Furnace Blower Cycling On/Off While Heating

For a few years now I’ve had problems with my furnace blower cycling on and off when it’s really cold out. Usually at night it would do it when I have my house set to 64 degrees. When I had it set above 70 it would work fine.

First I replaced the top plenum sensor being the only one I could find. That didn’t fix it. Later I thought it had something to do with my filters being too thick. So I replaced them with thinner ones. It seemed to help but when it got colder outside it started up again.

Finally this year I decided to fix it once and for all. So I opened things up and found the limit switches. There are two of them on my furnace. One is the high limit for shutting down the heat if it gets too hot. The other is to control the blower motor. First I figured that they just needed replacements. So I purchased replacements for them. It didn’t help. In fact it got worse.

This is when I started to really research this problem. The way the limit switch works is that when the flame comes on and heats up the plenum then the switch will turn on the blower. When the flame goes off the plenum cools down and the switch turns the blower off. There are different temperatures for the switches. There is a high temp that is for turning on and a low temp for turning off. My furnace had a 115/20 switch. That means it turns on the blower at 115 and turns it off 20 degrees below that. So 115 on 95 off. The new replacement switch I got was 120/20. 120 on and 100 off. And the new switch made it worse.

This is when I realized what was happening. The heat would build up to 120 then the blower would turn on. My house was 64 and that cold air would enter and cool the plenum down under the turn off temp. What I needed was a lower temp switch.

So I purchased a 100/20 and that was a problem in the other direction. The blower wouldn’t turn off now. I guess it’s too close to 100 degrees in the plenum normally. So then I found an adjustable switch. It was about $15 but it was worth it to save in fuel and electricity.

I put the adjustable switch in and set it for 110/20. That night it still had the problem so I turned it down until it worked. Finally ended up close to 105/20 on the switch and it’s been working out great!

I’m posting this because there wasn’t many answers on the internet when I was searching for this problem. Lots of people have the problem but nobody posted any solutions (other than call a professional). So if you are having the same problem then pick up an adjustable limit switch and go for it. Also, many other furnaces already have an adjustable switch. Mine did not.

Update – 2015-02-12:

My fancy adjustable fan switch stopped working. Lame. Woke up in the morning with the fan cycling over and over. Turned it up and that kinda fixed it but now it’s doing the cycling when it’s cold again. So I ordered another switch that has a 30 degree difference. So it’s a 115/30. I think the problem is the difference span is too small. We will see.