I would be very careful about using any appliance with a live flame in any enclosed space - carbon monoxide. We will probably get a media story about deaths from people who fired up their barbeque grill or other makeshift device in order to keep warm - this unfortunately is a very predictable occurrence.

This is one reason I stash a flameless heater in my kit, although those evidently are no entirely hazard free, either. I have used backpacking stoves (Svea or similar) briefly and gotten away with it. They will raise the temperature noticeably, but unless your space is well insulated, the heat goes away quickly.

It is much safer to just bundle up and snuggle with a warm, receptive partner than risk CO poisoning - voice of experience here. "There is no such thing as bad weather, only bad clothes."