sometimes furnace turns on and off when starting

[jaristotle]

For example, if the thermostat is set at 68 and room temp is 67, the furnace will turn on for about 2 seconds and the thermosat will show that the room temp is now at 68. Furnace will then turn off for about 2 seconds and the thermostat will show that the room temp is at 67 again. This on and off cycling will continue indefinitely, along with the cycling of the thermostat reading.
It doesn't happen every time the furnace turns on, and when it does happen, I'm not sure exactly how long it lasts. I've never hung around to find out...I usually notice it as I'm walking out the door in the morning. Is this some type of relay problem maybe?

[mrcaptainbob]

Well, that certainly can't be good for the furnace! Several things to consider....if it's the dial type thermostat, there may be dirt/dust inside. Pop the cover and blow out any that may be hanging around in there. If it's the digital type, try new batteries. Could also be a rush of the warm air affecting that 'stat'. Although I doubt it, but it is possible. I guess it it's located above register, or on the same wall as a heat duct....But, again, I doubt that it would affect it that quick. You could try getting a cheap t'stat and temporarily installing it just to see if that solves the problem. If it does, you can spring for a different one if you;'d like. Can't imagine it would be a furnace problem if the 'stat is changing that fast.

[Hube]

Check to make sure there are no cold 'drafts' behind (in the wall) the T stat.(Stuff some insulation in the wall cavity) Also, it should be located as far as possible from any influence from a supply /return outlet.. Your final remedy would be to simply replace the therm with a new one.

[jaristotle]

I'm not sure what you mean by "above register" Bob, but I don't think it's on the same wall as a heat duct. However, the digital thermostat is not completely secured to the wall (there is an open crack), so maybe that's affecting it? I will try new batteries and also try to fix it to the wall better.

But aren't most systems designed to stay on for a minimum amount of time though? In order to protect them from damage from this type of cycling on and off? Thank you.

[Dean]

First of all, when you say "the furnace" are you talking about the main burner, the blower, or both?

The burner will only stay on as long as it's getting a signal to do so from the thermostat so someone would have to be there checking for the cause at the time it was doing the short cycling to see if it is getting the signal or not.
A cold draft coming through a hole in the wall behind the thermostat would most likely cause the heat to be on when it should be off, not just cause it to go on and right back off again.
A register blowing on the thermostat might cause it to short cycle somewhat but probably wouldn't have the symptoms you describe because of the timing, the blower most likely hasn't had a chance to blow any warm air onto the thermostat that soon.
Return air near the thermostat is a good thing since it is sensing room air.

Is this a fairly new furnace that might have a flame sensor?
Sometimes A flame sensor will do that and removing it and cleaning it with steel wool or very fine sandpaper will fix it.

Most any reply would be just a guess when you can't be there with testing equipment to catch it in the act but I have seen thermostats be the cause of what you describe.

If you have any literature on the thermostat, you might check to see if there is a way to adjust the heat anticipator for longer cycles.

[jaristotle]

The main burner I believe, since the blower doesn't have a chance to turn on when it short cycles like this, since it operates on a timer I believe.
The furnace is about 12-13 years old, so I guess it doesn't have a flame sensor.
There is no register or return near the thermostat.
I'll look into that heat anticipator setting, along with the other thermostat advice. Thanks.