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8 Reasons Your Furnace Won’t Turn On

By May 21, 2021April 15th, 2024Furnace

We know it gets cold here in the northeast in the winter and when the temperatures drop, we expect the furnace to click on and warm up the home. But what if it doesn’t go on? What are the reasons the furnace won’t turn on? There are eight areas to check: circuit breakers, thermostat setting, a clean furnace filter, pilot light, blower motor, natural gas or oil supply, drain pan, and ignition switch.

Let’s take a look at why you need to know reasons the furnace won’t turn on, the eight reasons, maintenance, and when to contact a professional.

Why You Need to Know Reasons the Furnace Won’t Turn On

You want the house to stay warm in the winter and the way to do that easily is by running the furnace. It is a lot of work to put heaters in rooms throughout the house and dangerous as they can lead to electrical fires too. So by finding out what is causing the furnace to not turn on is important. A furnace can last about 15-20 years or longer so you want to be able to get the most amount of time out of it and keep it functioning. Cold days are no fun if it goes out or breaks down.

Circuit breaker

Reason 1: Circuit Breaker

Start with the circuit breaker. Find your electrical panel, which is usually in the garage, and check the breaker labeled for the furnace. If it is in the off position, switch it to on. Does the furnace turn back on? If yes, that is great! Wait a few minutes and make sure it stays on. Also, check other things plugged in inside the home to make sure that the power isn’t off.

If the breaker doesn’t remain in reset mode, trips frequently, or the electrical box smells like something is burning then you may have a bad circuit breaker. Contacting a professional to take a look is a good idea unless you are handy with things like this.

If you have a split bus panel or double tapped breakers, these could be the issue as well and you may need to contact a professional.


Reason 2: Thermostat Setting

Check the thermostat to see what the setting is for the furnace to come on. Perhaps it is set on cool instead of heat? If that is not it, then set the thermostat to 10 degrees warmer and see if it comes on. Sometimes if there is a power outage or a flicker of lights, the setting can be off.

In addition, check the following:

  • The batteries may need replacing.
  • The wiring may be dusty and have debris.
  • Check to make sure the thermostat is not on a cycle or schedule that prevents it from turning on.
  • Sometimes there could be an internal timer (up to 10 min, even) before a furnace switches from A/C.
furnace filter

Reason 3: Check the Furnace Filter That it is Clean

A dirty or missing filter can cause the system to not work well. Not sure where to find the furnace filter? It is usually inside the large metal (sometimes plastic) furnace compartment next to the blower, near the bottom of the door, or on the side of the door. It is important to change it out every 30 days. If it is dirty, it will block the air from going through and won’t allow the system to turn on.

When taking a look at the filter inside the furnace, if you notice a yellow or flickering flame, moisture on the windows or walls, or even rust on the vent pipe, the furnace may be leaking carbon monoxide. It is important to make sure the furnace is off and have this checked out right away.

Reason 4: Pilot Light Is Out

A pilot light that is out won’t allow the furnace to come on. It is a safety measure to keep the propane and natural gas from going into the home. This will need to be checked to see if it is the culprit and then replaced. On most furnaces, there is a pilot light viewing port through a small glass circle.

Be sure to follow your manufacturer’s instructions for reseting the pilot light.

Reason 5: Blower Motor Not Working

If the blower motor goes out on the HVAC or the furnace, then warm air won’t leave the machine and go out into the house as you want it to. Check to see if you can hear your blower turning on, you should hear a fan running. You could also look to see if there is a light that is blinking. If there is, it may be an indicator a professional needs to look it over and diagnose the issue.

Reason 6: Natural Gas or Oil Supply is Out

If you think maybe you are out of gas or oil in the home, try using another appliance that relies on one of those. Perhaps that doesn’t work either, then you may be out of oil or gas. If this is the reason, contact your gas or oil company.

Float switch

Reason 7: Drain Pan is Full

A float switch can turn an HVAC unit off if the condensation line backs up into a drain pan. This is also called a drip pan and is on an HVAC unit. If the pan is full of water, then the float switch activates when the water reaches a certain level and shuts down the HVAC.

You should be pouring 1/3 cup of vinegar down your condensation line every 1-3 months to prevent this from happening.

Empty the water in the pan and if it has a pump, make sure that the float switch is in the down position. Additionally, use a wet/dry vacuum to suck the water/clog out from the exterior of the condensation line.

If it is not working, call a professional to take a look.

Reason 8: Ignition Switch Isn’t Working

This should be your last area to check. If the other seven reasons are not the problem, it could be the ignition switch. However, this is something that a professional will need to take a look at as it can be dangerous.

  1. Look up the manual and find the furnace flame sensor.
  2. If the sensor is dirty, it won’t function and will need to be cleaned.
  3. A professional should clean the sensor.
Maintenance of a furnace

Here is a great video about more furnace issues and how to solve them (or tell a professional what you think the issue may be )

Maintenance of the Furnace

Here are some great tips to help maintain your furnace. Properly maintaining your unit can ensure your unit does not break down on you.

  • Try turning it on before it gets cold so you can make sure it works before it is freezing outside.
  • Clean the filter when it starts to get dirty or every 90 days.
  • Listen for noises and troubleshoot before they become issues.
  • Have a professional inspect the furnace twice a year.

When to Contact a Professional

Contact a professional if you have tried to resolve the issue or if you need assistance. A professional is trained and a furnace can be tricky to figure out so leave it to someone who knows the in’s and out’s. They may also be able to give you ideas for maintenance during the warmer months so that your furnace works in the winter months.


Having a furnace is a luxury to keep us warm in the winter months. Being able to stay warm under our covers is nice when the furnace also is providing warmth.

Be sure you are properly maintaining your furnace so you can prevent this from happening. However, sometimes there are things we can’t avoid. Hopefully, these reasons helped you!

As home inspectors, we look at furnaces during our home inspections in the New England Area.