Unusual Gas furnace problem

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Old 09-26-08, 12:53 PM
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Unusual Gas furnace problem

Hi all,
I'm new to this forum and trying it as a last resort before having to replace the entire unit. Any help/ suggestions is greatly appreciated.
Background:
-In fall 2006 the unit (Cumberland downflow / horizontal gas furnace. Now about 13 years old) started having a problem. It would sometimes work and not work, it would try to start 3 times before failing to work. By moving the on/off switch on the thermostat few times it was able to start and work properly for a while. (this was thru the winter season).
-Fall 2007 in december it stopped working. HVAC technician replaced the control board. Unit worked good for about 6 weeks then stopped again. Technician replaced the (electric fire start element). The unit worked for a while and stopped again. Technician came again, troubleshooted, checked connections and said to replace the whole unit.
-Fall 2008 the unit tries to start 3 times, it comes on and blows cold air thru the vents (gas is not burning). We are unable to turn it off with the on/off switch on the thermostat. We have to reset the fuse. Please let me know if you need any model #s or any other details. Thanks.
Daniel.
 
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Old 09-26-08, 05:41 PM
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You almost have to catch this in the act, wait it out, or keep throwing parts at it. There are a few intelligent guesses one can make.

You can't test things when they work, unfortunately. One has to be there to at least catch what phase is not energizing. Like: Does the draft inducing motor come on? I am guessing though it does, as I do not believe even say a fan timer would come on til that part of the sequence was acknowledged.

All the stuff the guy replaced may never have been the problem, even though you say it worked for a while after that. Maybe it would have anyway! Then again, maybe they WERE the problem and you just are really unlucky with so many things going wrong with this furnace.

All furture testing will have to be done with nothing taken for granted, as even any of thsoe new parts could be going bad again. For example, ignitors can go south in only one year. And if they are the glow type, they can get to where one day they work good enough to light the gas and on another day it isn't quite good enough. You'd have to ohms test it. Also, soem HSI ignitors double as flame sensors, so the ohms readign is important.

We also do not know if the tech put in identical one or adapted some universal one, and the one he mounted is not at the right position, exactly.

Does your furnace have a seperate flame sensor? That is most suspect on many furnaces. If yours has the probe style one, you will know by seeing if you have ONE thin (often white) wire leading in toward the combustion chamber, and see where the wire ends, where you can see where a small metal plate is screwed to the front entry to the burner compartment with one screw. These can be unscrewed, pulled out, and that probe lightly polished with a scotchbrite pad or real fine sand paper. This is THE first thing I would check if I were you.

Of all the furnace jobs, this (flame sensor) is one of your best shots at a furnace DIY job. Another is if you have a clogging up condensate drain line. If that does not flow freely, that can also cause the furnace to shut down. You can disconnect the tubing and blow through it the both directions. Blow the bulk toward the drain direction first.
 
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Old 09-30-08, 09:14 PM
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Excellent advice

We followed the white wire for the flame sensor probe and polished it. Turn the heat on and THE FURNACE WORKS!! Thanks for your advice, really appreciate it.
Daniel
 
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Old 10-01-08, 05:04 PM
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That's good news. Now you have something to tell your friends at work and how they can easily fix theirs, as this is a common problem. One of THE most common problems.
 
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