We just noticed this problem today, on the coldest day it has been this winter. Don't know if the cold has anything to do with this issue or not, but I don't think so after reading through the forum. The problem appears to be that our furnace (about 15-20 years old, Magic Chef/Comfortmaker Ultra II 97 gas fired/high efficiency model#eg6e100dc20-3) is turning on normally as it always has, but after 4-5 minutes, the burners go out with no attempt to relight, and the blower continues to blow cool air through the vents. The heat, of course, never reaches the setting on the thermostat so it goes on continuously unless turned off manually. After sitting 10-15 minutes and turning back on same result. We have cleaned the flame sensor, and double checked all the conections thinking that was the trick after reading the forums, but that hasn't seemed to change anything. The filter was new last month, and not the cheap fiberglass but the better paper kind. Last year, the ignition control module was replaced, and the burners were cleaned by a tech. I'm no expert so I can really use some help before taking the hit to the wallet again.
Thanks

After running for 5 minutes the opnly thing I can think of is a out of calibration high limit control. All of your registers open?

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Originally Posted by hvac1000

After running for 5 minutes the opnly thing I can think of is a out of calibration high limit control. All of your registers open?

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Hmm, well no, they're not as there are a couple rooms we don't keep heated. It's a good-sized home with a 20-foot ceiling in the living room so we try to cut heating anywhere possible. Is it actually not good to close some of the registers in rooms we don't use much?
Interestingly, a new development: This morning we decided we'd try again to clean the flame sensor, since we read on another forum that sometimes a good scraping is needed and that you can't necessarily see the deposits that form on it, and lo and behold the thing has stayed running for a good 25 minutes before the flame went out, briefly, then came back on and has been on for a bit now. I'm continually monitoring it to see what it does. Oh, we had also left the furnace off all night so I don't know if that had anything to do with this change. Is this calibration high limit control something that we should be worried about still, and can be easily remedied? This is not a term I'm familiar with. Thanks so much.

P.S. After another 20 minutes or so the flame did go off and didn't come back on. Now it has reached the minimum theromostat temp, though, and shut the blower off normally as it should. We'll see if it comes back on with heat or not...

From the sound of it I would just replace the flame sensor. They are not that expensive. Sometimes if to many registers are closed it can cause heat to build up in the furnace and activate the high limit.

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Originally Posted by hvac1000

From the sound of it I would just replace the flame sensor. They are not that expensive. Sometimes if to many registers are closed it can cause heat to build up in the furnace and activate the high limit.

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OK, so I just installed a new sensor and opened all the vents - ran the furnace, same result as before. So, I'm thinking this might be some other kind of electrical problem, such as a bad relay or something like that? I'm starting to think that I may be beaten on this one and will have to call someone in...

Admitted defeat and called in a pro. Turned out it was just a clog in the condensation drain line, who'd have guessed? Just blew it out with compressed air and it's good to go again. Well, I'm $90 poorer but a whole lot warmer, thanks for the help.

Not only that but a bit smarter. Next time you will know. Usually when the drain is clogged the unit will not really run for 5 minutes. They usually turn off then on. Interesting.