Quote Originally Posted by Tim K View Post
The products of combustion of propane are H2O and CO2. CO is only produced if there isn't enough O2 to support complete combustion. As long as there is enough make-up air so the O2 level in the garage doesn't get low, you should be fine. Clint's idea of a CO detector is sound and would provide nice piece of mind.

As others have stated, it's going to make quite a bit of water vapor which will condense into liquid water on any cold surface.
If you have an optimal burn with a blue flame and no smoke, no CO should be generated.

If the burner is running lean or rich, CO can be an issue, especially with poor ventilation.

Generally propane is very safe.