Vaulted cellar workshop approx 3.2m² useable floorspace. My boiler and Megaflow are down there too, with a radiator under the bench. Warmest room in the house at a pretty constant 18-20°C. I keep a dehumidifier running 24/7 to keep the damp at bay. It feels dry and I haven't had any issues with rust in the 6-odd years I've been using it.

(As pleasant as this sounds, it's not a great place to spend a lot of time - I'd rather have a space above ground where I could fling open the doors as and when needed..! Still need to sort out some sort of extraction/circulation down there...)

Wal.