Gas and Oil Home Heating Furnaces - fan cycles on and off for around 10 minutes

There is a weird phenomenom with my gas furnace. It seems to only happen when the temperature is too low. At night I adjust my thermostat to drop the temperature down to about 20 degrees, but when I do this, at some point of the night I can hear a fan come on in the furnace and cycle on and off for about 10 minutes. This will eventually stop and the furnace will eventually work properly and blow hot air and it even seems to be keeping the predetermined temperature fairly well. The only problem is with this fan (an exhaust fan I think) that keeps coming on. My fear is that it's going to break down by cycling like this.

Also, one other thing I noticed, when I took off the front cover to look around, I noticed that the ignitor wire is right in front of one of the gas pipes where the flame comes out. That wire is being subject to some pretty hot temps. Is that normal? I figured it should be out of the way ingniting a pilot rather than the actual burner. And one las thing, one of the pipes/burners seems to have some metal deflecting the flame from shooting completley inside the interior pipe. Some of the flame is instead being directed upwards onto a metal (heat shield?).

My first question, is the most pressing, but I thought I'd provide more info, as these may be related.
David

You didn't mention type or age of furnace..However I will assume you have a furnace with a "fan switch" not timed style..The furnace blower is pulling the cold return air thru the furnace across the fan switch making it think the air has cooled down & it is time to shut off fan...After home heats the return air temp is up enough it doesn't shut off fan until t'stat opens.

Thanks mbk3 for your help, but what would you suggest I do to remedy the situation? :confused: Or is it not really necessary to do anything? It seems to be keeping the house warm enough, I'm just worried about straining the furnance.

It's an:

Heil Ultimate II
Approx 10 years old.

I just did a search on this model, and found this: http://www.hvacmechanic.com/forums/resservice/messages/10875.htm Looks like the same problem as mine.

Thanks mbk3 for your help, but what would you suggest I do to remedy the situation? :confused: Or is it not really necessary to do anything? It seems to be keeping the house warm enough, I'm just worried about straining the furnance.

It's an:

Heil Ultimate II
Approx 10 years old.

I just did a search on this model, and found this: http://www.hvacmechanic.com/forums/resservice/messages/10875.htm Looks like the same problem as mine.

I'm not familar with that furnace..Where is your return air grille located? Next to front door, tile floor or some other cold area..Well you could always install a heat assisted fan switch..They work well in cases like this..Or keep house warmer

So we are talking about the draft induction fan.

Thanks guys, I think I'm getting somewhere. :thinker: I actually think I have the timed style not the other one. If I'm looking at the part that you're talking about it's model number: st9120g 4012 as seen here: http://keithspecialty.com/k/67-392.htm

I'm guessing for this type, I should change the dip switches to produce the same result. But how should I change them? :eek:

I'm guessing for this type, I should change the dip switches to produce the same result. But how should I change them?

To what are they set now?

Turn switches 2 & 3 on for a 3 minute (longest) cool down.




I don't see how this explains the symptoms you described.

Wait a minute... I'm not sure we're on the right track here... I thought the fan that was cycling on and off was NOT the blower fan, but was a fan within the furnace (the induction fan I would guess). If I'm correct, it has nothing to do with the fan control or return air temperature. I think the thermostat is cycling the unit on and off in short intervals after the setting is changed. You might try replacing it.

Doug M.

Wait a minute... I'm not sure we're on the right track here... I thought the fan that was cycling on and off was NOT the blower fan, but was a fan within the furnace (the induction fan I would guess). If I'm correct, it has nothing to do with the fan control or return air temperature. I think the thermostat is cycling the unit on and off in short intervals after the setting is changed. You might try replacing it.

Doug M.

I like your "wait a minute" Your right I may have miss-read..Oh we'll see. I did a lot of typing to be wrong..lol

(I see, wrong fan. :o )

Is the thermostat in a drafty area or on an outside wall?

Is this a mechanical thermostat that you can listen for a click?

I made an adjustment but haven't had a chance to see if it's working yet. Thankfully my 3 month old has woken me in the middle of the night the last couple nights. I'll post again if it still doesn't work.

:)
Cheers,
David

Thankfully my 3 month old has woken me in the middle of the night the last couple nights Thankfully? :confused: I still vividly remember my first full night's sleep after our now 7 y/o started sleeping all the way through. I'm fairly sure the furnace could have blown up and I wouldn't have noticed, although the night she rolled over onto her interactive Barney, whose speaker was apparently right next to the baby monitor, my wife and I both had to crawl down off the ceiling fan. Talk about bad dreams... Anyhow... :D

Most digital thermostats have settings to decrease the sensitivity and/or to limit the number of cycles per hour. If yours is digital and working correctly to begin with, either of these settings might help to resolve your problem.

Doug M.

Thankfully? :confused: Sorry, typo... that should have been "has NOT waken me up".

...anyways last night he did wake me up -Major fever. Anyhow it looks as if the setting that I changed worked!

Thanks guys!! :)

Okay looks like I spoke to soon. Definetly heard it last night cycling on and off, on and off. It's really quite nerve racking!! :wall:

I wish I could post of picture of what I'm looking at.

David

About the best way to do pics on this site is to host them on Photobucket, Yahoo, or a similar site & post a link here.

Dougm, Yep, I can relate to that. Kind of reminds you of the cartoon cats with their claws in the ceiling from fright.