Is Your A/C Running Non-Stop When It’s Quite Hot? (1 of 4)

A/C running non-stop tells you one of two things:

  • Your HVAC system is doing all it can to keep you cool & some corrective actions (not related to the a/c) need be taken.
  • There it has a problem with our HVAC System

So, How Do You determine If A/C Is Working Right Before You Call For Service?

Your a/c running non-stop does not necessarily mean it’s not working correctly.  To determine if it is, you must know the difference in the temperature between the air coming out and the air going into the HVAC system (Heating, Ventilation & A/C).

If the system is working properly, there will be a 14-20 degree temperature difference after the system has run for at least 10 minutes. NOTE: Higher outdoor temperatures will increase the temperature of HVAC output air  — this is why we state the 14-20 degree range.

How To Do The HVAC Cooling-Performace Test:

Step 1:

  • Turn on the air conditioner when outdoor temperature is at least 80 degrees.  Note: Start the test after system has been running for at least 10 minutes.
  • Set the thermostat 10 degrees lower than the room temperature at the thermostat (to keep a/c running non-stop during test).
  • Identify the supply-air duct closest to the furnace (air coming out of the duct).
  • Identify the return-air duct closest to the furnace (air going into the duct).

Step 2: Using a probe-thermometer, slip it into the duct as far as it will go (be sure duct is fully open).  After the a/c has run for at least 10 minutes, read the temperature of the air coming out of the duct.

Note: If you have a cook in the house, there is likely one of these thermometers in the kitchen.  They are also known as a meat-thermometer.

probe thermometer

ThermoPro TP01 $5.00 at

Step 3:  With the a/c running non-stop, move the thermometer to the return air duct closest to the furnace.  Leave it there until the temperature reading remains steady.

Step 4: Subtract the lower temperature from the higher temperature.

With the difference between the two readings, you know if the a/c is running well or not.  If you decide to call for service, advise the company of the two readings.  They should be able to tell you if it’s running ok or not, based on the current temperature outside.

You can make this process easier and faster by using a Thermal Heat Gun (thermometer) instead of a Probe Thermometer.  This also allows you to easily check many ducts.  The duct temperature will be higher as you get further from the furnace.  A rule of thumb is 1-2 degrees warmer for each additional 10 feet further from the furnace.

thermal heat gun

Here is a link to an inexpensive Thermal Heat Gun on    Thermal Heat Gun