The crawlspace at its highest is maybe 5 feet and lowest 1.5 feet with somewhat dusty dirt floor.
It is not insulated.
This winter I'll be working mostly in the larger section doing no flammable work (like foam or fiberglass).
The crawlspace is sealed up with expanding foam insulation, but imperfectly. For now, the large entry doors are slotted and maybe provide 1 sq foot of fresh air as well as various other leaks.
I expect to work for 2 to 3 hours at a time. I would bring a combined co2 / smoke detector with me into the space while working.
I have a few options:
Salamander or stand up kerosene heater - Mostly rejected as I don't want another fuel source to have to deal with, as well as the safety issues.
30000 - 60000 propane salamander - I have a 20LB tank on hand and could get another one. I could pipe via the supplied flexible hose to tank outside.
No confidence issues there - blast the place to 90F, turn it off and coast while I'm working.
Tank would be at ambient though, which in January is pretty cold.
I would construct a small pad in the largest section of the crawl space with concrete blocks to place this heater.
Enough power is there for 2 ceramic 1500W heaters - less interested due to cost.
I also have a 'Buddy heater', small one. Could plumb that to same 20LB tank outside or use single cylinders.
Buy Big Buddy heater, much higher output.
Other options ?
Buddy heaters have Oxygen Depletion Sensor but id be surprised if that condition occurs due to air leaks in basement.