Back in 2009 during a ice storm when we lost power I used the My Buddy heater for 8 days in the house (its the only heat my 92 yr old neighbor and I had). I did open the upper part of my window about a inch for ventilation.
I am hoping I won't need it, that the power will stay on.

https://homeinspectioninsider.com/mr-buddy-heaters-safe-use-indoors/

yes – Buddy heaters do indeed produce carbon monoxide. However, the difference lies in how Buddy portable propane heaters are made and designed.