Our one careful previous owner Mog camper had a 5KW hydronic Eberspacher which could happily get you past 35degC inside with snow out
The insulation was a bit rubbish too, single pane roof lights. That was with a calorifier and an air blown heater matrix with four outlets at floor level (noisy and dried your eyes when used to that excess). When you had just had a shower, the burner kicked off again to heat the calorifier, the air blown from the vents became cold, not nice on your wet skin as you towelled off! And on arrival somewhere, the 5KW would get you warm but not instantly. I seem to remember the calorifier is supposed to be the first thing downstream from the burner since you want that hottest, but once that has sucked the heat from the coolant your rads will then be chilled for a time. Perhaps a bigger burner than the calcs would suggest just to minimse the time you have with less than ideal heat? But we could have a fully hot calorifier 20 minutes after stopping so never a big deal.
That Mog camper had an overcab bed which made it probably too tall (but the height itself never became an actual issue). This leant itself to a false floor whose thickness varied from maybe 75mm to 500mm and storage and water tanks under. The hot pipes from the burner loop ran round within this which mostly gave a nice gentle floor heat as it heated the air in the gap.
There was a thread here where someone worked out that with the maximum temperature you could tolerate from the floor, and the area remaining once the furniture had been allowed for, it didn't give you enough output to maintain a sufficient cabin temperature. Maybe that is a also an insulation thing though? And time.
As I've bored you all with before, our Mog had the correct Eberspacher parts for connecting into it's coolant system similar to as described by some here but it didn't work so wasn't used. It was similar to what Joe's sounds like but was useless. The only few things that went wrong with that chassis was where someone had changed what Merc made. We went with a Mog because of it's reputation, so i came to the conclusion that buying into that but then fiddling with it was a little contradictory, KISS and leave well alone would be my view now. If you can get the bits that the manufacturer uses to plumb a pre heat system and exactly replicate that then I would go with that, but that system won't include heating your camper box. Changing the original according to a third party I would try not to do. Preheating an engine designed to start from cold at minus I can't remember what is required to reduce a bit of smoke? Well, quite a lot of smoke really, best leave early
We did have some issues with that old Eberspacher to begin with. For instance the fan inside the burner supposedly had the same part number for 12v or 24v and we were sent the wrong one which oversped and broke. The three screws sealing the burner chamber, undone every time it needs the soot cleaning out, are soft ally and easily deformed if set fast from repeated heat cycles. Finding and fixing a problem yourself may require changing a few bits to find the fault. On that basis we now have a second whole spare burner on our newer old camper.
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