I have a Hot Dawg 30,000 BTU unit that draws the combustion air from the outside. It is in a 14'x24' outbuilding that has 8' ceilings.
Temp is left at 15C(59F) all the time. When I turn it up when I am out there to 20C(68F), the air is warmer coming through the unit (but not screaming hot by...
http://www.snappower.com/
Got these off a kickstarter -works like a charm and took about 20 seconds to install.
Have 5 scattered through the house.
Leaves all the outlet plugs open as well.
Scott