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Unsafe air ducts: this HVAC diagnosis & safety hazard article describes
unsafe air conditioning or heating duct openings such as openings that may cause fatal production of
carbon monoxide and move it into the occupied building space. We also discuss air or return duct openings that may pick up and
distribute other gases, chemicals, mold or allergens throughout a building.
Air Conditioning (or Heating) Duct System Condition & Health/Safety Hazards
UNSAFE OPENINGS - Air Conditioning or Heating Duct System May Draw Dangerous Combustion Gases
Return air collected close to gas-fired appliance
Also see LEAKY DUCT CONNECTIONS for other examples of duct leaks and openings. 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.
This poorly-designed central air conditioning return duct was located in a cramped basement boiler room
only five feet from a large gas-fired, natural draft heating boiler.
It is a possible safety concern.
When the location of the system return air duct work and air handler
is such that the system may pull dangerous flue gases back out of the gas appliance flue vents,
piping and into the building heating or cooling air we cite it as a potential hazard: distribution of
combustion gases may be blown into the living area.
This return air location on a heating or air conditioning system is a dangerous carbon monoxide hazard.
Such a system should be
examined promptly and corrected by a qualified heating professional.
Return Air Openings Near Heating Equipment
Still more common is the presence of extra openings cut into the return ducts atop a
building furnace (perhaps also serving as the air conditioning blower system in cooling season).
Often these openings
are added to provide more return air to a system which is not providing sufficient cooling or heating to the building.
But making return air openings right at a heating appliance, such as shown in the photo here, risks drawing combustion gases into the building air supply as well as potentially
interfering with proper appliance draft and combustion.
This is the case particularly with gas fired furnaces, boilers, or water heaters, which operate at a lower
and usually natural draft, but it is also a potential safety hazard with oil-fired equipment.
Flue gases from nearby heating or water heater
appliances are easily drawn into the return air plenum and air handler. This would permit circulation of flue gases into the living
area and can be a safety hazard which could deliver potentially fatal carbon monoxide to building occupants.
"Hidden" Duct or Air Handler Leaks
Here is an interesting case of a surprise air leak into the return air plenum at a gas-fired hot air furnace and air conditioning
system. We removed the (not working) electrostatic air cleaner (first photo) to look into the return plenum where we saw a large
gap between the return plenum and the blower compartment (second photo).
When the gas burner was operating along with
the blower (in heating mode) this opening could certainly draw un-wanted gases into the duct system and might lead to CO production too.
In cooling or air conditioning mode, the blower was pulling in air from the basement (where we had a mold concern).
The third photo (shown here) of this surprise but quite large leak into the air duct system at the air handler was made by
placing our flashlight behind the blower assembly to make it easy to see the size of the opening.
Carbon Monoxide Production Caused by Improperly Located Return Air Ducts/Registers
Return air openings close to natural-draft fired appliances, again particularly gas, can also interfere with proper gas
(and possibly oil) burner operation by competing for combustion air, thus
causing carbon monoxide production when the burner is operating.
If openings are found in the duct system near fossil-fuel fired appliances it should be reported as
an indication of a system operating problem (inadequate return air) and as a safety hazard (potentially fatal carbon monoxide poisoning).
Review this potential hazard with a qualified service professional. For
example, should a stack pipe fail and flue gas be dumped into the furnace room
it would be picked up and distributed throughout the building.
Sample Air Conditioning Or Heating System Report Language for Unsafe Duct Openings
Sample inspection report language for dangerous HVAC ductwork openings:
*** Safety Hazard - additional details: when the heating system [or air conditioning system]
is running and/or when other nearby heating equipment is running (such as a water heater) there is
negative pressure around the furnace [or air handler unit] and at this return air inlet register,
(demonstrated during our inspection by seeing the furnace blower pull the utility room door shut)
risking pulling dangerous flue gases such as carbon monoxide out of
the flue vent pipe and into the building heating air through the barometric
damper or through other openings in the vent piping.
This item should be handled as soon as possible by an experienced and qualified heating professional - carbon monoxide poisoning
is a potentially fatal safety concern.
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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.
Air Diffusion Council, 1901 N. Roselle Road, Suite 800, Schaumburg, Illinois 60195, Tel: (847) 706-6750, Fax: (847) 706-6751 - firstname.lastname@example.org - 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 marking. 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 making 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.
Owens Corning Duct Solutions - www.owenscorning.com/ductsolutions/ - provides current HVAC ductwork and duct insulating product descriptions and a dealer locator. Owens Corning Insulating Systems, LLC, One Owens Corning Parkway, Toledo, OH 43659 1-800-GET-PINK™
"Flexible Duct Media Fiberglas™ Insulation, Product Data Sheet", Owens Corning - see owenscorning.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."
"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]
The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 25th Ed., 2012, 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. Field inspection worksheets are included at the back of the volume. Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Enter INSPECTAHRB in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
Or choose the The Home Reference eBook for PCs, Macs, Kindle, iPad, iPhone, or Android Smart Phones. Special Offer: For a 5% discount on any number of copies of the Home Reference eBook purchased as a single order. Enter INSPECTAEHRB in the order payment page "Promo/Redemption" space.
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 Special Offer: Carson Dunlop Associates offers InspectAPedia readers in the U.S.A. a 5% discount on any number of copies of the Technical Reference Guide purchased as a single order. Just enter INSPECTATRG in the order payment page "Promo/Redemption" space.