Furnace Problem

JASPER2008

I'm having a problem with my furnace and I hope someone will have some advice. I have a forced air furnace (LPgas) with an electronic continuous try pilot. My problem started about a month ago the furnace would not stay lit after the burners started. I replaced the pilot sensor and sparker and all seemed fine. But now the problem is: the thermastat calls for heat, the pilot lights, the burn runs for about 2-3 minutes (heats up the chamber), blower turns on and them immediately everything shuts off. It will not relight or restart unless the unit is turned off at the thermastat or at the breaker. Then it will once again go through the same cycle never actually heating anything. The blower works independantly when it is turned on. (no heat or cool just the blower).... I'm looking for some advice as to what may be causing the problem?? Any ideas or solutions???

kennzz05

will the heat stay on if the fan is left in the "on"position if you know how to diagnose a circuit with a meter check the limit circuit the one to suspect first would be the one on the back wall of the furnace behind the gas valve its got 2 wires (usually with spade clips) its low voltage you could just try putting those two wires in contact with each other(jumping thenm togeather) see if it stays on if so that limit is weak and needs replacement make sure filters are clean,although thats the first thing people do when the furnace dosent work,usually.

JASPER2008

so i tried to keep the blower running while the heat is turned on this is what happen....

with blower on i turned on heat it called and started pilot lit...gas ingnited and ran for only a few seconds and everything stoped and the blower immediately started up again....

i havent tried your other suggestion yet just wanted to get back with these results i never thought to try them at the same time

kennzz05

waiting

Slay

Your blower most likely has multiple speeds. The leads run to your control board. One to spade marked heat, one to cool. the others to spades marked park and are not used. these are 120v so turn off power at disconnect. try swapping one of the ones on park with the one on heat. this will probably be a lower speed and might not move enough air but worth a shot.

JASPER2008

thanks guys I think isolated the problem. I took kennzz05 advice and "jumped" past the pressure switch and all would work fine so I ordered a new pressure switch on line and once I install I expect all will work as it should. Thanks for all your help I never would have thought to try to issolate the problem by "jumping" past the circuit.

kennzz05

thanks for following up although i said to jump the limit circuit but it seems you took it to another level so now your on your way to being an hvac tech

veesubotee

Did you happen to look at the tubes connected to the pressure switch? Could be cracked/dry or blocked. You could be masking your problem by bypassing the switch.

V

key1cc

Good to know this is such a helpful site.








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burr

the pressure switch may be doing its job there could be some type of exhaust blockage.