Atwood Furnace Won't Shut Off

[Scott & Heather]

Finished my install of an Atwood Hydroflame 8500 series furnace. Furnace turns on properly via wall mounted thermostat, but it doesn't shut off when temp is reached. Doesn't even shut off when tstat is turned to the "OFF" position. I have to manually shut the unit down with the main switch on the motor. It was working perfectly when I bench tested it. Unit would reach tstat set temp and shut the gas off, and then the fan would blow for 20-30 seconds and then it would shut off. I know it isn't a wiring short cause it doesn't turn on on it's own. I have to tell the TSTAT to turn it on, but once it's going it doesn't quit. Any suggestions? I just bought this unit used from a fellow busnut so it will bum me out if I have to spend another $100 for a control board...

[luvrbus]

If you disconnect the wires on the T stat will it stop ?

[Scott & Heather]

Nope. Tried that.


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[Darkspeed]

Any chance it has enough current to start but not enough current to stop once it is running? Measure the pre-start voltage, start voltage ,and the running voltage, then do the same with current draw? Just an idea..

[thomasinnv]

Try disconnecting the tstat wires at the furnace, if the problem persists then it is most likely the board. You could also try disconnecting the low temp limit switch and see if it shuts off. And probably the least likely would be the sail switch improperly wired to the low temp limit circuit, with the high temp limit switch connected to the sail switch circuit. (very unlikely, but I have seen this once before. Scratched my head a long time on that one ) I would put my money on the board though.