pressure switch

Old 12-18-11, 07:57 PM
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pressure switch

furnace kept turning on and then immediately off, over and over - checked inside and watched like spauldini did and saw: led on, inducer motor working for 2-3 seconds, led off/inducer off, clicks, repeat and when disconnecting one of the wires going to the pressure switch, it would power up inducer motor, and then 3-blink the led (open pressure switch?). (i turned off power between each of these steps)

so i cleaned the vacuum tube and poked a paper clip into the two spouts where the tube connects, reconnected tube and same problem. lastly, i connected a paper clip to the two wire terminal points ON the pressure switch and this time it powered up inducer, gave 2-blink on LED (closed pressure switch?).

i then removed paper clip, turned furnace back on and it is now working normally.

also - there didn't seem to be anything inside the vacuum tube - looked pretty clean).

so i'm not quite sure what the problem was exactly, but i think that having an extra/backup pressure switch on hand might make sense. i wasn't able to check voltages since i don't have a meter on hand.

Last edited by hvactechfw; 12-18-11 at 08:17 PM. Reason: moved your post to a new thread, DO NOT HIJACK OTHERS THREADS!!!
Old 12-18-11, 09:21 PM
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When a pressure switch fails to close, only about 2% of the time is the reason a bad pressure switch. Almosty always the pressure switch is failing to close for a reason.

If the thermostat is calling for heat, the inducer motor should turn on and stay on as long as the thermostat is calling for heat. Since you haven't done anything that would affect that symptom, I would expect it to recur as a problem.

When the thermostat calls for heat, 24 VAC should be switched on to the W terminal on the furnace. Use an AC voltmeter to determine if that is happening ---- sometimes the thermostat can be shutting off the inducer motor when you aren't expecting it to do so.

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