Why Does My Furnace Fan Won’t Turn Off: Blower Keep Running

"Doesn’t that furnace ever stop running?" you ask. The answer may be, "Yes, the furnace stopped heating, but the fan won’t turn off." If this is a problem that you have in your home – or if it ever happens in the future – here are some things that might be causing it and what you can do.

Why the Furnace Fan Won’t Turn Off

Here are the most common reasons the furnace fan keeps running. Solutions are given. Some are easy. Depending on how handy you are, you might prefer to call an HVAC technician for some of these reasons the furnace fan won’t turn off. 

Thermostat Fan Switch Set to ON 

This is perhaps the most common cause of having the furnace fan runs after heat shuts off. It is also the easiest to find and fix. On your thermostat that controls the furnace – and perhaps AC as well – there will be a separate method to control the fan.

  • On manual thermostats it will be a separate switch marked, “FAN”.
  • On digital thermostats it will be a menu option labeled, “FAN”.
  • On Smart thermostats it will be a program feature labeled, “FAN”

Whatever type of thermostat you have, you will have a way to control the fan.

If it is set to “ON”, you have found the reason why your furnace fan keeps running after the heat shuts off. It is because the “ON” setting tells the fan to run all the time.

Why would you want that? This is an option so that you can use the fan to circulate air even when the furnace is not running if you want to, for example, to keep filtering debris and odor out of the air. But if you don’t want that, you will probably wonder why the blower motor doesn’t turn off. 

The way to fix this is to change the setting on your thermostat fan switch to “AUTO”. In this setting, the furnace fan will only come on after the furnace has warmed up a little, and it will shut off again after the furnace cools down a minute or two after a heating cycle. 

Shorted Thermostat Wires

When that fan switch on your thermostat is set to “ON”, a signal is sent to turn the fan on. But if the wires by which that signal is sent are shorted, the fan will run all the time even if the switch is set to “AUTO”. If you have a manual thermostat, it’s not too difficult to check to see if these wires are shorted inside. If the thermostat is a digital or smart model, you should probably contact your local HVAC professional to diagnose and fix the problem. 

The way you can check the wires inside are by simply pulling the thermostat cover off. Almost all of them come off by pulling straight away from the wall, perhaps with just a little wiggling action. The wires should be clearly visible and will be connected to a terminal with letters that usually correspond to the color of the wire. “R” for red, “W” for white and so on. 

Look carefully to see that the bare ends of the wires are not touching each other. You especially want to look at the Red and Green wires. Red is the 24V power and Green is the one that turns on the fan. If they are touching, gently separate them just a little with the tip of a plastic pen or some other small plastic or wood (not metal) tool. If you don’t see anything that looks suspicious here, replace the cover and call your HVAC expert. 

Defective Thermostat 

There is always the possibility that your thermostat is defective, especially if it has been in service for many years. New thermostats can be purchased at your local hardware or big box store in many different models. If you are even a little bit handy with tools, you should be able to install a new one yourself. 

Make sure you turn off the power to the furnace at the main breaker panel or at the switch on the side of the unit. If you also have AC, disconnect the power to that as well. Follow the instructions that come with the new thermostat, being careful to connect the wires to the correct terminals.

If you don’t feel comfortable doing this work yourself, just about any local handyman or HVAC technician will be able to do it quite quickly. 

Fan Limit Switch Set to Manual Override 

Furnace Limit Switch

This is a switch that is behind the front cover of your furnace. It is what normally allows your furnace fan to come on when the thermostat calls for heat and will shut the furnace down if it overheats. In older furnaces, it will have a manual override button that may be set to have the fan run all the time. On most newer furnaces, it will not have this button and this will not be something you can simply reset. 

If you are going to check this switch on an older furnace, be sure to turn off the power to the furnace first. The limit switch is normally a silver-colored box located toward the top of the unit and has a small white button on it. If this button is pushed in, the manual override is on. Push it in further and it should come back out. Replace the furnace cover, turn the power back on, and the fan should not be running all the time. 

Faulty or Stuck Fan Limit Switch 

The fan limit switch may have simply gone bad and need to be replaced. This can happen in any age furnace because the switch is constantly subjected to high temperatures inside the furnace as it monitors and protects against overheating issues. If you suspect this is the reason your blower motor fan is not turning off, contact a local HVAC technician to diagnose and repair the problem. 

Other Reasons Furnace Fan Won’t Turn Off

There are several other reasons why your furnace fan might not turn off that should only be handled by a trained HVAC technician. Some of these are:

  • Blown 3 amp fuse
  • Tripped flame rollout
  • Tripped low pressure gas switch
  • Fan limit control set improperly

If they sound rather technical, it’s because they are. That’s why you should let a professional deal with any possible problems that are inside your furnace. 

What to Do if the Furnace Fan Won’t Turn Off 

As you can see, the reasons why your furnace fan won’t turn off can be very simple or quite complicated. Before consulting with a good HVAC repair business, check those first few that you might be able to correct yourself. If those don’t solve the issue, go ahead and make that call.

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