It’s just a heating lamp (bulb), runs at 250 watts. I just put it into a porcelain light socket that usually holds a regular light bulb. It “only” got to 11 this morn (forecast was 8), but the temp inside dropped to 37.
The space is small enough that a bulb like that can make a difference. The cellar is half in ground, and has very good insulation around it. My late father in law built it about 25 years ago, down by the old house. We’re about to run out of space in it, so, I’d like to build a new one up here by us. And, have it deeper into the ground. One day.
Bulb looks like this, I got some of them at Tractor Supply for about $10 for a pair. They’re used a lot for keeping chicks warm. Also called a brooding lamp.