I had an issue with batteries in my garage thermostat going downhill in cold weather, switched to the "ultimate" version which are a difference chemistry (Lithium maybe?) and solved the problem. I'm keeping my garage temp at 40 and have not had an issue where before I would have especially with the current outside temps of 0.

 

If you remove the batteries and replace the thermostat back on the wall and the display comes back on (without the batteries) then more than likely the transformer is OK. Without the schematic that is about as far as I can get you.

 

Interesting, I think we are on to something here. It's been 5 hours since I stuffed cloth in the hole in wall behind where the thermostat wires come through....and it is working fine and the right temperature. Thinking about this, I had an old fashion mercury thermostat on that system over 20 years until last winter, when I went to a fancy digital-battery-programable style. Last winter was apparently warmer and apparently never tested the batteries under the stress of a long-term-really-cold-spell. So if that's cause then my rag-in-the-wall is in for a good test. Here's the forecasted lows starting with tonight: 5, 2, 3, -8, -11, and 2.

 

Home Depot/Lowes sells spray-in foam installation in an aerosol can. Won't take much to seal that hole. Wear rubber gloves as this is so sticky it will be on your hands for a week or more even cleaning with acetone. I was surprised at the cold air coming in via the electrical outlets in another home, Little foam pads are made to fit just under the wall plate and limit this.

Down to 5°F (five degrees) or a bit less here in Dixie last night. I'm sure it felt colder in Clemson. Glad we're on natural gas and not a heat pump.

-- Chuck

 

Amazing what info one can gather from a car forum!

Hope you found the problem, Dave!

 

Old Guys Rule.

 

Just checked and its a toasty 45°, so that cold draft thru the hole behind the wall mounting plate was the issue!

Thanks for the suggestion Chuck. I've very familiar with the spray and the wall outlet pads, having used both in the house and in the garage. If fact, I sprayed under each of the corners of the garage vertical strips just in case mice might think that a road to get in the garage attic.

 

Not something you will hear at the grocery check-out down here: "Hey, it's a toasty 45° out today." Nice detective work, Dave. Way to think outside the (thermostat) box.

 

Heck, at 45º there would be people around here out tanning!