You’re all ready for a cozy winter day binge-watching something on TV. You’ve rushed around and finished your chores and it’s finally time to sit.

But now that you’re still, you notice how cool the air is. You check the thermostat, and it seems to be working, but the room temperature is nowhere near your setting. The thermostat isn’t turning on your heating and you want to know why.

If this sounds something like your situation, you’re in the right place. I’ll take you through all the reasons why your heating may not be coming on, so keep reading.

Why Your Thermostat Isn’t Turning on Your Heating

Before doing a deep dive into the causes and fixes for a thermostat that doesn’t turn on your heat, let’s talk about how a thermostat works. It’s the hub of your HVAC system.

How a Thermostat Works

Thermostats take constant measurements of your home’s ambient temperature and using that information, they either turn on your furnace or air conditioner depending on your settings.

All electronic thermostats have a heat-sensing element and circuitry that senses changes in room temperature. Most are low voltage and used 24V of electricity, and they’re wired to different components of your system.

A very basic setup will look like the following:

  • White wire – Attaches to a relay that controls your heating
  • Yellow wire – Attaches to a relay that controls your cooling system
  • Green wire – Attaches to a relay that controls your blower motor/fan

For a detailed explanation of how your thermostat is wired, check out the following video.

When You’re Thermostat Is the Cause of Your Heating Not Turning on

There are several reasons for your furnace not to turn on, and some of them may be directly related to the thermostat itself. In other cases, the thermostat does what it’s supposed to do and communicates with your furnace. However, a problem with the furnace stops it from turning on.

  Your Thermostat Doesn’t Start Your Furnace: Possible Reasons
Your Thermostat Needs New Batteries
Faulty or Damaged Wiring to Your Thermostat
Check Your Thermostat Settings
Your Thermostat Needs to be Calibrated
You Have the Wrong Type of Thermostat

1. You Have the Wrong Type of Thermostat

All thermostats do not work with all heating methods.

The two most common types of thermostats are:

  • Line voltage
  • Lowvoltage

Low-voltage thermostats are the most common and operate under 30 volts. These are direct wired thermostats and are used with:

  • Electric furnaces
  • Single-stage gas heating
  • Multistage gas heating

Line voltage thermostats typically have single or double pole wiring and operate between hundred and 20V and 240V. They’re most commonly used for:

  • Direct wired electric furnaces

Solution: If this is a brand-new thermostat that you’ve just installed and it won’t turn your furnace on, verify you have the right type of thermostat. Most new thermostats should have a label on them that will tell you which types of systems they are compatible with.

Hand Triying To Set Temperature From Thermostat
Knowing the different types of thermostats is essential to maximizing your comfort and minimizing your bills.

2. Your Thermostat Needs New Batteries

Just because the LED screen on your thermostat is working properly, it doesn’t mean your batteries are good.

If the voltage in the battery has dropped below what’s necessary to start the furnace, the furnace won’t start. However, there may be enough voltage to run your screen.

Solution: If you have a battery tester, check your batteries. If not, simply install new batteries and see if that corrects your problem.

3. Faulty or Damaged Wiring to Your Thermostat

It’s possible that over time some of the wiring to your thermostat has come loose—or that it wasn’t installed correctly in the first place. It’s also possible that the wiring is damaged.

Solution: Check your wiring. Checking connections is fairly simple. Remove the faceplate of your thermostat and do a visual inspection. Is anything loose or damaged? If this is a new installation, have you connected everything properly?

You should also check the connections in the furnace. To do this:

  • Shut off all power to the furnace
  • Remove the furnace cover
  • Look for a bundle of wires, typically 3 to 8 wires of different colors, depending on your system
  • If you don’t see any wiring bundles, they may be hidden behind a control board cover. If that’s the case, remove that cover as well to expose your wiring connections
  • Visually inspect your connections and wires. Check for anything that looks loose or damaged

Finally, if you’ve had mice in your house recently, there is a good chance they have chewed through some wiring. Check out these signs that could indicate mice have damaged some of your wiring. 

4. Check Your Thermostat Settings

Sometimes the problem behind a thermostat that won’t turn your furnace on is your settings. The following can help you diagnose them.

Solution: First of all, do a simple test. If you have a thermostat that has HEAT OFF COOL or FAN AUTO OFF settings, make sure you have it set to HEAT and the Fan set to AUTO. Then set it to its highest settings and wait about 3 to 5 minutes.

  • If your furnace kicked in, your issue was likely with your settings. Rollback the temperature to your desired setting
  • If your furnace didn’t kick in, check your wiring and connections.
Hand Setting Thermostat
Your thermostat settings may be the source of the issue.

5. Your Thermostat Needs to be Calibrated

There are several factors that have an impact on how well your thermostat behaves and how accurate it is. To ensure accuracy and calibration, follow the guidelines below.

Solution: Your thermostat needs to be:

  • Located between 3 and 5 feet from the floor
  • In a spot where there is natural air circulation but not exposed to strong drafts from doors, windows, or HVAC registers
  • If you have an older mercury thermostat, it needs to be level

Your thermostat should not be:

  • In direct sunlight
  • Beside appliances or electronics that generate heat
  • Next to any concealed ductwork or pipes
  • Above or close to a table lamp

Conclusion

Having a furnace that won’t start on a cold winter day can be frightening but if the issue is with your thermostat, there’s often a simple fix.

To recap:

  Your Thermostat Doesn’t Start Your Furnace: Possible Reasons
Your Thermostat Needs New Batteries
Faulty or Damaged Wiring to Your Thermostat
Check Your Thermostat Settings
Your Thermostat Needs to be Calibrated
You Have the Wrong Type of Thermostat

Hopefully, this article has helped you diagnose and fix your issue.

Why not check out our related posts below. Perhaps we can help you with something else.

Thanks for reading!