Est. 1998 — 27 years of woodworking knowledge
Thermostat
John in NM
Does anyone know why a thermostat would run the furnace continuously when the batteries die? I don't think its being powered by 24V from the furnace, so I'm thinking that might be somewhere to start - then at least the batteries wouldn't fail in a year (always fall!) and burn through a third of a tank of propane before anyone notices 
Just looking for some confirmation before pulling a new wire or tearing into the current hookup to see how it was done.
Re: Thermostat
Stuart Johnson
This is a guess.
If the thermostat was calling fir heat when it died it / the unit might not know it had hit it so it would continue to run.
Re: Thermostat
Lee Schierer in McKean, PA
If you open the thermostat you can see how many wires are connected to which terminals. A battery operated thermostat normally only has two wires connected to it and it simply acts as a switch. Your problem may be that the fail safe mode for the contacts in the thermostat is to fail closed. You should be able to check this with an ohmmeter when the battery is removed.
Switch to lithium batteries and you will get more than a year from your thermostat.
Lee
Re: Thermostat
John in NM
The weird thing is that the instructions I downloaded say that the failure mode on 2 wire, dead battery should be open rather than closed. There's another unit in the back of the building that never does this, so I wonder if it's something in how the furnace is set up more so than the thermostat.
On the other hand, I think just running a 3 conductor wire would effectively fix the problem, so that I could use the third as a common and run the unit on the 24 VAC power from the furnace.
I've just never seen a failure mode like this. Since the building is frequently unoccupied, I kind of wonder if someone thought it would be better to fail closed than open - at least then you don't come by once a week to find frozen pipes rather than a half empty propane tank.
Re: Thermostat
Lee Schierer in McKean, PA
You are correct that the thermostats should be normally open and only close when the thermostat is calling for heat. That is how most thermostats work. Is it possible that this thermostat is actually closing a circuit with more than 24 volts, which could potentially weld the contacts closed? It is also possible that the relay in the furnace is bad and has permanently closed contacts. I've seen this happen where relays are undersized for the load and the high current on motor startup welds the contacts together.
I doubt your battery operated thermostat would work on 24 volts. You would probably have to replace it.
Lee
Re: Thermostat
John in NM
Its a digital model, instructions have a provision for adding a common to run the thermostat on 24VAC. These days they seem to use some kind of clipping circuit to make compact power converters that run low DC stuff from AC without that big transformer that us older guys remember.
The failing closed is sure a mystery. I think I'll have to get into the furnace for a look at that - bugs me that the other one doesn't do the same...
Re: Thermostat
Lee Schierer in McKean, PA
The Honeywell thermostat I installed recently required a common wire in order to operate. There were numerous warnings to not use it without the common wire.
Lee