I have an older Rheem gas furnace, converted to propane. Horizontal with forced induction air. Ser# CM 1 D302 F5092 1384. I installed it in 2000 but it has never worked right. It goes thru cycle perfectly but shuts down about 60 seconds after house air fan comes on. Then it repeats. Polarity is right and ground is good all the way to connector on controller board. Tried disconnecting AC signal. Tried moving ignitor a bit incase its overheating. Tried defeating the overheat sensor. Disconnected exhaust vent to eliminate possibility of obstruction. Troubleshooting chart says it's ground or ignitor overheat of the controller board. Time to buy a board?
Could be. But you should try changing the lower limit of the bi-metal thermostat switch located in the plenum for the induced (house) air. It could be that the plenum gets cooled off by the fan (too much air). The propane has lower BTU values than the gas the furnace was originally designed for. Because of this, the induced air plenum will warm,, the fan will turn on, but then the plenum goes below the low temp because the gas cannot maintain temp because of lower BTU values (or for some other reason), and the furnace "thinks" that it has done its job heating the house for a cycle. After shutting off, it is then rudely notified by a complaining wall thermostat that some more work must be done pronto. That is why it cycles. Good luck.
This could be the problem:
http://gasfurnacepressureswitch.blogspot.com/
If it shuts down shortly after the room air fan starts, have you thought about a cracked heat exchanger? If the flame pattern changes as soon as the fan comes on there is a very good chance you could have a crack or contamination. Or it could be a bad board but I don't think so.
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