For emergency use only, with a window cracked, I use;
If Nat Gas is still flowing I use a battery array and inverter to fire up the furnace.
Sine Wave inverter? (That's my major plan. I've only used the setup to run the refigerator and lights in the summer. I assume my furnace will eat the power output too.)
Diesel cars (with a large tactical Diesel fuel reserve) provide battery recharge/load coverage.
I use the set up in post 15. And a co detector. I don’t crack a window as the house is old and though it doesn’t feel drafty there is some infiltration. The detector has gone off twice in about 200 hours of backup use.
When buying a propane heater for indoors you prefer to get a model with the pressure reducer outdoors.
Many small emergency type heaters have the presure reducer on the heater so you have full pressure indoors which is a significant added risk.