Photograph of  duct work competing with itself for supply and return air Air Register Location
Proper Placement of Heating or Cooling Registers & Ducts

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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.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

Location of air conditioning & heating return & supply registers

Photograph of  duct work competing with itself for supply and return airSupply and return too close together result in short-circuiting

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.

Location of Heating or Cooling Return Registers in Basements

Photograph of  duct work competing with itself for supply and return air

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 RETURN AIR, INCREASE, 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.

Placement of Heating or Cooling Air Returns at Outside Walls

HVAC return duct on outside wall © D Friedman at

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.

Other Bad Locations for Air Registers

Return air at furnace © D Friedman at

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.


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Publisher - Daniel Friedman