If you have a closed cycle hot water heating system (circulating hot water) then here is something that may interest you.
This weekend Dave Emmett winterized our system with special anti-freeze that is good to -20 F. We discussed whether it was best to leave the heat on or off. On is best for the system but of course it is more costly and the propane can run out. Off could cause other problems if it does get really cold. I decided to leave it on although there was no perfect answer without risks.
However, Dave also pointed out that if the system is on, our system needs about 15 psi of water pressure to function properly and not get damaged. The pressure was zero so he sold us a used pump and some apparatus to pressurize the system again if necessary.
My suggestion is to talk to someone that understands these systems so that you can do what’s best in your particular situation. Dave has been maintaining our system for years and I can recommend him as one person to work with: 303.546.0448