Most garages aren't air tight enough to have to worry too much, anyway. I wouldn't stress out too much about it. Propane appliance are set to run at ideal burn, which produces very little, to no CO. Ideal burn is 1:24 propane to air. CO is produced by incomplete combustion of the propane, so if the unit is lean or rich CO occurs. You need to reach more than 70ppm of CO to start to feel effects, so I think even a slightly drafty garage and a brand new, well made propane heater will be just fine. You should always have a CO detector whenever propane or natural gas is your heat source, but the panic in this tread is a little much. I run my kilns off of propane and intentionally throw them into reduction (rich) and somehow I still live, breath and walk.