I have a propane hot air system with two zones. I have an issue that happens on rare occasion. Every now and then the house will heat up to the set temperature and will just continue to run. Twice it actually continued to run until the propane tank ran out. It's a vacation home. Last year I showed up one Friday night and the house was about 95 and the heat was still running. I then changed one thermostat thinking that might have been the issue. I just discovered it has happened again over the last few weeks. I changed the other thermostat but since the issue does not seem to be reproducible I don't know if it's fixed.

Thanks,
Russ

I have a propane hot air system with two zones. I have an issue that happens on rare occasion. Every now and then the house will heat up to the set temperature and will just continue to run. Twice it actually continued to run until the propane tank ran out. It's a vacation home. Last year I showed up one Friday night and the house was about 95 and the heat was still running. I then changed one thermostat thinking that might have been the issue. I just discovered it has happened again over the last few weeks. I changed the other thermostat but since the issue does not seem to be reproducible I don't know if it's fixed.

Thanks,
Russ

The cheapest thing to do is blow all the waterlines and treat the drains with the pink stuff. Then you will have no heat bill. That is what I do on both my other homes.

Now on to your problem.

Either you have an intermittent short in the thermostat/zone wiring OR a relay in the zone panel that is sticking once in a while or a relay or other control is sticking in the furnace. This might be a very difficult one to find so we need to break down the zone controls from the controls in the furnace itself.

My first suggestion is to not shut down the furnace the next time this happens but to leave it run and then disconnect the R-RH=RED and the W=White wire from the furnace terminal strip. Do not let these wires touch anything and be careful when you remove them so as not to hit the other wires in the furnace. This will effectively remove all other heating controls (zone controls/thermostats etc) from operating the furnace. If the furnace stops running then you know a control, thermostat or zoning relay/end switch of some type was causing it to run BUT if the furnace keeps running then the problem is isolated to the furnace itself and its internal parts. This needs to be done before you can actually start tracing the problem since there are too many controlling factors and the fact that this problem is intermittent.

Usually these jobs are a real pain and you might consider a phone dialer that will call your home or a service companies phone to let you know when the next overrun event starts. So far you probably spent a fortune on Propane that is why I no longer heat my places because of the cost and the fact that the structures hold up well without heat.

When I have had these types of problems in the past I install two little snap switches on the R-RED and W-White thermostat wires. Then when the event takes place all I have to do is flip the switches off to see where the problem is located. This lessens the risk of shorting the wires etc. Good luck.

One last note. While you are checking to see if the furnace turns off or keeps running remember the fan/blower will run for awhile after the burner shuts down to blow the residual heat out of the furnace. So what you are really looking for is weather or not the burner shuts down right away not the furnace blower.

Thanks for the reply. It's tough for me to catch it during the overrun since it's a 2nd home. I'll see if I can try some of the suggestions.

Good luck.