I have a D2 under driver seat. THinsulate + thermzite insulation, window covers, and altitude compensation on the D2. I regret not getting the larger D4 for the quieter operation at higher airflow vs the D2 running on high all night. Plus, we have run into really cold temps more frequently than one might think (-15 an colder).
Noise aside, it’s a better idea to run it on high than on low, unless you like to service it every 250-500 hours.
I find the D2 to put out a LOT of heat but it needs a little planning to get maximum efficiency out of it.
The conventional under the seat method unfortunately is not very good especially for long vans . Takes a lot to heat up the back where the bed is. Plus, it’s just one duct.
A way better approach is to mount it mid van on the fuel tank side if possible, get a ring connector from 2.5” to 3” and install 3” ducting . Have a Y connector direct the heat to 2 outlets one pointing to the front and one to the side or back.
Also take note where you install your temperature sensor or controller that has one built in. If it reads the temp next to where the air comes out of the heater...it’ll stop sooner than it has to.
I’ve been told all this by Cliff at AMC Cliffv’s marine in Seattle.
The guy knows his stuff...