Hi guys, I have a weird problem with my gas furnace and found this site while searching for a good HVAC troubleshooting site. I did a little bit of this work on a submarine in the Navy - but that was thirty years ago.
This has been an intermittent problem the last few weeks but its getting more frequent. The heat will operate normally for a few cycles, then at the end of a heat cycle the fan will not shut off, even with the thermostat turned off completely. After a while (an hour or more) the fan will shut off by itself. Same thing over and over. When it's in a properly working cycle the themostat controls work fine - you can turn the unit on, off, auto, etc. but when it's in a bad cycle the t/stat has no effect on the unit.
I assumed it was some type of overheat protection circuit. so far i have tried a new thermostat, bypassing the rollout limit switch, and the flow switch. I noticed when its in the "no heat" cycle the draft inducing fan runs constantly.
I'm thinking I have a bad board but thought I'd ask around.
The unit is a Tempstar / ICP Model # is PGME36G080E ...
Any input is greatly appreciated.