I put the VW Bus model gas heater in my 62 Beetle. The vent was above the passenger's legs in the well. The heater was mounted in the front right against the glove box, but outside the passenger compartment. That baby would heat the passenger compartment in about 30 seconds. My mechanic thought it used more gas than the engine. Scary to think back on what we did when were young, foolish and stupid.My '91 850i had that "rare" option … auxiliary heater. Those were Webasto. Of course there is also a auxiliary electric water & fuel pump ... even a exhaust with silencer … For that there is a extra batterie …. and a second alternator ! It first warms the engine and second the cabin heater …
Can be lux in cold regions … but not in Belgium , sure not when car sleeps in a garage . So i removed the complete package ... pump,many hoses and clamps could only make the risk on a water leak much higher. Not to speak of a " benzin" burner in your car …. ( on diesel .. yes)
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Frogish example is a air heater , not water , so more simple , but still on "benzin" . That or ,more on diesel ,are common used in boats and trucks.