There are in-the-floor mounted Propane/NG Heaters that are very compact
and are exhausted from underneath . Gas line is brought underneath as
well, so, the only thing you see from the topside is something that
looks like a typical floor register. Has a small fan that takes return
air and discharges the supply air all thru the topside opening. Saw
them in the W.W. Grainger catalogue. THis sounds like the way to go for
such a small sq. footage of 150.