I started thinking about this after I recently changed a cracked Heat Exchanger in a Gas-Pack. Of course I told the customer about the CO hazard that a cracked H.EX. presents & suggested that he get some CO detectors, etc.
I have a rental property that has Vangard N.G. space heaters & I began to wonder - don't they emit CO into the house, also? They aren't vented, so what happens with the combustion gasses?