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AIR CONDITIONING & HEAT PUMP SYSTEMS

A/C - HEAT PUMP CONTROLS & SWITCHES
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AIR CONDITIONER TYPES, ENERGY SOURCES
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BLOWER DOORS & AIR INFILTRATION
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BOOKSTORE - Air Conditioning "How To" Books

CAPACITORS for HARD STARTING MOTORS
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CLEANING & Legionella BACTERIA
CHINESE DRYWALL HAZARDS
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DEFINITION of Heating & Cooling Terms
DEW POINT CALCULATION for WALLS
DEW POINT TABLE - CONDENSATION POINT GUIDE
DIAGNOSE & FIX AIR CONDITIONER / HEAT PUMP
DIAGNOSE & FIX HEATING PROBLEMS-BOILER
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DUCTS - Asbestos
DUCT INSULATION, Asbestos Paper
DUCT INSULATION for SOUNDPROOFING
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DUST, HVAC CONTAMINATION STUDY

ELECTRIC MOTOR OVERLOAD RESET SWITCH
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OPERATING COST, AIR CONDITIONER
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REPAIR GUIDE, AIR CONDITIONERS / HEAT PUMPS
REPAIR & DIAGNOSTIC FAQs for A/C

SWAMP COOLERS

THERMOSTATS, HEATING / COOLING
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WATER COOLED AIR CONDITIONERS
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More Information

Photograph of added return air cut at basement AHU also may draw flue gases from nearby gas fired equipment Unsafe Air Conditioning or Heating Duct Openings
     

  • Unsafe Air Conditioning or Heating Duct Openings
  • Openings in the wrong place in a heating or air conditioning duct system can transport dangerous gases or other contaminants throughout a building
  • Common mistakes in cutting openings in ductwork such as attempts to improve air delivery by placing a return opening at the air handler
  • Questions & answers aboutHVAC air duct safety hazards
  • References

Click to Show or Hide Related Topics

  • DUCT SYSTEM & DUCT DEFECTS - home
  • AIR FILTERS for HVAC SYSTEMS
  • AIR FLOW IMPROVEMENT, HVAC
  • AIR FLOW MEASUREMENT CFM
  • AIR LEAKS in RETURN DUCTS
  • AIR LEAKs in SUPPLY DUCTS
  • ALLOY SYSTEMS FLEXDUCT
  • ASBESTOS DUCTS, HVAC
  • BALANCING AIR DUCT FLOW
  • DUCT & AIR HANDLER ODORS
  • DUCT in CONCRETE FLOOR
  • DUCT DAMAGE, MECHANICAL
  • DUCT ROUTING & SUPPORT
  • DUST, HVAC CONTAMINATION STUDY
  • DUST SAMPLING PROCEDURE
  • FIBERGLASS DUCT, RIGID CONSTRUCTION
  • FIBERGLASS HVAC DUCTS
  • FIRE DAMPERS in DUCTWORK
  • FLOOD DAMAGED DUCT WORK
  • FLOOD DAMAGE HEATING EQUIP
  • FLEXDUCT GOODMAN GRAY
  • FLEXDUCT OWENS CORNING
  • INCREASING RETURN AIR
  • LEAKY DUCT CONNECTIONS
  • LOCATION OF REGISTERS & DUCTS
  • MOLD in AIR HANDLERS & DUCT WORK
  • NOISES in DUCT SYSTEM
  • ODORS in AIR HANDLERS & DUCT WORK
  • OWL FLEXDUCT
  • RETURN AIR REGISTERS & DUCTS
  • RETURN DUCT AIR LEAKS
  • SOUNDPROOFING for DUCTWORK
  • SUPPLY DUCTS & REGISTERS
  • SUPPLY DUCT AIR LEAKS
  • TRANSITE PIPE AIR DUCTS
  • UNDERSIZED RETURN DUCTS
  • UNSAFE DUCT OPENINGS
  • VIBRATION DAMPENERS
  • WATER & ICE IN DUCT WORK
  • WET CORRODED DUCT WORK
  • ZONE DAMPER CONTROLS
InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.

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.

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

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

Photograph of  return in tight space next to gas fired heating boiler

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

Photograph of added return air cut at basement AHU also may draw flue gases from nearby gas fired 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

Photograph of surprise leak in basement AHU also may draw flue gases from nearby gas fired equipment Photograph of surprise leak in basement AHU also may draw flue gases from nearby gas fired equipment

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).

Photograph of surprise leak in basement AHU also may draw flue gases from nearby gas fired equipment

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:


*** 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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Technical Reviewers & References

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

Click to Show or Hide Citations & References

  • Thanks to AMark 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 - info@flexibleduct.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."
  • 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
  • Asbestos HVAC Ducts and Flues field identification photos and guide
  • Fiberglass: Indoor Air Quality Investigations: Fiberglass in Indoor Air, HVAC ducts, and Building Insulation

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.
  • Home Reference Book - Carson DunlopThe 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.

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  • GO TO Carson Dunlop's Home Study Course Information - How to Become a Home Inspector: Carson Dunlop's nationally recognized Home Study Course, selected by ASHI the American Society of Home Inspectors and other professionals and associations. This website author is a contributor to this course.
  • GO TO Carson Dunlop's Home Study Course Information - How to Become a Home Inspector: Carson Dunlop's nationally recognized Home Study Course, selected by ASHI the American Society of Home Inspectors and other professionals and associations. This website author is a contributor to this course.
  • GO TO Carson Dunlop's Home Study Course Information - How to Become a Home Inspector: Carson Dunlop's nationally recognized Home Study Course, selected by ASHI the American Society of Home Inspectors and other professionals and associations. This website author is a contributor to this course.
    Building inspection education & report writing systems from Carson, Dunlop & Associates Ltd
  • 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.

  • Complete List of Air Conditioning & Heat Pump Design, Inspection, Repair Books at the InspectAPedia Bookstore.
  • ...
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