Why Does My Furnace Blower Turn On and Off Constantly?

If you notice that your furnace blower is turning on and off when it shouldn’t, you’ll want to have that checked as soon as possible. Not only does this decrease your comfort, it can damage your furnace.

So what’s causing this? Well, it depends on your situation.

Is your blower turning on/off constantly during a heating cycle? If so, it’s most likely caused by

• Poor duct design
• Abnormally cold temperatures entering the heat exchanger
• Abnormally hot temperatures in the heat exchanger

Is your blower turning on/off when your furnace isn’t heating? If so, it’s most likely caused by a bad fan limit switch.

We’ll go over what you can do on your own to solve your problem and when you’ll need a professional.

But first, let’s take a quick look at how your furnace blower works in normal operation to determine whether you actually have a problem or not.

In normal operation, your furnace blower fan should come on 1.5 to 3 minutes after your thermostat calls for heat and should turn off several minutes after the heating cycle ends.

Your blower should run constantly until the heating cycle ends.

When your furnace isn’t actually heating, though, your furnace blower shouldn’t be running at all.

So, if your blower is turning on/off during a heating cycle or after the call for heat stops, you have a problem.

Now, let’s look at what’s causing your blower to “misbehave” in both of these situations…

...you have one of these 4 problems:

• Ductwork that is leaky or designed poorly
• A furnace that is oversized for the home
• Abnormally low temperatures in the heat exchanger
• Abnormally high temperatures in the heat exchanger

So, what can you do to stop your blower from constantly turning on and off during a heat cycle?

Unfortunately, you’ll need a professional to solve most of the problems listed above. But before you call a professional, two things you can do to try and solve the problem are:

1. Check for a dirty air filter and replace if needed. A clogged filter limits the amount of cool air that passes over the heat exchanger, causing it to overheat and shut off. At the same time, your blower turns on to help cool it down. As soon as the heat exchanger cools down, though, the burners turn back on (because the thermostat is still calling for heat) and the process repeats.

So, if your air filter looks like the one to the right, replace it immediately and see if that fixes the problem.

2. Check for a dirty air handler blower fan. Similar to a clogged filter, dirt on the blades of the air handler fan will decrease cold airflow over the heat exchangers, resulting in the same blower on/blower off pattern. So, if your air handler looks like the one below, try wiping the blades carefully with a dry towel or toothbrush.

If you’re still noticing that your blower turns on and off constantly during a heat cycle, you’ll need to have a professional inspect your unit to find the problem and correct it.

...you most likely have a bad fan limit switch.

Your furnace fan limit switch is basically a control system that determines when the furnace blower assembly should turn on and off. The system uses a “temperature probe” to detect temperature changes inside the heat exchanger (see image below) and turns your blower on and off in response.

But since temperatures inside the heat exchanger should stay relatively stable when the furnace burners aren’t on, your blower fan shouldn’t be needed at all.

So if your furnace blower is turning on and off when the furnace isn’t in a heat cycle, it usually means the fan limit switch is defective.

Unless you’re familiar with the components of your fan limit switch and how it works, we suggest letting a professional repair or replace the switch.

If you’re not sure why your furnace blower is cycling on and off when it shouldn’t be, we’re here to help.

If you live in the Minneapolis-St. Paul area, schedule your appointment with MSP today.

We’ll send a tech over immediately to take a look at your system and fix the problem in no time.