The subject of locating a wood-burner in a garage has come up before a few times. Unless your boiler is completely walled off in the garage from the rest of the garage space, with no access door into the then separate boiler room other than from the outside, you have a potentially dangerous situation. Gasoline fumes and flames are a bad mix. That's why the building codes I'm familiar with make this illegal and Danjahy's input from his insurance company was right on time. Just think of other flammable fume producers you might store in your garage like paint thinners and the like and you get the picture. I've got a gas fired DHW tank in my home in a laundry room. I made sure I removed anything that might produce flammable fumes from that room too. Why chance a potential disaster when you are literally playing with fire.