5 Reasons Why Your Furnace Fan Keeps Running With the Thermostat Off

When your furnace fan keeps running with the thermostat off, you may be scratching your head, wondering what’s going on. Before you start to panic about the cost of a new furnace, take a minute to see if you can diagnose the problem. Likely, it’s a simple issue to solve.

When the furnace fan keeps running, it could be an easy fix—like if your thermostat needs new batteries, it’s set to ON instead of AUTO, or the furnace fan limit is set to manual. However, other issues are best left to a professional. Read this guide to figure out what’s causing your furnace fan to keep running and how to resolve the issue.

1. Your Thermostat Is Set to ON

If your thermostat is set to ON, your solution may be a simple flip of a switch—literally. When your thermostat is set to ON, the fan will run continuously. But when you set the thermostat to AUTO, your furnace fan should circulate warm air until the temperature in the room reaches the temperature set in the thermostat. When the room warms up to match the set temperature, the fan should turn off.  

Try switching the thermostat from ON to AUTO. Then, turn the temperature down a couple of degrees. Make sure the programmed temperature is lower than the current air temperature. If the fan turns off within a few minutes, your problem is solved. 

2. Your Thermostat Needs New Batteries

Another reason your furnace fan keeps running could be that your thermostat needs new batteries. Good news: This is another easy fix. 

There are a few signs your thermostat may need new batteries. First, if you can’t remember the last time you replaced them, fresh batteries are probably overdue. If the display screen on the thermostat is blank or dim, that’s another sign you may need to replace the batteries. And if the thermostat doesn’t seem to be working consistently—or if it takes a long time for the furnace to kick on or off—you may need new batteries. 

Try replacing the batteries if your furnace fan keeps running when the thermostat is off. It’s an easy fix you can do yourself in just a few minutes.

3. Your Furnace Fan Limit Switch Is Set to Manual

Your furnace has a fan limit switch that signals when the blower should turn on or off. When the thermostat is on and the furnace needs to heat your house, the switch turns on. When the thermostat turns off—and no heat is necessary—the switch should turn off. However, if the switch is set to manual, the blower won’t turn off. 

Check to see if your furnace’s fan limit switch is set to manual. Locate the fan limit switch inside the furnace panel cover and check to see whether the white button on the switch is pressed in or not. If the button is pressed, that means the switch is set to manual and needs to be reset. If the button isn’t pressed, something else is going on with your furnace fan.

4. Your Furnace Fan Limit Switch Is Broken 

If you find that your fan limit switch is set to run automatically—not manually—and you've set your thermostat to AUTO and replaced the batteries, you may be dealing with a broken fan limit switch. If that’s the case, it’s time to call in a local furnace repair company. The pros will be able to tell you if your switch is faulty and replace it if that’s the problem. 

5. Your Thermostat Wiring Is Faulty 

If replacing your thermostat’s batteries, setting it to AUTO, and ensuring your furnace’s fan limit switch is set to work automatically don’t solve your problem, it could be faulty wiring in your thermostat that’s keeping your furnace fan from shutting off. 

You can pop off your thermostat’s cover to see if there are any obvious frayed wires, but fixing this issue will require a professional’s help. Even if you can’t see any frayed or disconnected wires, a pro can safely test the wires and fix any repairs. 

When to DIY vs. When to Call in the Pros

When your furnace fan keeps running with the thermostat off, it’s important to know when professional help is necessary to diagnose and fix a furnace problem. Checking your thermostat settings, replacing the batteries, and making sure your furnace fan limit switch is set to manual are all things you can safely do yourself. If you have questions, though, you can always consult a professional.

But if those three things are in order and you’re still having trouble with your furnace fan, it’s time to get professional help. The pros can determine whether it’s a broken limit switch or faulty wiring that’s causing you problems, and they can safely fix the issue. Attempting to fix broken wires can be dangerous, so it’s important to leave these issues to the pros.