Is It Bad to Block a Return Air Vent?

If your home has a forced-air HVAC system, it’s important to always have your return vents free, clear and open.

With the help of two industry experts — Jason Ryan from Boulanger’s Plumbing and Heating and Joshua Smith from Berkshire Heating and Air Conditioning —  learn what your return vents do, what happens if they’re blocked, and what sort of trouble this can cause.

What Is a Return Air Vent?

Return air vents, strategically placed in your ceiling, floor or wall, connect to ductwork in your forced air HVAC system. They draw air from a room to the furnace or air conditioner. where it’s warmed or cooled. This conditioned air is sent back into the room through separate supply vents.

“It’s a big cycle,” Smith says. “The supply side delivers the heated or cooled air, and the return side pulls the existing air from the living space.” This process is repeated until the thermostat reaches a set temperature and the system shuts off.

Forced air systems can have one central return vent or a vent in every room. These vents typically feature a slatted cover or grate. Unlike a supply vent, return vents will not blow air, only draw it in.

Is It Bad To Block a Return Air Vent?

Yes. Then the forced air system doesn’t function properly.

The return vent carries a certain amount of air to the furnace or air conditioner. This amount equals the amount of air carried away from the furnace or air conditioner and back to the room.

If this cycle is disrupted, Ryan says, “the system starves for air. You no longer have heat transfer from the heat exchanger or cooling from the cooling coil because there’s no new air to condition.” The system works harder with less output, which can lead to a host of problems detailed below.

You should never close your return vents on purpose, but it can happen accidentally. “It can happen from people not paying attention and throwing a rug over it or a couch that doesn’t have feet, and they cover the return,” Smith says. Keep furniture at least 10 inches from return vents to allow proper airflow.

What Happens If a Return Air Vent Is Blocked?

Nothing good. Here are some consequences of blocking a return air vent:

Lower system efficiency

Forced air systems with even one blocked return vent are less efficient due to limited airflow. “The system isn’t providing the air it needs, so it runs longer to try to get to a certain temperature,” Smith says.

There’s also a lower output of conditioned air. “If you cover the return, then you’re not pulling in as much air, which means you’re then not putting out as much air,” Ryan says. This makes the system work harder and longer with diminished results.

“Over time, this can shorten the life of your equipment,” Ryan says.

Inaccurate room temperatures

The air from the room never makes it to the furnace or air conditioner to be heated or cooled, so it struggles to change temperature.

“If you block off the return, then all you’re doing is pumping air into the room,” Smith says. “If you have your thermostat set to 70 [degrees] and you have no return, you may never get to 70 because you’re not cycling that air through.”

Broken heating or cooling elements

The furnace’s heat exchanger and the air conditioner’s cooling coils also depend on air from the return vents to regulate their temperatures. If a closed return vent reduces their air, heating elements can overheat, and cooling elements can freeze into a block of ice and stop working.

“When that block of ice melts, it can drip down into your system and short out blower motors and control ports,” Smith says.

On the heating side, if you block off the return, the exchanger can overheat. “In extreme examples, it can crack and fail,” Smith says. “They’re all designed to have that proper airflow and bad things can happen when that’s thrown out of whack.”

Lowered home air quality

“If the return air vent is blocked, the furnace is going to try and pull air from anywhere that it can,” Ryan says. If you have any sort of gap in your ductwork, the system will pull from these places instead, carrying contaminants into your home and reducing air quality.

“If you have gaps in your ductwork in the cellar, and you have a moldy, wet basement you’re going to be pulling that basement air instead,” Smith says. “Air is lazy, it takes the path of least resistance.”

About the Experts

• Joshua Smith is operations manager at Berkshire Heating and Air Conditioning in West Springfield, Massachusetts. He has more than 20 years of HVAC experience and currently holds a Massachusetts Oil Burner and Refrigeration License.
• Jason Ryan is HVAC project manager for Boulanger’s Plumbing and Heating in Easthampton, Massachusetts. He’s been with Boulanger’s since 2001 and holds a Master Plumber License in the State of Massachusetts.