Fireplace does not work when propane heater is on.

[LanerC]

Could these be return air passage for the basement areas?

[Redapple63]

Could these be return air passage for the basement areas?

I doubt it is an air return. Most likely it is a passive vent to the bathroom. Bathroom register looks like 4”, hole in floor looks like 1 1/2 or 2”. I would get a smaller register (for a 4” rough opening but 2” vent) then tape the flex hose listed in the previous post to both ends. That gives him the best chance of getting good passive heat. Only other option is to close it off completely which I think would be worse.