I came home from work and my girl railroaded me about the heat not working. Of course it's the first cold day in Saint Paul. Our thermostat is pretty old and missing its faceplate so I thought replacing it would be a decent first step. I bought a Honeywell programmable one for twenty five bucks plus a mail-in rebate. I installed it, tested it, no heat. I got out my volt meter and it registered 25vac from the red wire to any of the other 3. I shorted the red and white, waited a few minutes and fired up the green wire. Heat. Since it was kind of chilly in the house I left it that way and ran a 20 minute errand. Got home, no heat! I took apart the circuit and rechecked for voltage, it read 10 volts. I think something's rotton in Denmark. I went down and shut off the disconnect, 'course the meter flashed to zero, powered it back on, no it's at 25vac. I'm going to just slap the stat in now 'cause I'm missin' the Mets playoff game. I'll check back. Thanks

Could be the transformer that supplies the 25 volts is going bad. Quite often near the end of their life, they will work cold, but not after being under load for a while.

I swapped out the xformer this morning hoping to warm up the house, what a drag, same result. The furnace kicked on and warmed up, then the fan engaged, finally the whole thing killed like someone pulled the plug on it.

Try seeing if it will stay on in the fan on position with the heat off. If it only runs a short time and quits, it means the problem is in the thermostat circuit or blower, not the furnace itself. If it keeps running, try turning on the heat.

Yeah, good call. I can only get the fan to run intermittently, it should be on %100 of the time if the thermostat is calling for "fan on". Do you think I have to replace it? Also, on a different but similar subject, the filter my girl has in there is made of some sort of metal webbing that she cleans out every so often. Are these superior to the white disposable ones I'm used to. Thanks for your help Labman, we all appreciate it, even our two black labs, "Shanda" & "Dixie".