Why does our AC kick on almost constantly at night, running a minute or so and then turning off?

Asked by Liz
San Antonio, TX

We set our thermostat at 82-85 degrees at night.


Rich Franz-Ünder

Answered by Rich Franz-Ünder

Tucson, AZ

Pima County Development Services

June 17, 2010

It sounds like your AC is "short-cycling." This happens when the AC is oversized for the cooling load.

Unless you are fortunate to be living in an Energy Star home, your AC was probably over-sized when it was originally installed. What happens is the oversized AC comes on and sends a rush of cold air out into the house. This cold air quickly makes its way to the center of the house, where the thermostat is usually located. When the thermostat senses the cold air, it shuts the AC off again. This means that all of the air in the house does not get properly mixed with the cold air. The perimeter walls and the far corners of the house probably always feel warm.

By setting your thermostat high at night, you are actually increasing the over-sizing problem. Your big AC has to do even less cooling.

What is the fix? One do-it-yourself fix is to see if there is a supply air duct (blowing cold air) close to the thermostat -- if so, try temporarily blocking it to see what happens. (Don’t block the return air vent; this is one that is drawing air into your AC.) If this helps, then ask an HVAC contractor with a BPI certification to help you rebalance the air in the house. You can also try to leave different doors open or closed within the house to temporarily change the air pressure differential in the house. If this helps, again, seek the help of an HVAC pro with BPI certification.

One last thought: if you have a programmable thermostat that will let you change the increase or decrease in temperature that shuts off the AC, sometimes you can set the thermostat to shut off after the air cools down 1 degree, 2 degrees, etc. You might want to check the setting on your thermostat. If it is set at 1 degree, try changing it to 2 degrees.

Hope this helps.


For more information:

You should also read Steve Saunders' Ask A Pro Q&A, "Should I follow my service company's advice and replace my current AC unit with an 'environmentally friendly' Freon unit?"

Tagged In: heating cooling, summer energy

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