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sidb 10-28-2012 03:47 AM

Warm air Furnace
With new gas valve and thermocouple the furnace maintains pilot and when thermostat calls for heat furnace goes to main flame and keeps going for hours but when it has reached temperature main flam closes down and extinguishes pilot as well. Any ideas what is wrong?

beenthere 10-28-2012 05:37 AM

Moved to HVAC forum.

Marty S. 10-28-2012 06:14 AM

Could be a crack in the heat exchanger opening up. Anytime a problem could be a heat exchanger breach it's time to put the tools down and call a pro.

biggles 10-28-2012 06:45 AM

sitting in front of the furnace looking into the main burner section restart it ...with the main burner lighting up then note the supply fan coming on.if you see the slightest wave in the burner flames the heat exchangers is cracked...the pressure of that fan is leaking into the burner section and in turn the fle gases leak ino the supply air

ben's plumbing 10-28-2012 07:52 AM

after checking what the others said....if valve is new was manifold pressure set right. could be comming on hard and going off hard blowing pilot out.. is pilot strong enough...ben sr

how 10-28-2012 12:47 PM

Have someone turn the furnace on for 10 minutes and then off while you are watching the pilot flame through the entire heating cycle..

As others have said..does the pilot flame differ between when the blower is working and not.

Does the pilot flame lift off the thermocouple from the exchanger heat drawing air with a natural draft furnace when the main burners turn off.
(or at anytime through the heating cycle.)

Is there a small explosive popping when the main gas goes off that is extingushing the pilot.

Is the thermocouple presently fully enveloped by the pilot flame.

Send a photo of the new gas valve directed at the side that the pilot thermocouple attaches to. Some have eco connections that allow the metering of the thermocouple output while its still attached to the gas valve. This would tell you what the real pilot output is and whether the new pilot solinoid is actually strong enough.

Report back what you see as they all have different solutions.

sidb 10-29-2012 08:41 AM

Problem has changed, now main flame won't ignite, pilot still going but blew 500mlamp fuse, replaced but still won't go.

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