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No, closing doors won't save money on central A/C

Closing bedroom doors actually puts strain on most central A/C systems, which in turn leads to more expensive utility bills.

This summer, people in the U.S. and around the world have seen historic heat waves. Temperatures have surpassed 100 degrees Fahrenheit in large swaths of the U.S. in both June and July. With that, many are looking for ways to stay cool and save money this summer. 

Sally emailed VERIFY to ask if closing doors to rooms not in use will save money on central air conditioning bills.  

It’s a common belief that closing your doors is more efficient for central air conditioners because it funnels air into smaller spaces or cuts off rooms that don’t need cooled. Google data shows people have been asking this question online for years.

THE QUESTION

Will closing doors save you money on central air conditioning?

THE SOURCES

THE ANSWER

This is false.

No, closing doors will not save you money on central air conditioning, and may even cost you more. However, it does save you money if you only have a single-room air conditioner, like a window unit.

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WHAT WE FOUND

Your home likely has one of two kinds of air conditioners: a central air conditioner, which is designed to cool an entire home through ducts and vents, or a single-room air conditioner, the most common of which are the boxy A/C units that stick out of windows.

“Common central air conditioning problems occur when rooms are closed off and air flow through the home is disrupted,” the U.S. Department of Energy says. “On the other hand, if you have a room air conditioner, the opposite is true.”

Arizona-based Precision Air & Plumbing goes a step further.

“If you have a typically installed air conditioner unit with a central air return, shutting doors is one of the worst things you can do,” Precision says. “It’s right up there with closing vents and beating your outside unit with a sledgehammer.”

The reason for this is because HVAC systems rely on a careful balance of airflow to work effectively. To understand this, we first need to understand how a central air unit works, and what a “typically installed” unit looks like. 

Central air conditioners use a system of supply and return ducts to circulate air throughout a home. The air conditioner sucks in warm air from outside and cools it. Supply ducts carry cool air from the A/C unit into the home through supply vents. This cooled air becomes warmer as it circulates through the home before it flows back into the A/C unit through return vents and ducts. That air is then cooled again and dehumidified in the A/C unit and its heat is pumped back outside while the air re-enters the supply ducts to continue the cycle.

RELATED: No, you should not turn off your air conditioner to save on your power bill

It’s pretty easy to tell the difference between a supply vent and a return vent. Summers and Zim’s, Inc., a Pennsylvania-based HVAC and plumbing company, says supply vents are the grilles on walls, ceilings or floors from which you can feel conditioned air come out. Return vents are typically larger than supply vents, and there won’t be any air coming out of them.

In a perfectly designed duct system there should be a roughly equal amount of supply and return vents, says Official Heating & Cooling, an HVAC company based in northern Illinois. But the “vast majority” of homes usually have less airflow on the return side. SuperTech Heating & A/C, a Baltimore-based HVAC company, says builders often opt to install one large central return to save on costs. This could be in a hallway, just above the stairs in a two-story home or part of a door to the utility closet.

Since central air conditioning units are designed to cool the entire home, most rooms will have supply vents to blow the cool air in. It might sound like this means cool air will go into the room and stay there, but without a proper return system, this is not the case.

Closing interior doors while the A/C is in a cooling system traps the pumped air in the room, says Hyde’s, an HVAC company in California. This will increase the room’s air pressure, which will force the cooled air out of any small openings or cracks around the room’s windows, walls, ceiling and floor. The opposite then happens when the A/C is not in a cooling cycle — because the closed off room will be cut off from the airflow of the rest of the house, the room’s air pressure will drop below that of the rest of the house, and the hot outside air will enter the room through the same openings and cracks the cold air left it.

Additionally, the return vent is making an extra effort to suck in enough air to cool the entire house, not just the rooms with the open doors, Precision says.

These all add up to dirtier air, higher utility bills, increased HVAC wear, more energy loss and a less comfortable home.

RELATED: No, the federal government is not giving away free solar panels

Is there a fix if I want to close my doors and have a single central return?

If you have a properly balanced supply and return system — one in which return vents are present in every room supply vents are — closing your doors still won’t save money, but they won’t cost you any money either.

But simply adding return ducts and vents to every room in your home where there weren’t any before isn’t always an option. Instead there are far easier, simpler solutions that will fix your home’s air imbalances.

The Department of Energy’s Building America Solution Center says a combination of transfer grilles, jumper ducts and door undercuts can allow airflow to continue between rooms, even when doors are closed.

Transfer grilles are grilles on either side of a wall between two rooms to allow the air from the two rooms to pass between each other, according to the Building Science Corporation, a consulting firm for building technology. A jumper duct accomplishes the same thing, except the vents are located on the ceilings of both rooms. A door undercut is a space between the door and the floor below it that’s large enough to allow air transfer between rooms.

“While undercutting doors can create part of the return air path, wall transfer grilles or jump ducts should be installed to prevent the return problems,” the Building Science Corporation warns. It adds that bedrooms should have a vent directly connected to the return ducts or transfer grilles.

Don’t close the supply vents to unused rooms as a long-term solution, either. Most central air conditioners are going to try to cool the entire house no matter what and have no way to detect when a vent is closed. That means air will continue to travel to the closed vent, build-up and increase pressure in the duct and damage the duct, causing leaks that lose you money.

But what if I have a window unit?

Closing off unused rooms in houses and apartments with window units will actually reduce cooling costs and make the space you’re using more comfortable, according to Garland Power & Light, a municipality-owned utility provider in Texas.

Window units don’t have supply ducts and vents blowing cool air into every room of a home, and they aren’t built with cooling an entire home in mind. Therefore, trapping the air in the room the window unit is in actually does give it less air to cool and makes it run more efficiently.

More from VERIFY: How to stay cool in a heat wave

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