How to Measure the Indoor Temperature Difference of the AC Unit

AC window units and forced-air systems work in the same manner, but the window unit works in a smaller area instead of cooling the whole house. Room temperature air is drawn into the unit where it is cooled and recirculated into the home at a colder temperature. This closed system keeps cooling the air with each pass through the air-conditioner until the temperature setting on the thermostat is reached. Measuring the difference of the room temperature air and the cool air returning to the room can alert you to the need for a service call.

Clean filters in an AC return air vent supply cooler air to your home.

Step 1

Turn a window unit or forced-air system to the “On” and “Cool” position.  Lower the thermostat or temperature setting so the air compressor activates.

Let the AC unit run for about 15 minutes. 

Step 2

Hold an outdoor thermometer in front of the return air vent on a whole house unit or in front of the return air area on a window unit.  These areas are where the air filters install to filter the internal air and recirculate it into the house after it is cooled.

Hold the thermometer in place until the temperature remains constant for at least two minutes.  Observe the temperature of the return air.

Step 3

Hold an outdoor thermometer in front of an air-conditioning vent that distributes cool air to a room on a forced-air system or on the top air vents of a window unit.  Keep the thermometer in place until the temperature remains constant without moving colder for at least two minutes.

Observe the cool air temperature. 

Step 4

Subtract the return air temperature from the cool air temperature to find the difference between the two.  The difference should be between 14-20 degrees.

If the difference is less than 14, the AC is below the efficiency at which it should run and may need cleaning or the addition of Freon to cool the air in the home better. 

Things You Will Need

  • Outdoor thermometer

About the Author

Mary Lougee has been writing for over 10 years. She holds a Bachelor's Degree with a major in Management and a double minor in accounting and computer science. She loves writing about careers for busy families as well as family oriented planning, meals and activities for all ages.

Photo Credits

  • air conditioner vent image by Tammy Mobley from Fotolia.com