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Old 12-01-2009, 02:47 PM   #1
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problem with Furnace staying on.


Furnace is a DMO Industries. Around 15 years old. Unit does the full cycle, but after ignition of burners, flames go out after a minute or so. changed flame sensor, same problem. Bypassed the pressure switched, works fine. Replaced pressure switch and I still have the same problem. The only thing different between the pressure switches is .05WC. One was .80WC and another is .85WC. Could that be the problem? Also, after the burners shut off, I see a fluctuantion in the voltage on the pressure switch for a split second, then goes back to normal. So it looks like the switch is loosing presure for a split second.

Thanks in advance.

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Old 12-01-2009, 04:38 PM   #2
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problem with Furnace staying on.


DMO ?? DuoMatic OLsen is it? Post a pic of it with both doors off and the burner area so we may recognize it. Where do you live?

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Old 12-01-2009, 07:16 PM   #3
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problem with Furnace staying on.


Sounds like a venting problem. When flame occurs the negative pressure created by the inducer will drop.

Is this a condensate furnace? .85"wc is a high setpoint.
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Old 12-01-2009, 09:55 PM   #4
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problem with Furnace staying on.


Here are the pics that I could take. Not really that great since I could not get too far behind the furnace. The cover is off the Limit switch because I was checking to see if it was getting too hot, which was not the case.

I assume it is DuoMatic OLsen since it says Olsen on the cover. I live in NY.

What is a condensate furnace? I check the venting only from the furnace to the chimney and it was fine. I guess my next step is to check the chimney.











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Old 12-01-2009, 10:19 PM   #5
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problem with Furnace staying on.


Put your volt meter across the pressure switch terminals and start the unit. Should have 0 volts. If it opens you get 24 volts and we know why the burner shuts off. The switch needs to be mounted vertical and not to the side like now. Remove the pressure switch hose at the inducer fan and poke a small drill bit or wire thru the port on the fan. May be partly blocked. Blow thru the hose with both ends open. Check your chimney and venting and remove the smokepipe directly where it attaches to the furnace and look for a dead bird. Sounds like the burner is firing and a quantity of exhaust is building up and the pressure switch is doing its job and shutting down the furnace. Or you are getting a downdraft down the chimney and causing spillage and the switch to trip. Check your chimney liner. Some get old and rotten/holes in them and can downdraft on certain days. If all else fails call a Pro as you may have a serious problem and we don't want any CO poisoning events in your area.

Last edited by yuri; 12-01-2009 at 10:22 PM.
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