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What is an Air Return Duct?

By Mary Lougee
Updated May 17, 2024
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An air return duct is basically a concealed air passageway in a home or other building that acts as a channel for pumping air out of an enclosed room and into the air conditioning or central heating system. These ducts typically start with vent grates in the wall, on the ceiling, or in the floor. Where exactly these vents are located is usually a matter of whether the system is focusing on heating, cooling, or both. Different structures have different ducting capabilities, but builders and contractors often try to put these sorts of channels near doors and windows to maximize their effectiveness. Sometimes these ducts share the job of air input, but not always. A lot depends on how the building was constructed and how difficult it is to heat or cool.

Location and Main Goals

Return ducts are an integral part of nearly all heating, ventilating and air conditioning (HVAC) systems. Generally, HVAC systems use a forced air process that blows either heat or air conditioning through ductwork into a home or business. Return ducts are usually located either in a hallway or in the ceiling, and their purpose is to extract air from a room and recycle it through the system where it will be further conditioned, either through heating or cooling.

The main thing this duct does is pull air out of a room. In the summer it will pull air that is cold, whereas in the winter it will pull air that is hot and humid. In most cases, different ducts will re-circulate warmed or cooled air to bring the building to the optimal temperature.

How the Larger System Works

Most central air and heat systems have a large condenser unit that is outdoors and a smaller coil that is located indoors, usually tucked away in a closet or utility room. Together, these two components can both heat and cool a building by controlling the air that flows to various rooms through a series of ducts.

In the summer, the return duct removes warm air from inside and transports it to the outside to be conditioned. The ducts through which the air travels are part of a sealed system that usually is located in the attic for structures with slabs, or under the floors on structures that are raised off the ground. These sorts of systems can’t really work without ducts; the ducts give air a passage through which it can flow to reach the outdoor components, undergo the conditioning process, and return to the building. Once back in the building, the air is usually distributed by a fan through registers into the rooms.

HVAC systems typically use a refrigerant to cool the air by carrying the heat outside to the condenser. The compressor in the outside unit compresses the refrigerant into a high-pressure gas. Once the gas travels through the coil and on to the outdoors, it loses heat and transforms into a liquid. It then enters the evaporator coil and expands into a low-temperature gas to absorb heat that enters into the system from the return duct. This process repeats itself until the temperature that is set on the thermostat is reached.

Importance of Good Sealing

The importance of a sealed HVAC duct system can be attributed to the temperature of the air that is extracted by the return duct. In nearly all cases, warmer air rises to the ceiling and colder air remains closer to the floor. In the case of cooling, the air return duct provides circulation of cold air as it pulls the cooler air upward, passing through the warm air that is closer to the ceiling. If there is a leak in the ductwork in any portion of the system, it could pull in unconditioned air, which will take more time and electricity to effectively condition. This usually causes the HVAC system to run longer and use more electricity to achieve the desired temperature.

Common Problems

Air return ducts can take up a lot of space, though when they’re built alongside a house or other structure they’re usually concealed within crawlspaces or behind walls. Retrofitting an existing building with an HVAC system can be troubling, at least when it comes to finding a place for all of the ductwork. Creative builders are often able to tuck these into existing spaces, and many can help redesign rooms so that a bump-out in the corner or in a closet isn’t as noticeable as it might otherwise be.

Trouble with proper temperature might be a sealant problem, but it could also be an issue with vent locations and overall force of the suction. Getting a professional to rework some aspects of the airflow can help ensure that as much is being pulled out as is pumping in. Homeowners often also find that the air return duct works more efficiently with clean filters. Experts usually recommend replacing the filters at least seasonally, though monthly or bi-weekly may be needed for people with a lot of pets or environmental dust.

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Discussion Comments

By anon1006044 — On Jan 20, 2022

Is the duct leading into your heat unit supposed to be warm after the heat kicks on?

By anon1001762 — On Jun 20, 2019

Does the return air come back in the house after it is filtered? Reason being I'm having mold issues in my house, and the return gets half of its return from the basement.

By Mcwick1968 — On Sep 27, 2015

Should I cover the central air vent in winter, if I have a separate heating system (hot water baseboard). I close the central air room vents, but I wonder if warm air is going up the open vent. I live in New England, where it gets very cold in winter.

By anon320392 — On Feb 17, 2013

Our house has a thin layer of insulation spot- glued to the inside surface of the return air duct. Does this serve any function other than a dust collector and health hazard?

By JackWhack — On Oct 01, 2012

@giddion – I know! I didn't know it was a return vent, either. I thought that air came out of it and into the room, but apparently, that's what the air registers are for instead.

I have an air register in every room of my house, and if it gets too cold in one room, I can shut the register. I like being able to adjust the air flow like this, because I am rather cold-natured, but my roommates are not.

By giddion — On Sep 30, 2012

I had no idea that's what the big vent in my hallway was for! I knew it had something to do with the HVAC system, but I never knew that air was actually being sucked into there.

By healthy4life — On Sep 29, 2012

@feasting – Yes, a dirty filter in your return air grill can run up your electric bill. Some filters need replacing once a month, and some only need to be replaced every three months.

I vacuum the dust out of my air return duct every time that I change the filter. I don't want any of that dirt to get on the new filter.

Since I have dogs in the house, there is often a lot of dander and pet hair in there. I notice a big difference after cleaning everything out, because I stop sneezing and wheezing.

By feasting — On Sep 29, 2012

Can having a dirty air return grill affect the efficiency of the air conditioner? My electric bill was really high last month, and I just discovered that the filter in my air return grill was coated in dirt and pet hair.

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