What Is an Air Return Duct?

An air return duct is an important part of most HVAC systems.
The air return duct pulls air back into the HVAC system.
High-pressure gas is turned into liquid that enters the evaporator coil to turn into a low-pressure gas.
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  • Written By: Mary Lougee
  • Edited By: J.T. Gale
  • Last Modified Date: 03 September 2014
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An air return duct is 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. An air return duct usually is located either in a hallway or in the ceiling, and its purpose is to extract air from a room and recycle it through the system to further condition it by cooling or heating.

Most central air and heat systems have a condenser that is outdoors and a coil that is located indoors that allows one unit to both heat and cool a building. In the summer, the return duct removes warm air from inside and transports it to the outside through ductwork to be conditioned. Ductwork is 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. The purpose of a ducts is to 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 usually is 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 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 air return duct. This process repeats itself until the temperature that is set on the thermostat is reached.

The importance of a sealed HVAC duct system can be attributed to the temperature of the air that is extracted by the return duct. For example, for cooling purposes, warmer air rises to the ceiling and colder air remains closer to the floor. 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.


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Post 5

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?

Post 4

@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.

Post 3

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.

Post 2

@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.

Post 1

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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