Wolffarmer wrote:The batteries in my home thermostat died last night ...
Idea #1, failsafe thermostat in parallel.
I've thought about this problem with the new digital thermostats---batteries are required to make them work. I've decided that if I ever get one, I'll keep my old mechanical one wired in parallel with it. (Yes this means there will be two thermostats on the wall.)
I'll keep the old one set at 55F. Then, if the fancy new one fails while I'm away, the old one will take over as the temperature drops and protect the house until I get home and use it (the old one) to raise the temperature.
Full disclosure: My home thermostat is an old Honeywell ChronoTherm. It is two mechanical mercury thermostats in one unit and a Sony clock mechanism to switch between---maximum of six high/low temperature changes per day, and every day is the same. The battery only runs the clock. If the battery dies, it simply functions like an old-style, one temperature mercury thermostat---a built-in failsafe. These can still sometimes be found on Ebay.
Idea #2, external power supply.
I remember reading the instructions for the new digital thermostats. I remember they can be powered by an external source.
I seem to remember there being a "B" terminal inside the thermostat that accepts power from the furnace/thermostat 12v transformer. All you need is a wire to get it from the transformer to the thermostat.
Check your digital thermostat's owner's manual. Look for the external power feature and how to wire it.
Then check your thermostat connecting wiring to see if there is an unused wire. Follow the recommendations for color codes so any HVAC technician working on your system later will not be confused.
I believe one of the thermostat pair of wires is used like a ground connections. This is why you only need the one extra "B" wire to complete the external power circuit for the thermostat. The other thermostat (switched) wire controls the 110V furnace relay that runs your furnace.
Wired this way, your thermostat battery becomes a backup to the external power connection. I don't know if your thermostat programming is stored in permanent memory, or temporary battery-backed memory---meaning it goes away if the battery fails. You will learn this when you replace your battery.
When I talked to an HVAC technician about digital thermostats and any failsafe programming, we tested one and learned that as soon as the battery was installed, it preset itself to 55F. So whether from a battery failure or the restoration of external power after a power failure, it should set itself to no less than 55F.
It's been a long time since I looked into this so take everything with a grain of salt and reconfirm everything for yourself.