I don't already have the Webasto, it's an expensive item and won't be needed for awhile so I'll wait to purchase it until later but I will install the rest of the system in its entirety...blowers, hose, WTW (water-to-water) heat exchanger, expansion tank, etc.
The blowers are pretty much a copy of a typical car-type unit; a bunch of tubes with fins with low draw fans blowing across them. There are a few manufacturers but so far I'm leaning toward the Heatercraft units. I'll likely use the toekick model (9H); it's rated at 8087 BTU and draws .36 amps. Heat-wise this unit puts out just a bit more than a 1500-watt electric space heater and has a 3-speed fan. Street price should be around $175.
The distribution lines are 3/4" industrial heater hose. The fittings for copper tubing are too restictive for a low-flow system like the Webasto although one can use some short sections of copper tube where you want to radiate a little heat...like in a closet (especially on a boat with its "wet" locker for foul weather gear); I'll use all heater hose.
The buses heater hoses (supply and return) run up the driver's side of the bus from the rear engine comparment to the dash area. In the rear port corner of the bus I'll install the water-to-water heat exchanger in the supply line coming from the engine. This keeps the engine's coolant system (and the heat to the driver's area) separate from the Webasto system so a failure in one doesn't affect the other.
The Webasto system is just a simple loop; starting with the water-to-water heat exchanger the next item is the expansion tank (under the rear bed above the heat exchanger), then the Webasto DBW2010, then each of the three (planned) Heatercraft 9H blower units, then back to the heat exchange for another lap. Total temperature drop for the loop is about 7 to 10 degrees (F) so all the heater blowers work about the same.
Here's the Webasto:
There are a couple of other considerations. Right now I have a 3-way water heater...AC power, propane, and an engine heat coil. If I use this unit the Webasto loop will go through it. I may bite the bullet and get the RV500 propane-fired demand water heater; 6-gallons of hot water is not enough for the tub. The tub is a total luxury and will most often be used as a shower but on those "special" occassions where water is plentiful it would be nice to have enough hot water for the tub. Also the RV500 takes up less space inside the bus but it's expensive and you're beholden to the one and only company that makes it for support and parts.
Another consideration is the coolant pump. When the engine is supplying hot water the Webasto's coolant pump keeps the Webasto loop running (the coolant pump can run independently of the boiler), but if I don't install the Webasto right away I'll need to install a coolant pump to circulate water through the heater blowers.
Like this guy:
Most likely I'd leave it in parallel with the Webasto once it is installed as a backup pump.
What I don't like about the Webasto system (other than cost!) is that it can be finicky. It is diesel-fired and problems with fuel, high altitude, or the fuel nozzle getting crudded up can shut the system down. I don't want to be wholly dependent on one fuel source (if I have a major fuel problem, like fuel gelling) I may not have the engine or the Webasto for heat so I'm also installing the Dickinson Marine propane heater.
This unit is rated to heat a 36' boat so will provide enough heat to keep the temps in the habitable range in an emergency. It will mostly be used for ambiance and to provide a bit of extra heat up forward (where most of the windows are). It's a direct vent unit so it doesn't consume oxygen from the interior. Because of it's vent configuration it will not be used underway and isn't needed; the bus' heat system will provide more than enough between the Webasto loop and the original front heater/defroster.
That's it for the heat systems.