be beaded.

Sheet metal sleeves used for joining two sections of flexible duct should be a minimum of 4 inches

(101.6 mm) in length and should be beaded on both ends.

- Sources: [13][14][15][16] Illustration adapted from [13].

Frequently Asked Questions (FAQs) about HVAC air ductrouting, support, lengths, sizes & installation

Question: how much efficiency do I lose if air returns are not located close to thefloor

My house is one story on a slab, 1600sq ft. I am converting froms electric baseboard to a gas forced hot airsystem. All ducts are located in the attic. Since the return ducts will also be in the ceiling how many returnsshould I have the contractor install? How much efficency will I loose because the returns cannot be locatedclose to the floor? - [email protected] 5/29/11

Reply:

I agree that you're asking an important question about proper supply and return duct design, but I cannotanswer it by email and with so little information. I'd ask the contractor to go over the design with you and toshow you how the return air and supply air will be both adequate and balanced. Also pay attention to thelocations of the supply and return registers - you don't want the return too close to the supply or you'll have ashort circuit in the system that will prevent good operation.

I also agree that pulling cool air from close to floor level is preferable to pulling it from a ceiling, but I don'tknow an actual efficiency number. In part that's because you need to consider the effect of good (versus poor)location and distance between supply and return outlet and inlet.

Question: what is the maximum recommended length for runs of flex-duct?

how long I can use the fex duct work?

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A home inspector say can not used mor the 12 feet is that true?

Reply: A summary of Flex Duct Installation Specifications & Recommendations

Angel, I think you are asking what is the maximum recommended length for flexduct in an HVAC system.

Flexible air duct systems should be sized and designed in accordance with ACCA Manual D for residentialduct systems or other ICC referenced engineering standards. An engineering answer to the question ofHVAC duct design length restrictions for a specific installation would have to consider the resulting air flowthrough the system, determined by a number of factors including:

a calculation of the required heating or cooling load for the room served by the duct

the supply air that the room requires

the static air pressure provided by the air handler blower

a translation of all of the duct bends, fittings, and terminal devices (supply air control registers) into anequivalent "length of ductwork)

the duct diameter (which you did not specify),

the number of turns or bends (those reduce air flow rate). Bends in flex duct should not be acute andshould not be less than one duct diameter in radius; and we are assuming there are no squashed,crimped, nor disconnected duct sections.

the smoothness of the duct interior (flex duct is more resistant to airflow than a solid duct surface)

the blower fan capacity in cubic feet per minute.

for cool air distribution from a blower assembly located below the area served, the height of ductworkdetermines how much cold air the blower has to push "up" - don't use flex-duct for vertical air supplyrisers past more than two floors in a building.

Adding to all that complexity are other factors that can snarl up the delivery of air flow by a heater or airconditioner into a room, such as

the presence or absence of return air ducts in the same room,

their locations or placements on floors, walls, ceilings, and

where central air returns are used, how easily air can flow out of individual rooms to the central airreturn.

OK so those are a bunch of factors that tell us how well a duct system will work.

But there are more: assuming you are describing using flex-duct to deliver air to a single supply register insome building room, is the flex duct routed directly to the room from the supply plenum, or did the designerinclude a larger diameter trunk line that brings conditioned air to the general area of the building, from whichyou are using a smaller diameter flex duct to get from the trunk line over to the actual air supply register?

Flex duct run length rule of thumb

In addition to duct design calculators and worksheets provided by duct manufacturers and HVAC industryassociations, people often also do a "sanity check" on the duct design by applying a rule of thumb - much asyou are asking about in your question:

The routing and length of flexible duct, the numbers of degrees of each bend and the amountof sag allowed between support joints will have serious effects on system performance due to

the increased resistance each introduces. Use the minimum length of flexible duct to make

connections. Do not install excess lengths of ducts to allow for possible future relocations of air

terminal devices.

Flexible air duct trunk lines should not be limited in length. It is recommended as a guide that

flexible air duct branch takeoffs (run-outs), flexible air ducts direct to boots from the plenum and

flexible ducts in radial duct systems be limited in length to 25 feet (7620 mm). The preferred

duct geometry is a “Trunk and Branch” system (see ACCA Manual D Figures 1-4 and 10-7).

... Terminal devices [air supply registers] should be supported independently of the flexible

duct.

- using the International Mechanical Code & the Georgia state construction codes [13] as an example.

...

Questions or comments about this article

Questions & answers or comments about proper HVAC duct routing and support.

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Technical Reviewers & References

Related Topics, found near the top of this page suggest articles closely related to this one.

[1] Thanks to Mark Cramer, Tampa Florida, for assistance in technical review of the "Critical Defects"section and for the photograph of the deteriorating gray Owens Corning flex duct in a hot attic. Mr.Cramer is a Florida home inspector and home inspection educator.[2] Thanks to Jon Bolton, an ASHI, FABI, and otherwise certified Florida home inspector who providedphotos of failing Goodman gray flex duct in a hot attic.[3] Air Diffusion Council, 1901 N. Roselle Road, Suite 800, Schaumburg, Illinois 60195, Tel: (847) 706-6750, Fax: (847) 706-6751 - [email protected] - www.flexibleduct.org/ - "The ADC has produced the 4th Edition of the Flexible Duct Performance & Installation Standards (a 28-page manual)

for use and reference by designers, architects, engineers, contractors, installers and users for evaluating, selecting,

specifying and properly installing flexible duct in heating and air conditioning systems.

Features covered in depth include: descriptions of typical styles, characteristics and requirements, testing, listing,

reporting, certifying, packaging and product mark ing.

Guidelines for proper installation are treated and illustrated in depth, featuring connections, splices and proper support

methods for flexible duct. A single and uniform method of mak ing end connections and splices is graphically presented for

both non-metallic and metallic with plain ends."

The printed manual is available in English only. Downloadable PDF is available in English and Spanish.

[4] Owens Corning Duct Solutions - www.owenscorning.com/ductsolutions/ - provides current HVACductwork and duct insulating product descriptions and a dealer locator. Owens Corning InsulatingSystems, LLC, One Owens Corning Parkway, Toledo, OH 43659 1-800-GET-PINK™[5] "Flexible Duct Media Fiberglas™ Insulation, Product Data Sheet", Owens Corning - seeowenscorning.com/quietzone/pdfs/QZFlexible_DataSheet.pdf "Owens Corning Flexible Duct Media Insulation is a lightweight, flexible, resilient thermal and acoustical insulation made of

inorganic glass fibers bonded with a thermosetting resin."

[6] Modern Refrigeration and Air Conditioning, A. D. Althouse, C.H. Turnquist, A. Bracciano,Goodheart-Willcox Co., 1982[7] Principles of Refrigeration, R. Warren Marsh, C. Thomas Olivo, Delmar Publishers, 1979"Air Conditioning & Refrigeration I & II", BOCES Education, Warren Hilliard (instructor), Poughkeepsie,New York, May - July 1982, [classroom notes from air conditioning and refrigeration maintenance andrepair course attended by the website author][8] Refrigeration and Air Conditioning Technology, 5th Ed., William C. Whitman, William M. Johnson,John Tomczyk, Cengage Learning, 2005, ISBN 1401837654, 9781401837655 1324 pages[9] Carson Dunlop, Associates, Toronto, have provided us with (and we recommend) Carson DunlopWeldon & Associates' Technical Reference Guide to manufacturer's model and serial numberinformation for heating and cooling equipment ($69.00 U.S.).[10] Air Conditioning SEER - New DOE Air Conditioner and Heat Pump Efficiency Standard[11] Asbestos HVAC Ducts and Flues field identification photos and guide[12] Fiberglass: Indoor Air Quality Investigations: Fiberglass in Indoor Air, HVAC ducts, and BuildingInsulation[13] "Design and Installation of Residential Flexible Ductwork Systems", Georgia State ConstructionCode, retrieved 8/12/12, original source:http://www.dca.state.ga.us/development/constructioncodes/publications/1ONE.pdf, [copy on file asGA_Ductwork_Code.pdf] The information presented in this document comes primarily from sources inthe International Mechanical Code including referenced standards and Georgia Amendments.[14] International Mechanical Code, 2000 Edition, with 2001, 2004 & 2005 Georgia Amendments[15] Flexible Duct Performance and Installation Standards (3rd ed.) – Air Diffusion Council[16] Manual D—Residential Duct Systems – Air Conditioning Contractors of America (ACCA)[17] HVAC Duct Construction Standards—Metal and Flexible (1995 ed.) – Sheet Metal and AirConditioning Contractors’ National Association, Inc. (SMACNA)[18] Uniform Mechanical Code, 2003 Edition, Appendix A: Standard for Installation of Factory-Made AirDucts[19] Florida Mechanical Code, Section 610: Air Distribution Systems

Books & Articles on Building & Environmental Inspection,Testing, Diagnosis, & Repair

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