fan short cycles but it isn't the switch

I have a problem with the fan short cycling when the house is cool. The house may be around 65 and the furnace kicks on and then the fan kicks on and off in maybe 30 second cycles for maybe 10 minutes and then finally runs till the house is warm. It does this a little when the house is warm.
Details: Interthem gas forced air heat only in a doublewide manufactured house.
This worked fine in its early years and gradually developed this problem and gradually getting worse.
Had a licensed furnace guy service it and said everything was fine but maybe the fan switch which he replaced (with a t-o-d60t82 f110-20f). Couldn't test it then because it was a warm day and it didn't exhibit the problem.
Called him back and he said to check from the roof for anything blocking air in or out (a two layer flue pipe). Nothing there.
Flame stays steady and seems to be fine.
I just want to get some input to minimize troubleshooting time with the furnace guy.
Just for laughs I should switch to electric heaters. With electric at 7cent/kwhr and propane at 2.15 I'd be better off with electric!
Thanks, Tom
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Well don't let the house drop to 65 degrees. If the fan moves cold air across the furnace too fast it causes the fan to cycle. Is there a fan off setting you can lower? Or a fan on setting you can raise? Did somebody change the blower motor recently? Have the speeds been changed? Whats changed since last year, or has this always been a problem?
-Canadian Heat
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The fan switch on this furnace is a klixon type switch
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I'd want to watch the dial in the fan limit switch. If the fan is blowing out all the heat, could be the heat exchanger is cooling and reheating. Or at leat the fan limit switch is cooling and reheating.
Could also be a motor problem. But, with a regular on and off like this, I doubt that to be the case.
Don't know from here if you have a three speed fan, might use a lower speed. I've also heard of a device called a camstat, never used one. It delays the fan turn on, and then runs the fan continuous until the end of the call for heat. Or so I remember.
Please let us know how things turn out.
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Christopher A. Young
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There are no separate fan on/off settings to adjust. No new motor or speed change. The fan is one speed. I've always let the house get cool at night both as a preference and to save gas. When the furnace was young this wasn't a problem and it very slowly started this problem and got worse. Now it does it some even when the house is warm.
A handy friend wondered if carbon had built up somewhere. I saw where the whole panel where the valve/burner goes in could come off so I looked in there and the burn chamber looks clean to me, except for a very thin coating of a white chalky substance. Very thin like a too thin paint job with watery paint. I assume this is just burn residue and if it's white it must be burning itself off as it goes along?
Thanks, Tom
Stormin Mormon wrote:

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Well, you got me puzzled.
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On Tue, 07 Nov 2006 15:35:09 GMT, "Stormin Mormon"
Oh my! There's a first! Bubba
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Okay, lemme get this straight. Your furnace fan has been short cycling for a while. You get a "tech" out to look at it. He cant figure it out. He guesses at the fan switch. He replaces it but tells you he cant test it because its too warm out? You pay him. You call him back and his remedy is to tell you to crawl up on the roof and check the flue for obstruction? Wow! I love it. I guess Ive been running my business all wrong. Bubba
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