Mechanically damaged heating or air conditioning ducts, especially flex ducts, may have more than one cause and effect
Crushed flex duct blocks airflow
Damaged duct interior increases release of airborne fiberglass
Damaged duct insulation increases heating or cooling system operating cost
Air Conditioning (or Heating) Duct Defects, Defective air duct products and materials
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This article describes damaged HVAC ducts due to some mechanical activity such as stepping on a duct and crushing it, or improper and too-aggressive duct cleaning methods that damage duct interior surfaces, liners, or binding resins. This article series discusses duct system defects 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.
At left our photo, provided by reader Steven King, shows the interior of fiberglass-lined flexduct that has been crushed, perhaps by having been stepped-on?
The result is reduced heating or cooling air flow, reduced building occupant comfort, and higher heating or cooling system operation cost.
We were also concerned that the exposed fiberglass in the duct appears to have been mechanically damaged, perhaps by a too-aggressive attempt at duct cleaning that may result in higher levels of airborne fiberglass duct insulation fragments in the building.
Other crimping and blockage or support problems found in flexduct installations are discussed at DUCT ROUTING & SUPPORT.
This photo shows the connection of fiberglass flex duct to a metal HVAC duct component. The fiberglass flex duct has been badly damaged.
As this photograph of duct damage was taken at the same installation as the example above, we suspect that an inexperienced duct cleaner has been at work here.
This photo shows a common return air duct system in some older homes: space between the floor joists was used as an air passage, sometimes also for supply air.
But when the metal sheeting nailed across the floor joist bottoms has rusted out, severe air leaks occur. In a supply duct the result is higher heating or cooling costs.
In a return air duct a hole such as the one shown in our photo can cause unhealthy or unsafe conditions by drawing other unanticipated air into the duct system (in this case next to a crawl space vent we are basically heating outdoor air and blowing it into the living area).
Watch out: large improper openings in return ductwork, whether from rust or any other causes, can cause building backdrafting and dangerous carbon monoxide hazards. See UNSAFE DUCT OPENINGS and see BACKDRAFTING HEATING EQUIPMENT.
Other Duct System Damage Topics are listed in the links at page left; some of those links are described below.
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
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
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
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.
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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.
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, 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
Thanks to reader Steven King for photographs and discussion of damaged ductwork, November/December 2010.
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
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 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
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.
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.
The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 2010, $69.00 U.S., is available from Carson Dunlop. The Home Reference Book is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. InspectAPedia.com ® author/editor Daniel Friedman is a contributing author. Field inspection worksheets are included at the back of the volume.
"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]
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.).
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