The Cost Of Air And Cooling: Risks Of AC With Open Windows

Every spring in San Antonio there are a few perfect weeks when the mornings are cool and the afternoons are warm, and it is tempting to open the windows and leave the air conditioner running to take the edge off. It feels like the best of both. In reality it is one of the most expensive habits a homeowner can have, and it is hard on the equipment in ways that show up months later.

What actually happens

An air conditioner works by removing heat and humidity from a closed volume of air and running it past the coil over and over until it reaches the set point. Open a window and that volume is no longer closed. Outside air pours in, the system never catches up, and it runs continuously. On a 78 degree afternoon with 70 percent humidity, an open window can double the system's runtime compared to a closed house.

The humidity problem

Temperature is only half of what an air conditioner does in South Texas. The other half is pulling moisture out of the air. Outside air in April and May in San Antonio often carries more water than the air conditioner can remove, so with the windows open the house feels clammy even at 74 degrees, and the coil stays wet around the clock. A constantly wet coil grows mold, the drain line runs nonstop and is more likely to clog, and the musty smell people blame on their duct work often starts here.

The cost on your bill

The math is simple. A 4-ton system draws about 4 to 5 kilowatts while running. At CPS Energy summer rates, an extra five hours of runtime a day for a month is roughly $60 to $90 added to the bill, and you got nothing for it except a slightly breezier living room. Over a spring and fall of open-window afternoons, that is a few hundred dollars.

The wear on equipment

Compressors are rated for a certain number of running hours over their life. Doubling runtime in the shoulder seasons quietly uses up that life. Continuous operation also keeps the outdoor fan motor and contactor cycling through heat they would otherwise avoid, and the evaporator coil never gets the dry time between cycles that keeps it clean. We see a noticeable difference in coil condition and drain problems between homes that run closed and homes that do not.

What to do instead

If you want fresh air, turn the air conditioner off and open the windows. On a 72 degree morning with low humidity, that is the right move and the house will stay comfortable for hours. When the afternoon heats up, close the windows first, give the house fifteen minutes to settle, then turn the AC back on. Ceiling fans make a closed house feel three or four degrees cooler and cost pennies to run. If the goal is better indoor air rather than a breeze, a clean coil, a good filter, and a UV-C light will do more than an open window ever will.

A note on older homes

Homes in Alamo Heights, Olmos Park, and Terrell Hills built before central air often have windows that do not seal well even when closed, and the same math applies at a smaller scale. Weatherstripping and a duct leakage check make a bigger difference in those houses than a higher-efficiency unit. If your system seems to run all day even with everything shut, our inspection service can find where the air is going. Call 210.549.4839.