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This article explains how to inspect and diagnose trouble with heating or cooling air flow, air ducts, air registers and zone components of heating and air conditioning (HVAC) duct systems.

We explain how to find and correct air duct problems such as missing air conditioning cool air supply or return air registers, undersized air conditioning duct openings, improper cooling duct routing, cooling (or heating) air duct corrosion, leaky air duct connections, defective heating or cooling ductwork materials such as Goodman gray flex-duct, some Owens Corning Flex-duct, and asbestos-containing air conditioning or heating duct work.

Also see our explanation of problems with reduced air flow, dirty air handler blowers, blocked cooling coils, blower leaks and mold, frost and ice build up in ducts, and blower sizing problems of air conditioning retrofit projects which we discuss at: AIR HANDLER / BLOWER UNITS, see our discussion of LOST COOLING CAPACITY and see our discussion of various problems with AIR FILTERS for HVAC SYSTEMS since all of these can involve diagnosing and curing problems with the adequacy of distribution of cool air in a building. Readers should also see our INDOOR AIR QUALITY IMPROVEMENT GUIDE article series.

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Diagnostic List of Heating & Cooling Air Duct Defects: Things to Inspect, Check, Repair, or Replace

This chapter continues with a discussion of common defects found in air conditioning duct work (see links below) such as incomplete supply or return ducts, poor choice of location or size for supply and return registers, locating ducts in problem areas, leaks into and corrosion of duct work, leaky duct connections, and special notes on product failures by two manufacturers of flexible duct work. The chapter concludes with important safety warnings about dangerous openings in duct work.

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

The master document, of which this is a chapter, describes the inspection of residential air conditioning systems (A/C systems) to inform home buyers, owners, and home inspectors of common cooling system defects. Asbestos in or on HVAC ducts is a possible hazard for which we provide links to a separate document - see "Asbestos HVAC Ducts" at below left.

Our master list of HVAC duct system inspection, diagnosis, and repair articles continues just below and at the left of this page.

Below we list some common sources of poor air flow from air conditioning or heating duct systems: check these first before ordering a more costly survey or repair job.

HVAC Supply & Return Air Duct Leaks & Obstructions - Theory vs Practice

Technical note: The D'Arcy-Weisbach equation for pressure and head loss can be used to calculate the actual pressure loss due to friction in a building piping or air duct system. The Engineering Toolbox provides the D'Arcy-Weisbach formula: Δp = λ (l / d_h) (ρ v² / 2)  where Δp = pressure loss (Pa, N/m²), and l = length of duct or pipe (m) and d_h = hydraulic diameter (m) and finally, ρ = density (kg/m³).

Watch out: But keep in mind that even this apparently accurate calculation of the effect of piping on air pressure and airflow loss will not include the effects of obstructions in the building return air or supply air duct system such as

• Air filter or other item that has been sucked into the duct system will block air flow and can risk a fire if drawn into the blower assembly fan, Dirty Air Filter Problems are perhaps the most common cause of unsatisfactory airflow in an HVAC system.
• Blower Fan: dirty blades on a squirrel cage blower assembly fan significantly reduce the blower fan's ability to move air into the HVAC system from the return-air side as well as reducing its ability to push conditioned air into the occupied space. DIRTY A/C BLOWERS
• Fire damper that has become stuck in the closed or partly-closed position interferes with proper airflow through the system FIRE DAMPERS in DUCTWORK
• Flex duct defects: Collapsed sections of flex duct block or stop airflow in either supply or return air systems DUCT ROUTING & SUPPORT
• Insulation loose in air ducts: Collapsed duct interior insulation, (FIBERGLASS DUCT, RIGID CONSTRUCTION
• Leaks in the supply air duct system are a very common HVAC duct defect that results in poor heating or cooling air flow. LEAKY DUCT CONNECTIONS
• Return air inlets: Return air inlet grilles that are obstructed with dirt, debris, or furniture or that are improperly located or are just too small mean that because the heating or cooling system is "starved for air", the supply air flow into occupied spaces will also be reduced. RETURN AIR REGISTERS & DUCTS
• Transite air ducts: Crushed or collapsed transite duct TRANSITE PIPE AIR DUCTS
• Zone dampers that are stuck partly closed obstruct air supply into that building area, or if stuck "open" when the zone damper should be closed, airflow to other building areas will be reduced. ZONE DAMPER CONTROLS
• Also see UNDERSIZED RETURN DUCTS.

Illustrations of a leaky cast iron sewer line that sent sewer gases into the building's transite (asbestos cement) heating duct system can be seen at TRANSITE PIPE AIR DUCTS and at CAST IRON DRAIN PIPING.

Questions & Answers About Air Duct & Air Flow Problems on Heating & Air Conditioning Systems

Question: How do I Diagnose Too Little Cool Air Flow Downstairs?

I have a 2 story house the air flows out the registers heavy upstairs but downstairs hardly any flows and from one there seems to be none. i inspected ductwork but find no visible pinching.

- Stephen

Reply: Tips for diagnosing uneven air conditioning air flow between floors in a building

Tips for diagnosing uneven air conditioning air flow:

Stephen hasn't provided much detail about how the air conditioning system in his home is designed and installed except that he doesn't see any duct pinching [DUCT ROUTING & SUPPORT], so pending some feedback we start with some general cool air flow diagnostic tips.

In a two story house at which air conditioning is provided by a single air handler, balancing the delivery of air flow can require extra attention to duct routing, use of balancing dampers in the ductwork, return air register location, and supply air register location and even multiple air filters or air filter condition.

In contrast, if the home has separate air handlers, one for upstairs and a second for downstairs his cool air flow problem would be simpler to explain.

• Is this an A/C retrofit job? Just double check - if this is an A/C retrofit job onto a home that already had warm air heating, and if the A/C air handler was put in the attic, perhaps it is ONLY connected to the first floor ductwork. Rule that condition in or out first.

Stephen reports heavy air flow upstairs and poor air flow downstairs.

• Check for balancing dampers [see BALANCING AIR DUCT FLOW] : since it is easier to push cool air down through ducts from an upper level to a lower level in a building than it is to push cool air upwards from lower to upper floors, the installer may have included airflow balancing dampers - look for these in the visible and accessible ductwork as well as at individual supply and return registers.
• Check ductwork connections. Perhaps there are none or too few delivery ducts to downstairs? In some air conditioning diagnosis jobs we found that an air duct had simply become disconnected entirely.
• Check for return air obstructions: if you are trying to blow cool air into rooms whose doors are shut and if your system uses a centralized air return register, air supply may be blocked.

Let us know what you find. It may help other readers.

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Technical Reviewers & References

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• Additional technical contributors & reference sources for this article are listed below.

Use links just below or at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

DUCT SYSTEM & DUCT DEFECTS
DUCTS - Asbestos
DUCT SYSTEM DEFECTS
  AIR FILTERS for HVAC SYSTEMS
  ALLOY SYSTEMS FLEXDUCT
  ASBESTOS HVAC DUCTS
  ASBESTOS PAPER on DUCTWORK
  ASBESTOS TRANSITE DUCTWORK
  BALANCING AIR DUCT FLOW
  DUCT & AIR HANDLER ODORS
  DUCT DAMAGE, MECHANICAL
  DUCT INSULATION - Asbestos Paper
  DUCT ROUTING & SUPPORT
  DUST CONTAMINATION FROM HVAC?
  FIBERGLASS DUCT, RIGID CONSTRUCTION
  FIBERGLASS HVAC DUCTS
  FIRE DAMPERS in DUCTWORK
  FLOOD DAMAGE in DUCT WORK
  GOODMAN GRAY FLEXDUCT
  INCREASING RETURN AIR
  LEAKY DUCT CONNECTIONS
    Leaky or Missing Air Ducts
    Hidden Missing Air Conditioner Duct
    Duct Retrofit Leaks
    Rooftop Duct Leaks
    RETURN DUCT AIR LEAKS
    SUPPLY DUCT AIR LEAKS
  LOCATION OF REGISTERS & DUCTS
  MOLD in AIR HANDLERS & DUCT WORK
  NOISES in DUCT SYSTEM
  ODORS in AIR HANDLERS & DUCT WORK
  OWENS CORNING FLEXDUCT
  OWL FLEXDUCT
  RETURN AIR REGISTERS & DUCTS
  SOUNDPROOFING for DUCTWORK
  SUPPLY DUCTS & REGISTERS
  TRANSITE PIPE AIR DUCTS
  UNDERSIZED RETURN DUCTS
  UNSAFE DUCT OPENINGS
  VIBRATION DAMPENERS
  WATER & ICE IN DUCT WORK
  WET CORRODED DUCT WORK
  ZONE DAMPER CONTROLS

• 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.
• Thanks to Jon Bolton, an ASHI, FABI, and otherwise certified Florida home inspector who provided photos of failing Goodman gray flex duct in a hot attic.
• Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. (416) 964-9415 1-800-268-7070 info@carsondunlop.com. Thanks to Alan Carson and Bob Dunlop, for permission to use illustrations from their publication, The Illustrated Home which illustrates construction details and building components. Carson Dunlop provides home inspection education including the ASHI-adopted Home Inspection Training Program (home study course), publications such as the Home Reference Book, report writing materials including the Horizon report writer, and home inspection services. Alan Carson is a past president of ASHI, the American Society of Home Inspectors.
• Carson Dunlop, Associates, Toronto, have provided us with (and we recommend) Carson Dunlop Weldon & Associates' Technical Reference Guide to manufacturer's model and serial number information for heating and cooling equipment ($69.00 U.S.).
• Engineering toolbox properties of water - http://www.engineeringtoolbox.com/water-thermal-properties-d_162.html and email: editor.engineeringtoolbox@gmail.com web search 09/16/2010
• Thanks to Scott at SJM Inspect for suggesting this EPA document and for technical editing remarks regarding our air conditioning website, SJM Inspection Service LLC, serves the entire state of CT, sjminspect.com 203-543-0447 or 203-877-4774 5/16/07
• Reference: Modern Refrigeration and Air Conditioning, A. D. Althouse, C.H. Turnquist, A. Bracciano, Goodheart-Willcox Co., 1982
• Reference: 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 and repair course attended by the website author
• Mold in the air handler or duct work is discussed at AIR HANDLER / BLOWER UNITS chapter
  HOW TO PREVENT MOLD: Correct the Causes of Mold and Prevent Indoor Mold or other indoor environment problems
• OPTIMUM INDOOR AIR QUALITY - Air Filter Choices & Placement for Optimum Indoor Air Quality
• Thanks to Chris Van Rite, Vice President Sales, M&M Manufacturing Company, 4001 Mark IV Parkway Fort Worth, TX 76106, Office (817) 348-2241 Cell   (817) 825-2363 - cvr@mmmfg.com - Mr. Van rite provided the HVAC & Duct System references listed just below 2/9/2009
• Consortium for Energy Efficiency (CEE) - http://www.cee1.org/resid/rs-ac/hvac_press_kit.pdf
• Air Diffusion Council (ADC) - http://www.flexibleduct.org/
• Static Pressure Losses in 6, 8, and 10-inch Non-Metallic Flexible Ducts, Weaver, K.; Culp, C.
  Texas A&M University Energy Systems Laboratory http://txspace.tamu.edu/handle/1969.1/5275
• A Study of Pressure Losses in Residential Air Distribution Systems, Bass Abushakra, Iain S. Walker, Max H. Sherman, Lawrence Berkeley National Laboratory - http://eetd.lbl.gov/ie/pdf/LBNL-49700.pdf
• ENERGY STAR® Homes Technical Standards - http://www.energystarhomes.com/documents/tech_standards.pdf
• ASHRAE Advanced Energy Design Guide for “Small Office buildings”, “Retail buildings” & “K-12 School buildings” available currently as a free download from ASHRAE Publications Store http://www.ashrae.org/publications/page/1604 (do not copy this PDF to networks)

How to diagnose and fix an air conditioning system that is not working

Since the failure of an air conditioner to turn on, loss of air conditioner cooling capacity, reduced air conditioning output temperatures, loss of cool air supply, or even loss of air flow entirely can be due to a variety of problems with one or more components of an air conditioner or air conditioning system, after reviewing the lost air conditioner cooling diagnosis procedures described in this article, be sure to also review the diagnostic procedures at each of the individual air conditioning diagnosis and repair major topics listed just below. To return to our air conditioning and refrigeration home page go to AIR CONDITIONING & HEAT PUMP SYSTEMS.

If your air conditioning or heat pump system has lost its cooling capacity or won't start select one or more of the diagnostic articles listed below.

• A/C REFRIGERANT LEAK DETECTION: how to use a TIF5000 to detect air conditioning refrigerant gas leak
• A/C DIAGNOSTIC FAQs: air conditioning system diagnostic FAQs: Q&A about air conditioner repair - a detailed air conditioning system diagnostic checklist
• AIR HANDLER UNIT: problems with the air handler, air filters, and the cooling coil itself
• COMPRESSOR CONDENSER: problems with air conditioner compressor/condenser units. Also see FAN, COMPRESSOR/CONDENSER UNIT for help in diagnosing and fixing problems with the outdoor compressor/condenser fan and fan motor.
• A/C - HEAT PUMP CONTROLS & SWITCHES: air conditioner controls and switches - begin here if your A/C won't start. Here's an important tip: most refrigeration problems, in air conditioners, refrigerators, or freezers, are electrical, not mechanical. In air conditioning school, we used to drive out and collect abandoned refrigerators that people were tossing out during our community's spring cleanup week. Taking these appliances back into the shop we found that almost always the problem that had caused the owner to dispose of their air conditioner or freezer was in an electrical connection or electrical control. So it's worth checking out switches and controls on an air conditioner before replacing more costly components.
• DUCT SYSTEM DEFECTS: problems with the air duct system, air filters, supply registers, return air registers
• Fire dampers, and Heating and Cooling Air Duct Controls such as manual and automatic duct dampers, zone dampers, and fire dampers are discussed and distinguished at DRAFT REGULATORS - barometric damper
• LOST COOLING CAPACITY: what to do when not enough cool air comes out of the system
  • What to check first if there is no cool air or not enough cool air
  • Compressor failure diagnosis: basic checks of the air conditioner compressor
  • Ducts & Air Handler diagnosis: basic checks of the indoor air handler (blower), air ducts, and filter system
• OPERATING DEFECTS: major air conditioning problem symptoms and how to get the air conditioning system working again,e.g. compressor or fan noises, failure to start, and inadequate cool air volume
• Automatic Fire Dampers for Duct Work discusses fire safety controls - a different type of automatic duct closure device than heating zone regulators.
• ZONE DAMPER CONTROLS . discusses manual and automatic air duct zone controls

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

• Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.
• Complete List of Air Conditioning & Heat Pump Design, Inspection, Repair Books at the InspectAPedia Bookstore.

• Modern Refrigeration and Air Conditioning, A. D. Althouse, C.H. Turnquist, A. Bracciano, Goodheart-Willcox Co., 1982
• 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 and repair course attended by the website author]
• Refrigeration and Air Conditioning Technology, 5th Ed., William C. Whitman, William M. Johnson, John Tomczyk, Cengage Learning, 2005, ISBN 1401837654, 9781401837655 1324 pages
• Carson Dunlop, Associates, Toronto, have provided us with (and we recommend) Carson Dunlop Weldon & Associates' Technical Reference Guide to manufacturer's model and serial number information for heating and cooling equipment ($69.00 U.S.).
• Air Conditioning Inspection, Diagnosis, Repair, Efficiency all the basics for home owners, inspectors, new repairmen
• Air Conditioning SEER - New DOE Air Conditioner and Heat Pump Efficiency Standard
• Asbestos HVAC Ducts and Flues field identification photos and guide
• Fiberglass: Indoor Air Quality Investigations: Fiberglass in Indoor Air, HVAC ducts, and Building Insulation
• ...