I've been camped out at Mom's off and on since the Polar Vortex event and noticed something going on with her Tempstar T9MPV furnace installed in 2008. The filter is clean (verified by the furnace guys) and the batteries in the thermostat are good (verified by my DMM). I change those things plus the humidifier pad religiously over the Thanksgiving weekend so I didn't think those were causing a problem. Since the extreme cold weather ended it's been acting erratically - the draft inducer starts up, I see the igniter light up, gas comes on, and so far all is good. The blower doesn't kick in when it's supposed to, and after about 4 minutes the burners shut off and the blower kicks into high mode to dissipate the heat. The system then sits for 5 minutes and tries again. It looks like the high limit switch is kicking on and, at least in that case, the system is working as it should to keep the house from burning down. Called the warranty company Mom has through the local utility. First visit the guy says that the draft inducer had water in it and that was affecting the system. Claims to have drained it out and the furnace seemed to work fine for about 8 hours then started the sequence I outlined at the start. Called the warranty company again, same guy gets sent out, now he decides it's the limit switch and replaces it. This seems to solve the problem for another day, then the sequence starts again. Call the warranty company again, this time the HVAC company sends out someone different. This time they decided that the furnace is too big for the house (which it may be) and the electronics were not set up properly for the thermostat so they fixed that. System seemed to have worked fine for a day and now it's really erratic - things like the draft inducer motor running for 45 minutes at a time, burners cycling on and off, blower cycling on and off but at the high-speed it did when it was sensing the high-heat condition and not in the manner it would be if the system was running properly. Contractor is coming out again to check on the gas pressure and some other items but I get the sense that he's going to throw in the towel after that and the warranty company will probably not do any more if he declares the thing non-repairable. I'm obviously no expert but I'm wondering if the control board is at fault here? It's weird to me that the system worked fine for 10 years and is acting up now, so while I can't argue things like the furnace is too large, I have to wonder why we didn't see this before now. Any suggestions?