Heats my 32 x 36 attached garage perfectly fine. R15 in the walls 24 on center studs, maybe 6 inches of blown in on a 9 ft ceiling. 1 small window, 1 16 ft wide R11 insulated overhead door and 1 8 ft wide overhead door. I could probably get by with a 45,000 BTU but I like the fact that I can...