Stoves, charcoal bbqs, etc. if non vented (and the gas stove in the 
Westfalia is non-vented) emit all of the products of combustion including 
water vapor and carbon monoxide, and they consume oxegen. So they can 
kill you either with toxic gas or just be depleting the oxegen you need 
to breath.

Non-vented catalytic heaters emit only water vapor (which can cause 
corrosion). But they do consume oxygen. So a window needs to be open for 
safety.

Vented catalytic heaters emit noting inside the van. And their vent fan 
creates a negative pressure inside, so a small amount of fresh air is 
drawn in through every vans' air leaks. In addition, the Thermal Systems 
heater shuts down in the event there isn't enough oxegen, or the vent is 
blocked. It really is fail safe.

--Dave