Separated combustion shop heater
I'm pricing a heater for my new shop, looking at the hanging style propane heaters. Since I plan to use solvents and paint when the heat is on, I'm looking at separated combustion heaters where the combustion air is drawn from outside the building using a separate tube instead of from inside the building. Most vendors (Modine, etc) sell both conventional vent and separated combustion versions of their heaters, but the separated combustion versions carry a 60%-100% price premium. As an example, the one I'm considering is about $450 for conventional and $750 for separated.
I've downloaded the manuals for these units, and the only difference between the two is that where the conventional units simply have louvers in the sides where the heater draws combustion air in, on the separated units these louvers are blanked off and a 4" pipe connection is provided instead that is routed to outside the building. The only other thing is that the separated units have additional gasketing inside the case to ensure that no inlet air can be drawn from inside the building. Everything else in the unit - blower, burners, propane nozzles, etc, all have the same part numbers between the two units.
$300 extra to NOT punch louvers?! Seriously? Yeah, I suspect a large part of that price difference is for product liability insurance and lawyers, but you know what my next question will be. Why can't I seal up the inlet side of a conventional heater, blank the louvers, and add a pipe connection to outside? Assuming the airflow is the same (and the separated units all use standard 4" galvenized duct), the burner won't know the difference. This is even more attractive because used conventional vent heaters are readily available for even less money, but I have yet to find a used separated combustion unit.
Thoughts from HVAC guys?