Square footage of heated area for an average insulated garage would be approximately up to 550 square feet for this heater.
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My garage is about 600 square feet, insulated walls and ceiling (just fiberglass, nothing exotic), plus R-18 insulated garage doors. 17,000 btu works fine, but add a small fan blowing downward to pull the stratified hot air down to the concrete slab. Getting the concrete warm takes 24 hours, but after that the shop is comfortable for all kinds of work. The stored heat in the concrete is valuable, since if you are doing painting, spraying, and handling anything flammable I turn off ALL heat sources and air out the garage for a few minutes, and the concrete holds the heat until I can put the heater on again.
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