What is the lowest temperature to leave a thermostat so that the pipes don’t freeze? by Randy Crawford

The best answer is a formula, not a single number.

The best answer is a formula, not a single number. Find the coldest pipes in your house (on an outside wall, in the basement, next to an inner cold space like an unheated chase, etc).  Measure the temperature there (use an IR heat gun or leave a thermometer there until the reading stabilizes).  Then check the setting on your furnace thermostat.  Subract the difference between those two numbers. That’s your delta (D).   In theory, you’ll want to set the thermostat to be that much higher than freezing (freezing + D).

Of course that’s the minimum temperature.  If your thermostat/furnace is slow to respond to a fast temp drop, or the house’s heat is slow to reach your coldest pipes, the pipes may fall below freezing for a while.  Frankly that probably won’t be long enough for them to freeze, but I’d add a little extra margin for error (D + a small sigma), just to be safe.  It’s also possible that if your temps drop well below normal, the coldest spot in your house may differ from your thermostat more than you expect (differ by more than D).  If there’s a chance of that, make your sigma a bit bigger.

Alternatively, you might want to turn off the water to the house and drain the lines.  This is quick and easy to do.  Then you need to worry only about the temperature of your water cutoff valve and meter.  If you wanted to wrap these in heated tape monitored by its own thermostat, you could probably shut off all heat to the house too.

Personally, I set my house’s thermostat to 55 when I’m away.  Setting it lower just isn’t worth the risk, IMHO.  AND I turn off the water.

One more thing.  IIRC, some older thermostats actually turn off when you set them to their lowest temperature (like 50F).  Don’t accidentally turn off your furnace that way.

What is the lowest temperature to leave a thermostat so that the pipes don’t freeze?