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How to Fix a Broken Thermostat

Sometimes what you think might be a faulty furnace is actually a malfunctioning thermostat. Rather than assuming you have an expensive HVAC problem, it's a good idea to take a look at your thermostat first.

Thermostat Troubleshooting Tips

In our last blog post, we talked about 5 Furnace Troubleshooting Tips that can help you solve some of the most common heating problems. On the list, we talked about having your thermostat cleaned, set to “heat,” and changing its batteries, but we didn’t go very much into detail.

Sometimes, your thermostat needs a little more attention. What you think is a broken thermostat may just be a neglected one. But sometimes, you may just have to break down and buy a new thermostat because of aging, faulty wiring, an old transformer, or other issues. In this case, we highly recommend investing in and learning to use a programmable thermostat.

You may also be interested in the new, high-tech “smart” thermostats, which claim to learn your habits and automatically adjust the temperature as needed. If you do end up replacing your thermostat, do not throw your old, broken thermostat in the trash, especially if it has mercury in it. Find a way to properly dispose of it or give Cassel Home Comfort a call!


How to Fix a Broken Thermostat:

Before you do anything, check to see if your HVAC system has power. You don’t want to be that guy. Go over to your breaker panel and make sure your HVAC system is getting power. The next step is checking to see if your thermostat has power:

1. Change the Batteries

This is definitely the first thing you will want to check before moving on to the more complicated stuff. All thermostats are different, with some requiring battery changes and others not needing batteries at all. For wireless systems, use AA Lithium batteries instead of the weaker regular ones. Watch this video to figure out which kind of thermostat you have and how you can change its batteries:

2. A Good Dusting

Another simple fix, usually for older electromechanical thermostats, is a simple dusting with a small paintbrush, or other soft brush. Now that you know how to open up your thermostat’s housing from the video above, you can now open it up for some light cleaning. Dust and dirt are often the cause for inaccurate temperature readings and other problems. Lightly dust the inside of your thermostat, including the metal coils and contact plates. If you brush can’t fit in between the contact plates, try sliding a soft paper back and forth to clean them.

3. Check Location

This may seem obvious, but you’d be surprised how many homeowners overlook this problem: your thermostat needs to be away from heat sources and direct sunlight. If your thermostat is near a lamp, computer, or any other source of heat, the thermostat’s temperature reading will be off and will send the wrong commands to your furnace or air conditioner.

4. Check Balance

Using a level device, make sure your thermostat is perfectly level. If your thermostat isn’t completely straight, then your mercury switch will not work properly/accurately.

5. Adjust the Anticipator

Oh the anticipation! Well, not really, but it will be exciting to see if this fixes your problem. To adjust the anticipator, open up your thermostat’s housing and look for a small metal tab next to a scale from shorter to longer. If your HVAC systems cycles on and off too frequently or not frequently enough, adjusting your anticipator will usually solve the problem.Mercury Switch

  • If the heat is cycling on and off too frequently, your anticipator should be moved closer to the “longer” setting. Make adjustments one calibration mark at a time.
  • If your furnace never seems to reach the desired temperature, move the anticipator away from the “longer” setting.
  • Once making these small adjustments, wait a couple of hours to see if the system has stabilized itself. If you still feel as though your anticipator needs to be adjusted, make the necessary changes, one calibration mark at a time. Usually, the anticipator will work properly when set between .2 and .8 amps.

photo source: HomeRepairGeek


Bonus Tip:

There is one last thing you can try to get your heat and thermostat working again:

  1. Set your thermostat to “heat,” but turn the temperature setting down to about 60 degrees, so that your furnace is not getting any signals from your thermostat to turn on. Your furnace should be off at this point. If it hasn’t turned off, keep lowering the temperature setting until it does.
  2. Go to your breaker panel and turn off power to your thermostat/furnace system. If you have not labeled the breakers in your panel, now might be a good time to test them all and label them correctly for future reference.
  3. Leave the breaker switch off for about 30 seconds to a minute and then turn the power back on to your HVAC system.

Hopefully, this “reboot” was what you needed to get your heat back on. In any case, it is highly recommended that you have annual HVAC inspections for your heating and air conditioning.

Most manufacturers warranties require them for warranty claims and they have been proven to save homeowners time, money, and energy in the long run.If you are still confused about thermostats and how they work, let Mr. Wizard teach you:

If these 5 Thermostat Troubleshooting Tips didn’t fix your broken thermostat and/or HVAC system, the professionals at Cassel Home Comfort will be glad to help. We will also show you simple maintenance tasks that you can do yourself to maximize the efficiency and effectiveness of your HVAC system and thermostat. For more information, see our FAQ page.


Call Cassel Home Comfort Heating & Cooling today at (217) 866-1596 to schedule a service for thermostat repair!


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