I have a 550 square foot cabin in Michigan. Right now I am using a mr heater 30,000 btu natural gas heater. It heats the place great on setting 1. The problem is that it will shut down when the oxygen level drops to around 18%. I want to heat the place all winter to save the drywall from cracking. I just gutted and redid the place. I was thinking about cracking a window or putting in a vent so i can get enough fresh are in there to keep it running, but that isn’t a good long term efficient plan. I’m thinking a mini split may be best option. The only downside of a mini split is that they don’t work well when it gets extremely cold. The cabin has spray insulation in walls and ceiling. Looking for other options.