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HVAC heating & cooling air supply & return register locations:
Air Conditioning (or Heating) Duct supply or return air register placement mistakes can increase cooling or heating costs, limit system capacity, and can even be unsafe.
This article describes the proper location and placement of air supply and air return registers in HVAC systems and addresses problems such as misplaced or missing air conditioning cool air supply or return air registers, improper cooling duct routing, cooling (or heating) air duct corrosion, and defective heating or cooling duct work.
This article is part of our series How to Inspect, Diagnose, & Repair the Air Conditioners or Heat Pumps.
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You don't have to be an HVAC design engineer to see that in the photograph at page top and shown again here the air conditioning supply register is above and just a few feet from the central air conditioning return grille.
Cool air delivered to this attic room from the supply register over the door will mostly fall down to be simply drawn right back into the return.
Ventilation and HVAC experts call this air movement short-circuiting: the air being supplied is taking a shortcut right back into the return duct rather than mixing with air in the cooled or heated space.
Short-circuiting of ventilation air occurs when ventilation air enters and leaves a space or duct before it has a chance to mix well enough with the room air to do the job it was intended to do - that is, to adequately dilute pollutants, or to break up stratified air in the occupied space. Occupants are left too cold or too hot or they are left complaining about poor indoor air quality. - adapted from Prozuments(2017).
Poor supply and return duct locations like this can severely reduce the effectiveness of the cooling system and increase its operating costs.
In this particular home the installer was confronted with a shoehorn retrofit of the air handler and duct work into a sub-standard attic bedroom closet in an area where s/he was not permitted to open cathedral ceilings nor to construct a delivery duct along the ceiling or under the floors.
It was a costly to operate and poor performing air conditioning installation.
The photograph above shows a basement door into which an installer cut two return air inlets to feed basement air back to an air return located at the basement air handler.
As we discussed at INCREASING RETURN AIR, this is a poor design that increases heating or cooling system operating costs. In addition to that issue, placement of return air inlets in basements, depending on their location, risk other potential hazards including CO hazards.
Carbon monoxide hazards: Return air registers too close to oil or gas fired equipment may draw combustion gases or carbon monoxide into the air duct system, sending dangerous gases into the living space
Carbon monoxide production may be increased and heating fuel combustion incomplete at nearby heaters, water heaters, or even gas clothes dryers, if
the air handler is pulling return air from a confined space where combustion equipment is also located.
Air-starved equipment may not only work improperly, but may be unsafe, producing dangerous carbon monoxide. We've also found this problem in basements where the owner, attempting to improve basement air quality, ran powerful exhaust fans continuously.
Watch out: you should not normally detect persistent CO in indoor air. Carbon monoxide gas indoors is odorless and colorless and can be fatal. Be sure your building has properly installed and working CO detectors and smoke detectors.
Heating or cooling return air duct systems which place the return register at outside building walls may perform poorly.
Some heating authorities opine that more effective and economical design places these registers on the interior walls - the outside walls and perimeter of some rooms may be chilly even when the heat is operating.
Our photo above shows an octopus furnace located in the basement of a pre-1900 home. All of the return air to this system is drawn from the un-heated basement floor - a "one-way" air movement heating design that increases heating costs as well as risking pick-up of dust, debris, or anything else undesirable from the basement area.
At UNSAFE DUCT OPENINGS we describe other air register location mistakes that can be dangerous, such as cutting a return air opening near heating equipment or in hazardous areas like a wet moldy crawl space.
Continue reading at AIR FLOW IMPROVEMENT, HVAC or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.
Or see RETURN AIR REGISTERS & DUCTS
Or see SUPPLY DUCTS & REGISTERS
Or see DUCT SYSTEM & DUCT DEFECTS - home
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(July 24, 2014) rachael kelly said:
Wehad smoke damage a couple of years ago and now after cleaning the walls smoke is still blown out from furnace causing more
Clearly your furnace needs repairs.
(Aug 8, 2014) Dave said:
I'm having my house rebuilt, I have 800 sq ft on first floor (using 3 ton unit) and 1200 sq ft on second floor(using 4 ton unit). They are only putting returns in my ceiling and none closer to the floor, my concern is more about heat and not so much AC...Will those returns in the ceiling heat my house? Doesn't heat rise?
My contractor states because of how the supports for the second floor were installed it is extremely difficult to rn duct work down to the floor and that just the returns in the ceiling and oversized units will be sufficient to cool and heat the house. Any opinions? Thanks.
Yes Dave, warm air rises, cool air falls.
If your design priority is for heating, not cooling, you might prefer to place return registers either in floors or in walls close to floor level. You don't have to take the register all the way to the floor to make this improvement, just close to floor level.
I can't comment about the overall adequacy of the heating system or cooling system designs, lacking adequate information and possibly expertise. You'll want to have advice from an HVAC designer, engineer, or a similar expert.
(Jan 4, 2015) Anonymous said:
this page shows where NOT to place registers. Where do you discuss correct or best placement?
Thanks Anon - good point. I'll work on the article above to add some "DO's". I suppose I avoided those details for a couple of reasons: many locations are OK except the DONT's we listed, but still where to place an air supply or return register for best operation varies by whether we are most interested in heating (place those registers low on a wall and in an unblocked location) or in cooling (those registers are optimally placed high on walls or in ceilings).
In many buildings there are reasons that the installer will use non-optimal locations including for duct systems that have to deliver both heated and cooled air in different seasons.
Still I'll collect and add some other suggestions.
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