SERVING 
So you know your heating system has satisfied your thermostat setting, but the furnace blower won’t turn off. Now you’re wondering, “Why does my furnace fan keep running?”
You’re probably also wondering how to shut the fan off. Keep reading. We’ll tell you how to proceed.
Let’s take a look at some of the reasons your furnace fan might keep blowing air even after the furnace has completed a heating cycle.
1. The furnace blower won’t turn off because you’ve set it to “on.”
There’s a good chance you or someone in your house set the fan to “on” using the little switch on your thermostat.
Most thermostats have a switch that lets you toggle the furnace fan (aka the blower) between “auto” and “on.” When set to “auto,” the blower only runs while the furnace is completing a heating cycle. In other words, it’s only blowing heated air through your supply registers.
But when you toggle the switch to “on,” the blower will continue to run even after the furnace is done heating. The fan is circulating the same air that the furnace just heated; however, the air loses some heat as it passes back through your HVAC system while the furnace isn’t engaged in a heating cycle. Your home might cool down a bit faster than if you’d set the fan to “auto.”
In most homes, most of the time, it’s better to set the fan to “auto.” This is true during winter and summer.
Why would you ever set the fan to “on?” Sometimes you might want to blow some air around if your home is feeling a little stuffy during the shoulder seasons when neither the heat nor the AC is on. The fan can help with that. Other times, you might want to cycle some air to help eliminate cooking smoke or some other bad smell from your home. Homeowners with fresh air systems might also want to run the fan more often to better distribute fresh air within their living spaces.
Some people whose homes are equipped with HEPA filters also prefer to keep the fan on because they want to constantly filter their air. Constantly running the fan certainly isn’t a requirement if you have a HEPA filter, but some people prioritize having super clean air over the downsides of keeping the furnace fan on.
But if someone toggled that switch to “on” and you want it set to “auto,” just toggle it back. Your blower will probably turn off.
2. Your smart thermostat keeps the fan on by default
Some smart thermostats have a setting where they turn the fan on automatically every hour. This setting might be active on your smart thermostat by default, meaning it shipped to your home with that setting toggled to “on.”
Basically, the thermostat will tell the fan to turn on for a set amount of time even if the system isn’t heating or cooling. If you don’t want to keep running the fan, you can modify this setting on your smart thermostat.
The solution is to set the fan to “auto,” just as you would with a typical thermostat.
3. The fan limit switch is in manual override mode
Yikes! That sounds complicated! What’s the fan limit switch?
Relax. You don’t have to know what it is or where to find it. Also, this is an issue that only occurs in older HVAC units. We don’t see it much anymore. Still, if you have an older unit and can’t figure out why the fan keeps running, you might have this problem.
A fan limit switch is a safety mechanism that comes with furnaces. It can determine whether the fan is on or off; it also turns off the furnace burners if it senses that there’s too much heat. Anyway, the switch has two different modes. One is “auto” and the other is “manual override.” “Auto” is what you want most of the time. In this setting, the blower will run whenever the furnace is heating – kind of like the switch on your thermostat. However, it’s possible to manually override the fan limit switch by pushing in a little button found under the furnace hood.
If the switch is pushed in, the furnace fan will run continuously. If the button is out (that is, not pushed in), the fan will turn off when the furnace isn’t cooling.
It’s possible to check this yourself, but call an HVAC pro if you’re not sure you can identify the switch or you don’t have any experience with furnaces. Opening your furnace hood can be dangerous if you don’t know what you’re doing!
4. Other causes
If the fan is set to “auto” on the thermostat and the fan limit switch isn’t pressed in (older units only), there may be a problem with your furnace or thermostat.
Likely culprits include a bad thermostat, a shorted wire, or a bad circuit board inside the HVAC unit. In these cases, it’s best to call an HVAC professional to diagnose and address the problem.
Otherwise, you’ll be stuck with a furnace fan that never turns off! Not good.
Got a furnace blower that won’t turn off? We can help!
At PV, our team can help you sort out any problems with a misbehaving blower or furnace fan. We service HVAC systems at homes all across Metro Atlanta! Just give us a call at (404) 798-9672 or schedule service online!
Our team travels with all of the equipment and parts needed to address this issue. Get in touch today to get your furnace working properly again!
