How To Test If Your Furnace Is Heating The Way It Should

Have you ever wondered if your furnace was working the way it should? Well, there’s a simple way to tell if it’s heating right. In this article, we’ll go over how to test your furnace’s temperature rise. We’ll also explain what the temperature rise is and what to do if it’s out of range.

What Temperature Rise Is And Setting Up To Test It

measuring temp split

The air that leaves the furnace to heat the house is called the supply, and the air that goes back to the furnace after heating the home is called the return. You subtract the return air temperature from the supply air temperature to get the temperature rise. Each furnace has a temperature rise specified by the manufacturer that shows if it’s heating the way it should. It’ll be listed on a nameplate inside the doors of the furnace as a range of temperatures from low to high.

In order to test the temperature rise, you’ll need a probe thermometer and something to make a small hole in the metal. Then you’ll need to figure out where the return and the supply air are. Look for the arrow for airflow on your air filter. The filter should point towards the furnace. The air flowing towards the furnace from behind the filter is the return. If you follow the furnace past the heat exchanger, that’s the supply side. With that information and the range from the nameplate, you’re ready to test.

How To Test The Temperature Rise

Failed Fill Valve

In order to run the test, you’ll need to let the furnace run for at least 10 minutes to normalize its temperature. Then you’re going to need an access hole in both the return air ductwork and the supply air ductwork. A small drill bit or metal screw should be enough so that you can pass the probe of the thermometer through. If you’ve had your system serviced, chances are the tech already made access holes.

Now it’s time to test the temperature. Stick the thermometer in the return-side hole and wait for the temperature to stop moving. Then record the result. Repeat this process for the supply side. Once you have those numbers, subtract the supply temperature from the return temperature. That's the temperature rise. With that, you can refer back to the temperature rise range to see if your furnace is within the range listed.


If the temperature is within that range, it should be heating right. If it’s out of range, the most common cause is restricted airflow. Dirty filters, dirty evaporator coils, or too many closed vents can all cause that kind of problem. If your temperature rise is good, but you aren’t feeling comfortable when it runs, it’s possible that your system wasn’t designed right for your home. In any case, if you have any questions about your heating system, Any Hour Services would be happy to help. You can give us a call or schedule an appointment online.




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Author: Nathanael Stuver
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Nov 16th 2022

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