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?
If not adjusted properly or lack of combustion air yes co in house. And yes products of combustion in house. But I'm pretty sure you already new that if you are changing out heat exchangers. Personal opinion Get rid of any unvented heating appliance. The ones I have seen pose another danger and that is potential burns. Especially for children. We would always take these off line.