If you’re having issues getting your furnace to turn on, we’re willing to bet you want to know why (and how you can fix the issue as soon as possible).

A few of the most common reasons your furnace isn’t turning on include:

  • Thermostat issues
  • Filter is clogged
  • Electrical issues
  • Condensate line is clogged
  • Gas supply to the furnace is closed

In this article, we’ll look at each of these issues in more detail. We’ll let you know which one you can fix yourself and which you’ll need to reach out to a professional for.

Don’t like the idea of diagnosing and repairing your furnace on your own?

Contact Roy’s! Our goal is to take the stress out of any furnace issue you’re having and get your system back up and running as soon as possible.


See furnace repair details

Reason #1: Thermostat issues

Thermostat in a Buffalo, NY home

Your furnace won’t turn on if your thermostat is on the wrong settings or simply needs new batteries.

During the winter, your thermostat should be set to “HEAT” and the fan should be set to “AUTO.”

If your thermostat is set to COOL, it won’t turn on, even if the temperature you’ve set on your thermostat is higher than your home’s current temperature.

If your furnace is turning “on” but not heating your air, your thermostat’s fan setting is likely set to ON instead of AUTO. On AUTO, your furnace will blow air all the time, even when that air has not been heated by your furnace.

If your furnace isn’t turning on at all, it might just be out of batteries. Try replacing the batteries; if your thermostat turns on, this was likely your issue.

Reason #2: Filter is clogged

If your filter is clogged, your furnace can start to overheat, which can cause your system to shut off.

Your furnace needs a certain amount of air blowing over the hot internal components of the system in order to work properly. If your air filter is clogged, it will reduce the amount of air your furnace can pull in, which can cause the inside of your furnace to get too hot. When your furnace begins to overheat, it will automatically turn off.

Your furnace may start up again when it has cooled down. But if your filter is clogged, it’s only a matter of time before this happens again. So, if you notice that your furnace is running for short periods of time and then shutting down, check your air filter. If it looks like the filter on the right, it needs to be replaced.

Dirty HVAC air filter

Reason #3: Electrical issues

If your furnace is not cycling on at all, your breaker may have tripped or the disconnect switch may be off.

Tripped breaker:

A tripped breaker is a fairly common situation for most homeowners. It happens when the circuit is trying to carry a higher electrical load than it was built to handle.

To see if a tripped breaker is your issue, head to the breaker box and check the breaker labeled HVAC or furnace. If the breaker has moved to the neutral (or central) position, it means your breaker has tripped.

To reset your breaker, you’ll need to turn the breaker off and then back on. If your furnace turns back on within a few minutes, problem solved.

However, if your furnace does not turn on or the breaker trips again a short while later, you’ll want to reach out to a professional. A breaker tripping this often is usually a sign of a larger electrical issue, and resetting the breaker again could pose a safety hazard.

Disconnect switch

A disconnect switch turns off (or disconnects) the electricity to your furnace. This switch is usually on or near the side of your furnace, and is an easy way for technicians to “turn off” your furnace for maintenance.

If you think this may be your issue, simply check the disconnect switch to see if it’s ON. If it’s not, turn it back ON and your furnace should start working.

Note: These issues apply to both gas and electric furnaces. Even though gas furnaces use gas as their fuel source, they still need some electricity to function.

Reason #4: Condensate line is clogged

High-efficiency furnaces produce moisture as part of the heating process. This moisture must be removed from the system by way of a condensate drain line.

If this drain line becomes backed up or clogged, your furnace’s safety switch, or float switch, will turn your furnace off to prevent water damage.

If you notice water around the floor of your furnace, your condensate line is probably clogged. To unclog it, try to find the end of the drain line (located outside). Once you find the drain (usually a PVC pipe), hook a wet/dry vac to the end of the line and turn it one for 1-2 minutes.

If your furnace starts working again shortly, a clogged line was probably your issue. However, if you notice that your furnace is still leaking and/or it doesn’t turn on, you likely have another issue, like a broken condensate pump. In this case, you’ll want to reach out to a professional who can accurately diagnose and repair your furnace.

Reason #5: Gas supply to the furnace is closed

If you have a gas furnace and the gas supply to your system is cut off, your furnace will not turn on.

There are two ways to check your furnace’s gas supply:

  1. Check the furnace shut-off valve
  2. Check the main shut-off valve

Each individual gas appliance you have will have its own shut-off valve, which will turn off the gas supply to that one appliance. Check the gas supply to your furnace and make sure that it’s open.

If the gas supply to your furnace is open but your furnace isn’t turning on, it’s time to check your main shut-off valve. The main shut off valve is located outside and will shut off gas supply to all gas appliances in your home. If this shut off valve was closed, turn it back on.

If both shut-off valves were open, you will want to reach out to a technician for help. There could be another issue with your furnace or there could be an issue with your gas line, either of which you’ll want a professional’s help for.

Still having issues with your furnace? Call Roy’s!

If you’re still having trouble getting your furnace to turn on (and/or stay on), it’s probably time to reach out to a team of pros who can accurately diagnose your furnace issue and repair it. At Roy’s, we have years of experience helping Buffalo homeowners get their heating systems back up and running, and we’d be happy to help you too. Stop worrying about your furnace and call Roy’s!

Scroll to Top