anything that burns, water is the by-product.

simple chemistry!

any type of "fuel oil" gasoline, diesel, kerosene, liquid propane, natural gas, even wood... the hydrogen (H) in the item that is burning combines with oxygen (O) in the air, and you get H2O -- Water. the Carbon in the stuff you are burning produces CO and CO2...carbon monoxide--poisionus, and carbon dioxide--what we exhale.

so yes, any propane, diesel, kersone type heater can produce lots of humidity. Unless, as mentioned, it uses a heat exchanger, and the damp, moist exhaust is vented outside, and the heat is kept inside...which is what most home furnaces do. which is why the air can be so dry.

knowing the explosive hazards created by spraying a large amount such as an entire car, i think I'd want to wait until the ambient air temps were above 60 degrees rather than risk blowing my shop up, or spending big bucks on all the explosion proof heaters.

small parts here and there, I wouldn't have a problem with though...so use the winter time to do your underbody, suspension parts!