DocJim
Looks like I am the first to answer and I am sure you will have several different opinions on this subject, so here is mine.
the 400D does not have the engine pre-heat nor can it be added to the unit. The heating capabilities are excellent and for me I have camped in weather that was in the teen's at night and around 30 or so for up to a week, It used about 2.3 gallons of fuel every 24 hours of running. While doing that, we had the electric element on as well, and the fireplace as well(wife likes the effect) but it does throw out a lot of heat. I and some others on here have added a thermostat (and some, another heat duct) into the water bay. (The factory has now done this on the new models). So in the winter we run both the diesel and the electric to keep heat and hot water as hot as you want it and plenty of it.
In the summer I only run the electric but from cold to hot it takes about 1 to 2 hours to heat up the water, when the diesel is on, it takes about 20 minuets to heat up the water. So, I have had mine now for almost two years (this will be the third winter). I have had it serviced twice (new filter, new nozzle, clean the burner, check the electrodes, and check the % of boiler fluid. I did have some problems when I first got the unit (It had air in the unit) which was hard to diagnose, but after I raised up the expansion tank and removed the air from the unit, it has been fine (knock on wood)
Here is a link to the service manual if you don't have it, it will help you immensely.
http://www.aquahot.com/files/service...nual_Rev_D.pdf
If you need any help let me know
Linear Mode
