How To Tell If Your Thermostat Is Bad (6-Step Guide)

Whether it’s a stifling hot summer day or a bitterly cold winter night, a properly functioning thermostat is essential for your HVAC system to keep you comfortable.

In my experience as a licensed contractor, I’ve seen all types and models of thermostats. Just like other parts, over time components wear out and need to be replaced, so checking your thermostat on a regular basis can help prevent break downs and keep things running when you need it most.

Proper Disposal of Broken Thermostats

Most thermostats today are digital, but some may still be bimetal thermostats with mercury switches.

There’s nothing wrong with that type of thermostat, but they aren’t very accurate and usually have a larger temperature swing.

If you’re going to replace it, you have to dispose of it properly because of the mercury. Most HVAC distributors are able to recycle old thermostats so check with local distributors or local contractors to see if they will dispose of it for you.

Read Also >> Which Inexpensive Programmable Thermostats Do We Recommend?

Supplies & Tools You’ll Need

  • Digital Thermometer – To check how accurate your thermostat is

6 Steps To Know If Your Thermostat Is Bad

Step 1 – Visual Check

The easiest thing to start with is checking the screen. If your thermostat screen is blank there’s definitely something wrong. It could be as simple as dead batteries, but its possible there’s a bigger problem with the thermostat or the furnace.

Step 2- Replace Batteries

Almost all digital thermostats have batteries, usually double A or Triple A. Some thermostats have a small door that flips open to access the batteries, and others you have to remove the thermostat from the back plate. Replace them and check to see if the numbers return to the screen.

Step 3 – Turn The Temperature Up

Set your thermostat to heat and turn the temperature up. Make sure it’s above the current room temperature and listen for a subtle click. After a minute or so, your furnace should be running and heat should be blowing from the registers.

Step 4 – Turn The Temperature Down

Turn the temperature back down below room temperature and listen again for another click sound. There’s usually a blower off delay, so the air will still blow from the vents for a couple of minutes. 

Step 5 – Check The Temperature 

Take a digital thermometer and place it on or very close to the thermostat. Leave it set for a couple of minutes and make sure the temperatures are reading almost the same. 

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Step 6 – Determine If The Thermostat Is Working

If your system is turned on and off properly and the temperature is reading accurately, your thermostat is most likely working correctly. If it’s inaccurate or doesn’t come on, you may need to replace your thermostat. If it doesn’t shut down when you turn the temperature down, the thermostat is most likely faulty and should be replaced.

Other Signs The Thermostat Is Bad

No power

If your thermostat screen is blank, you definitely have a problem. Dead batteries are a pretty common reason for this, but there are other causes. Some thermostats don’t use batteries and receive power from the furnace. You may have a different furnace issue that’s preventing the thermostat from getting power and keeping the screen from showing up. If you rule out everything else and think you have a furnace problem, you should call a professional to look over your furnace and diagnose it properly.

System Won’t Come On

Another symptom of a bad thermostat is that it won’t turn on your furnace or A/C. The contacts in the thermostat can fail, and even though it says it’s running, it may not be telling the equipment to come on. 

System Won’t Turn Off

This isn’t as common, but it does happen from time to time. Sometimes the contacts in the thermostat can stick closed and will cause your furnace or A/C to run continuously. 

Inaccurate Thermostat Temperature

Your thermostat may be turning your HVAC equipment on and off, but not at the right temperature. If your home feels significantly colder or warmer than what the thermostat is telling you it is, it’s probably time to replace it with a newer model. You can check this with a digital thermometer to see how far off the temperature is reading.

Changing Your Thermostat

Replacing your thermostat isn’t overly difficult. If you turn the power off to your furnace, and take a picture of how it was originally wired, you can most likely replace it yourself. If you aren’t comfortable or confident in doing so, call a contractor or technician to take care of this for you.

Read Also >> What To Do If The Furnace Pilot Light Goes Out?

Frequently Asked Questions 

What Are The Symptoms Of A Bad Thermostat?

It may be something obvious like a blank screen or running your unit continuously, or something a little less obvious like not turning the furnace on or misreading temperature. There are several different ways a thermostat can fail, so knowing what to look for is a helpful skill. Follow the steps above to find out if your thermostat is faulty!

How Do You Check A Thermostat If It’s Working?

It’s hard to fix something that isn’t broken, but you can check a few things to see if it’s beginning to fail. Check the accuracy with a digital thermometer and cycle your system on and off by turning the temperature up and down. If you can hear the thermostat clicking and your heating equipment turns on and off appropriately, your thermostat is most likely just fine. If you’re still questioning it, call a pro and have them check it out for you.

How Do You Know If You Need A New Thermostat?

If you still have an old mercury thermostat, it’s probably time for an upgrade. If you’re beginning to have issues with temperature accuracy or if it isn’t turning your system on or off, it’s definitely time to replace it.

Can A Bad Thermostat Cause No Heat?

Absolutely. If your thermostat isn’t telling your heating equipment to come on, you won’t have heat. The furnace doesn’t know what temperature your home is, it relies on the thermostat to tell it when to come on. 

Final Thoughts

A good quality thermostat is an essential part of your HVAC system. It’s reading the temperature in your home and telling your equipment when to run. Following these steps will help you determine if your thermostat is bad and can help make your home more comfortable! 


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