I currently only have wood heat in my home, and would like a backup to use when I go away. I don't need the house to be that warm - just 12C or so. I was told that heat pumps should be set to at least 18C or else their defrost cycle would not work. Could someone please confirm this is true, and explain how this works?
Thanks!

Some heat pumps steal heat from the room in order to operate the defrost cycle, but it's not that much and you aren't there to feel it anyway. The final authority would be the operating manual for the system you are addressing.