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Old 03-17-2012, 01:46 AM   #1
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Furnace fan won't switch on--UNPREDICTABLE INTERMITTENT problem


Hi and thanks for the input on my January 22 8:59 p.m. post, including especially from Member HOW in Vancouver B.C.

It did it again for the first time since my original post on 1-22-2012. Thanks to How's ideas, I was more prepared to see what is going on. Just to briefly recap the prior post, our HVAC contractor replaced the pilot module twice and the gas control valve and did some other tests described in my original post.

Well, now armed with Member HOW's info, I took the service cover off. Then after a long interval with the unit off, I turned it on at the thermostat. What it's doing is the burners are cycling on and off at about 7-10 second intervals. It never stays on long enough for the fan limit control thermostat to even get close to switching the fan on. So I'm thinking that kind of rules that out. (I also don't think it has anything to even do with the blower.) Eventually, I gave up and was going back inside to turn the thermostat to off. When I got inside, the blower was running! And it's been working fine all day. I'm sure it will keep going for anywhere from a couple weeks to several months before it screws up again. It's ALWAYS on a damp early morning that it goes out.
I'D BE GRATEFUL FOR MORE INPUT ABOUT WHAT THIS COULD BE. Could it be the pilot module has gone out a third time? The first time it lasted about eleven years, and now that its been replaced twice, it still has the problem. QUERY: Is there some other part not easily accessed that could go bad? If so, WHAT? QUERY: Should I replace the pilot control (S8610U 10030) with a non-OEM one. There's one called an ICM made in USA with led lights apparently that blink out a trouble code; apparently the Honeywells have that too, now. FINALLY, does anyone have a recommendation on brand and model for if we replace this with a new super-high efficiency unit? (What we have now is a Rheem dual-pack a/c w/ lp furnace only about 16 years old, but lp and electricity were relatively cheap back then, so it's a low-efficiency unit.)
MANY THANKS for any input and many thanks to HOW for prior great ideas!

Last edited by arvonon; 03-17-2012 at 01:51 AM. Reason: spelling/mechanics. Add info.
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Old 03-17-2012, 05:48 AM   #2
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Furnace fan won't switch on--UNPREDICTABLE INTERMITTENT problem


what style stat do you have there...?brand and model.first off remove the stat OFF the subbase and do this test.your heating wires are R to W so by jumping them you are simulating the stat controlling the furnace.the purpose will prove is it the stat or the furnace causing intermitant cycling.if you do a jump and it stays consistant(need to catch it when its doing cycling problem) then it is the stat if it continues with the direct jump it's the furnace...to narrow it down...
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Old 03-18-2012, 07:38 AM   #3
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Furnace fan won't switch on--UNPREDICTABLE INTERMITTENT problem


Keying off your statement:

'the burners are cycling on and off at about 7-10 second intervals. It never stays on long enough for the fan limit control thermostat to even get close to switching the fan on'
try this easy, inexpensive task.

http://www.youtube.com/watch?v=I86joWL66aE
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Old 03-18-2012, 11:07 AM   #4
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Furnace fan won't switch on--UNPREDICTABLE INTERMITTENT problem


Biggles jumpering of the stat connections and Divorpay's sensor cleaning are easy tests to try.
If that doesn't help, a 7 - 12 second burn cycle without the fan coming on is usually an ignition sensing problem or a borderline pressure switch problem. There are lots of variables so
I think some photos of the inside of your furnace would help.
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Old 03-18-2012, 05:11 PM   #5
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Furnace fan won't switch on--UNPREDICTABLE INTERMITTENT problem


I'm not sure if you're having the same problem that I had, but I'll explain my problem and my solution anyway:

In my old house, our furnace occasionally would turn on, ignite and then shut off before even turning on the fan. This started out once a month and then became more frequent until it happened at least once every night. Especially on really cold nights. I had to go into the basement, shut the power off and turn it back on for it to work again. I researched the issue a lot, and then tried just one thing: I located the flame sensor, took a very fine grained sand paper and slightly sanded its surface. This fixed the issue, and the furnace worked flawlessly again.
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