Bad pressure switch

Problems related to residential installations.

Bad pressure switch

Postby Beezfun » Fri Dec 09, 2011 9:32 am

Got home and found house cold, thermostat calling for heat. Went to circuit board and found "OK" light blinking 4 times, which translates to closed pressure switch. Tested switch with wires connected and it showed open, hmmm. So I disconnected wires and it still showed open. Disconnected rubber tube and drew slight vacuum by sucking on switch and confirmed contacts close. So it seems like at least now switch is working properly. Cycled the power on furnace to clear lockout and it came on and functioned normally.

Am I correct in diagnosing this as a bad pressure switch? I'd feel better if I could find switch in the failed condition. Is there any other possibility I'm not thinking of?
tnx
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- Bad pressure switch

Postby heatseeker » Fri Dec 09, 2011 9:34 am

You did find it in the failed position. When you pulled on the tubing it unstuck it. If it happens again I would consider replacing. You never know your inducer motor might have malfunctioned.
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- Bad pressure switch

Postby Beezfun » Fri Dec 09, 2011 10:54 am

Not sure I understand what you're saying. When there is a negative pressure in the vent, the switch closes. Normally it's open. So the logic is that the furnace won't come on if the switch is closed because it should only be in the closed state when the inducer motor is running. I found the switch in the open position, which is the position it should be in when the system hasn't started running. I did order the switch, they're only $35. Cheap insurance to swap that out.

heatseeker -
> You did find it in the failed position. When you pulled on the tubing it unstuck
> it. If it happens again I would consider replacing. You never know your inducer
> motor might have malfunctioned.
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- Bad pressure switch

Postby heatseeker » Fri Dec 09, 2011 11:50 am

okydoky
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- Bad pressure switch

Postby Beezfun » Fri Dec 09, 2011 3:14 pm

OK, I caught the switch in the failed mode, but the switch is very sensitive to motion. I carefully touched my meter leads to the base of the spade terminal that was exposed and the switch was closed. Just touching the connected wires caused the switch to open and the relay for the induction motor pulled in and the furnace started. I feel confident this is the problem and the replacement switch will solve it.
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