Furnace RUNS by itself, without interaction from wall thermostat

Old 09-18-09, 05:00 PM
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Furnace RUNS by itself, without interaction from wall thermostat

My “Day & Night” gas furnace, model 80cu12B,
series 152, “runs” by itself, without interaction from the wall thermostat. I tested the thermostat, and its OK. The furnace is the problem.
Any ideas?
Old 09-18-09, 05:29 PM
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I have lots of ideas on numerous subjects. Why don't you tell us what you mean by "runs by itself" and also describe how you "tested" the thermostat. If you mean it is firing anytime the power is applied it may be in the thermostat wiring.
Old 09-18-09, 06:53 PM
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We need to know if the fan just runs?, or the heat along with it?

If it is heating without the stat, and you are sure the stat is disconnected (you can remove R wire on furnace terminal board - that wire feeds 24 volts ac current to the stat - to find out, or turn off furnace and ohm test R to W terminals and it should be dead. If continuity, this means thermsotat is still energized or thermostat wires are shorted), then you must be getting power through the W terminal connection anyway.

And if just the fan runs, since you are in CA?, maybe it is hot in the house and your fan switch setting is not right. Or you have a short somewhere between R(24 volt supply to stat) and G(to fan, from stat) terminals or wires.

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