Why Is My Furnace Not Kicking On When Temp Drops?

It’s the middle of the winter season, with temperatures approaching 40°F, maybe even colder. Fortunately, you have a reliable furnace that has served you well for years. So, you’re not worried about the cold conditions at all. 

The problem is that the heater refuses to kick on automatically when temperatures drop below the thermostat setting. So for the umpteenth time now, you’ve had to manually ignite it when temperatures drop below your thermostat setting, say 70°F. 

What’s the problem? Is the furnace damaged? Do you need to replace it? Or is it the thermostat? Could it be broken?

What Would Cause a Furnace not to Kick ON?

The truth is that sometimes it may be the thermostat or something else. The following are some of the most common causes and potential solutions.

1. Thermostat issues 

If your furnace isn’t responding to temperature changes, the most likely culprit is the thermostat. Why? Because the furnace only takes temperature directions from the thermostat.

It’s the thermostat that tells it to raise or lower the temperature depending on indoor air conditions. So, what could be wrong with the thermostat? It could be one of the following;

  • Is the thermostat ON? The thermostat can only communicate with the furnace if it’s ON. So, check the thermostat switch. Is it ON? If not, turn it ON and turn the furnace ON to see if the problem goes away. 
  • Incorrect reading: Perhaps the temperature reading on the thermostat is too low. For instance, if the thermostat is set to 50°F, the furnace won’t kick on even if temperatures drop to 55°F. So, check the thermostat reading again to make sure it’s set to 68°F to 72°F. 
  • The thermostat is damaged/dead: Finally, you may be dealing with a damaged or dead thermostat. You’ll know that your furnace is damaged/dead if it doesn’t come ON no matter how much you try or if it doesn’t have readings on display. A damaged thermostat may also keep fluctuating. 

If the thermostat is damaged, you may attempt to repair it or call an HVAC pro to take a look. However, you may need to replace it in the end. 

2. Ignition issues 

If the thermostat is in good working condition, then the next thing you want to check is the ignition system. Modern furnaces have electronic ignition systems that allow them to ignite automatically. However, the ignition system is a complex structure that can malfunction without notice. 

The three main components of the ignition system that you want to pay special attention to.

  • Ignitor problems: In most modern furnaces, the ignitor is a spark or heat-activated system that ignites the pilot system that in turn ignites the burners. However, this ignitor can malfunction due to natural wear and tear or if it’s broken. You may need to replace it. 
  • Flame sensor issues: The flame sensor determines the presence of a flame before opening the gates for gas to reach the burners. If it’s damaged or dirty, it may malfunction. It may need general replacement and proper replacement. 
  • The pilot light: The pilot light is the little flame the ignites your furnace burner. Without it, the furnace cannot work. The flame can go off for many reasons, including dirt buildup, windy conditions, and gas supply issues. 

Troubleshooting ignition issues can be a regular DIY process or an extraordinarily delicate and risky process. We recommend adequate cleaning and regular maintenance. However, don’t hesitate to call a professional if the problem seems beyond your capacity. 

3. Gas supply issues 

If the issue persists even after fixing the ignition issues, it’s time to consider that the gas supply may be compromised. This doesn’t necessarily mean that your home isn’t receiving gas, as sometimes the furnace may cut gas supply to the burner for safety reasons. The following are a few key factors to consider;

  • Do you have gas? It should be relatively easy to know whether or not you’re receiving gas. If you use propane, you can verify whether the tank is empty. If it’s natural gas, the meter reading will tell you whether the supply is cut off. The only solution, if you have no gas supply, is to wait or contact your supplier. 
  • The gas valve is turned off: If you have enough gas/supply, perhaps the gas valve is turned off. The gas valve is a small lever that controls gas flow from the main piping to your appliances. Make sure it’s turned ON. It’s ON if the lever is parallel to the piping and off if it’s perpendicular. 
  • Is the limit switch triggered? Gas furnace limit switches are safety mechanisms designed to protect you and the furnace from fire risk and gas poisoning. For instance, it will cut off the gas supply if leaks or overheating is detected. The switch usually remains off until you fix it, which means that the furnace wouldn’t come ON even if temperatures drop. 

You can fix many gas supply issues without involving a professional. However, don’t hesitate to call the technician if your DIY attempts are unfruitful. Most importantly, know your limits. 

4. Electrical issues 

The furnace may also refuse to kick on when temperatures drop due to electrical issues. You’re probably wondering why. The reason is that, although gas furnaces run on gas, they need electricity to keep some of the parts operational.

For instance, the furnace control board is powered by electricity. So, if you have a blackout, it will not run. The following are a few electrical issues to consider. 

  • No power supply: Do you have power? If you don’t, the furnace will not run because most internal components won’t run. 
  • Is it a blown fuse? It’s plausible that you have electricity, but one of the fuses on the control board is blown. For instance, the primary line connecting your furnace to the electric supply has a fuse to protect it from power surges. If there’s a surge, the fuse will blow, effectively cutting the electricity supply to the furnace. 
  • The electric supply is switched OFF: Finally, check to ensure that the furnace’s electric switch is turned ON. 

If you have a blackout, you’ll need to wait for the power to return. However, if the switch is OFF, turn it back ON and turn on the furnace to see if the issue goes away. If it doesn’t, then you may have a blown a fuse. You may need to involve a professional electrician at this point. 

Other Reasons 

The furnace may refuse to come ON when temperatures drop for many other reasons. For instance, blocked air filters may cause the gas valve to shut, effectively cutting off the gas supply to the burners. 

The heating failure may also be the consequence of drainage problems. The furnace condenses moisture in a special condensate pan. This water needs to drain freely. Otherwise, the drain pan can become filled. Furnaces are designed to suspend heating when the drain pan fills. 

Blower fan issues and airflow are other issues that can cause the furnace to refuse to come on even in the middle of a cold winter night. 

The bottom line is that you should troubleshoot and ideally fix many of the fundamental issues. At the same time, though, always have an HVAC technician’s contacts at hand so you can call them right away if things get out of control.