Should I leave my interior doors open or closed when running my AC?

Cooling your home can be expensive, especially if you live in a hot climate. Some months it seems like the air conditioner runs more than it doesn’t, making the sheer thought of opening your utility bill horrifying. People have devised a number of theories on how they can save money on air conditioning costs, but no theory is as pervasive as keeping interior doors shut.  Basically, the theory is: if you shut doors to rooms you’re not using, then you won’t have to cool them, which should save you money.

Unfortunately, this well-meaning wives’ tale will end up costing more — and it can create other indoor air problems.

A modern air conditioning system is carefully balanced as a whole. When you close down one part of the system, you’re not making the other parts stronger — you’re actually disrupting the system’s operation. Not only will this force the system to work harder and potentially damage it, but your indoor air quality will also suffer significantly as your air conditioner tries to compensate for the sudden air imbalance.

What Happens Behind Closed Doors

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. (It’s right up there with closing vents and beating your outside unit with a sledgehammer.) When you shut a door, the air that’s still blowing out of the vent builds in the room, pressurizing it. At the same time, the return is making an extra effort to suck in enough air to cool the entire house (not just the rooms with open doors).

The positively pressurized cool air behind the door is looking for a way out, and it eventually finds its way through cracks in walls, behind trim, and around windows. The return is still trying to take in air, despite cool air flowing outside from the closed bedroom, and takes it from anywhere it can find it. This often results in a negative pressure in the part of the living area that’s still open to the return, and outdoor air rushes into cracks and crannies.

As this cycle continues through the summer, you may notice increasing humidity, your indoor air quality sinking, and no real improvement in your power bill. Because so much outside air is pulled into your home, allergens, mold colonies and other outdoor pollutants can build up inside. Only when you finally balance the pressure by opening up those closed-off rooms will your air conditioner be able to clear the air and cool your house at peak efficiency.

Relieving the Pressure of High Utility Bills

Closing doors is a recipe for disaster, but there are plenty of things you can do to help keep utility bills down. Adding insulation, caulking around trim, and sealing visible cracks will help reduce the amount of air transmission from outside.  If you also keep your curtains and shades drawn during the hottest parts of the day, you’ll help prevent heat from entering your home.

Air conditioners that sit out in direct sun often run less efficiently because they have trouble getting rid of the heat from inside your home. Moving your air conditioner or installing a shade to keep it cool will help it work better. More permanent solutions like planting trees will help in the long run.

Turning on the fans in your home will increase indoor air circulation. Many people find they can raise their thermostats several degrees and remain just as comfortable without the air conditioner running as often. Fans are extremely cheap to run, especially when compared to your air conditioning unit.

It can be tempting to slam the doors and hope the cool air builds up in the areas where you want it, but the fact is that modern air conditioning units are designed as whole-house systems. So when you shut that door, you’re only making it much harder for your system to cool your home.