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I have a ComfortMaker combination furnace/AC for the back part of my house. The furnace kicks in, but never heats up. No errors are reported. Started troubleshooting and took a peek at the pressure switch.

When disconnected, it's properly open (OL). When it's running and the inducer motor has kicked in, the circuit closes, but it reads all over the place, constantly resampling, usually well over 1kΩ (and often as high as 20kΩ).

I've not been able to determine what a good read would be, though I'd expect it to be significantly lower than this (somewhat close to 0?). I understand that the switch "not working" usually means it's doing it's job rather than it being faulty, but with that sort of constantly changing reading, and being so high, is it safe to assume that the switch is probably shot?

I don't have a manometer, and I suppose my inducer could just be incredibly faulty or somehow the negative pressure from the airflow is... I guess wavering? Occam's points at the switch, but HVAC isn't my area of expertise.

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Measuring resistance on an element in a live circuit can produce weird results.

Disconnect one of the wires from the switch (or both if they're molded into a single connector), then measure the switch again while the inducer blower is running. The furnace controls won't detect the draft and will probably throw an error, but that's fine.

Switches can fail to a relatively high resistance state, but I'd guess it's unlikely.

  • Thanks for the feedback. I had been measuring it with the connector off the board. Looks like the igniter is dying. Was able to get it up and running - should last through the rest of the cold (I mean, it's May, it'll warm up soon, right?). – Jesse Williams May 1 at 14:21

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