Gas and Oil Home Heating Furnaces - Magic Chef gas furnace blower cycles on and off without burner

I have a magic chef gas furnace that is about 22 years old. When heat is called for it lights, then the blower comes on a minute or so later. It comes up to temp and turns off as it should. About 2 minutes after the system shuts off, the blower will turn back on for about 10 or 15 seconds without the burner. this happens a couple of times. Is there a limit switch that I can adjust? if so where do i find it?

This is an old furnace I'm supposing?

Does it have a fan/limit switch with a round dial that turns as the furnace temperature increases and decreases, with three adjustable pointers around the dial?

I will take a closer look at it as soon as I get home from work. Thanks

yes , there is a honeywell switch on the heat exchanger that has a round dial and three set points

The pointer on the highest temperature control the limit switch, when turns the burner off if the furnace overheats.


The pointer in the middle sets the temperature at which the fan turns on. The pointer at the lowest temperature is the fan off temperature.

If you watch this dial turn as the furnace heats up and cools down, you are probably going to see the fan turn on as the temperature get to the fan on temperature. After the burners shut off, the fan will continue to run until the temperature declines to the fan off setting.

Then you are probably going to see the temperature increase again to the fan on temperature because there is still residual heat in the heat exchanger.

Make a not of the temperature settings for the fan off and fan on settings and report those settings here. It may be that the fan off temperature needs to be reduced or the fan on temperature increased. Which one needs adjustment depends on the current settings of both.

the fan off is set to 100 degrees and the fan on (middle pointer) is set to 130. I let the furnace go through a full cycle and watched the dial, after the burner cut off the blower continued to run until the the unit cooled enough to shut down. I watched the dial climb four times to the fan on point and it ran about 15 seconds each time. should I raise the middle pointer up some?

You could set the first one down lower than 100 (say 90), to recover more heat. And also, by leaving the 2nd one at 130, rather than raising it, you are not wasting as much heat up the chimney before the blower turns on.

Personally, I'd raise the fan on temperature to 140 and try that.

Reducing the fan off temperature to 90 runs a risk of creating cool drafts in the house as the furnaces circulates cooler air.

That's my bias, anyway.




Seattle Pioneer

I will try increasing the fan on temp. Is it possible that the limit switch is actually bad, causing the sensor to increase to the setting of 130 when it is not actually that warm in the heat exchanger? The reason I ask is this was not happening until this year and no settings have been adjusted. Thanks for all your advice.

Very possible it has lost some of it's calibration. I also had a hunch that if I picked down, SP would pick up. :D

You could check what the temp is down in a nearby heat register to see at what temp it is when it quits blowing. Also you could also see what the temp actually is when it first starts to blow.

I do know that if it is in the 80's, it can make you feel drafty. Then it is a folly to think you are recovering all the available heat, while you sit there at the end with your sweater on.

Do as the instructions say and remember to hold the round dial firmly when moving the pointer(s)!

I moved the fan off pointer to 140ish, seems to be OK now. Parts are wearing out, might be time to just bite the bullet and get some quotes to replace it before something big lets go ( it was installed in 1987 ). Thank you for all of your advice! very helpful!