Originally Posted by DanielC
Have a working CO detector in the same space as the heater.
That's obviously good advice, but expect the detector to go off if you've got an engine running, even with the door open. These heaters give off very little CO, and all new ones include an oxygen depletion sensor that turns off propane flow if the O2 level drops. The bigger problem is that the primary byproduct of these heaters is water vapor. That's not necessarily a bad thing in the winter when the air is very dry, but expect high humidity in an closed space.