We use an Aqua Hot for bus heat and to heat the 8-92. On Sunday afternoon at around 0 C or a little lower 1 hour with the heat on and the engine fired on the first turn. My system is routed with all the radiators & the engine in parallel. That isn't the best system - I would put at least some of the rads in series. The way mine is set up, if the engine is in the loop then very little heat goes to the coach because the path through the engine is such a low resistance. There is a bypass at the engine but I actually close that off when I want maximum heat to the coach. With the engine running and the valves open it provides lots of heat for the coach.
What they did on mine was set up a lines running the full length of the bus with one line out from and one line returning to the Aqua Hot. Then they ran radiators and the engine preheat "across" the legs of the loop. It's not a real good layout but it would be hard to change now.
When I looked at Aquahot and Hydrohot surplus systems, I found they cost a minimum of $2000. Too rich for my blood.
OUCH!!!! That was one of the things I did not know! OOOOOWEEEEEEE! Well, I guess I will have to really start looking around or getting creative. (I've been known to do that)
The downside is the chimney through the roof that makes you look like a hippie (nothing wrong with being a hippie you understand, but you might get thrown off campsites).