Some people seem to have bodies that crave a lot of heat.
Sometimes in the heat of the summertime you will see them wearing snorkel parkas zipped all the way up with the hood on.
I personally have questioned a couple of these people with the parkas on, They both replied similarly. They just enjoyed being that hot in this summer.
Maybe it's the same with the heat turned way up and the doors propped open or the windows open. I have not questioned anyone like this because I have not had the opportunity to.
But I have seen this. I assume it's the same reason. One man was from the South Pacific where it's very hot and he was paying the bills and kept it like that.
Will it ruin the furnace???
Maybe you could talk to the furnace manufacturer customer service and ask that question.
My thought on it is everyone is different, some people in the winter time chop a hole in the ice take off all their clothes and jump in the ice water.
Others seek to be very very warm all the time.
Why they leave the door open? Maybe they don't realize it's costing them extra money. Not everyone understands heating and cooling like a landlord would.
Have you tried verbally educating them about closing the door?
Good Luck!
https://www.youtube.com/shorts/MuaLaOTCHxw