Just to add i haven't actually used hot water so not an expert, just my thoughts. Spruce is the man to ask i think, as i believe he has looked into all this before and tested it, and of course the people who actually use these system.
If i was to get one then i wouldn't buy it from a wfp supplier. I would buy the webasto heater itself and install it, you would probably do it for at least half the price these companies charge as i believe they just use the same type heater.
I found on a test bench that the Thermo Top C only raised the water from 9 degrees to 35 degrees at the brush head. The initial flow was 1.5lpm but the water flow increased with heat. I dear say that if I reduced the flow back to 1.5lpm I may have got a little more heat. I set the thermostatic temperature control valve to 40 anyway.
I used minibore hose and found that the particular hose I use happily with cold became very stretchy.
According to physics I should only have used 2.65kw to raise the water temperature through 26 degrees at 1.5lpm. But the heater had nothing else to give. Although some heat would have been lost by the normal process of a hot item in a cold place, radiation, I believe that the majority of remaining heat lost was out of the exhaust as that gets very hot.
One needs to be very careful if going down buying a second hand Thermo Top C . They are usually auxillary heaters from various cars from the breakers yards.
Most are vehicle specific. So the BMW one I originally purchased wouldn't work as it needs to get instructions from BMW's canbus system. A new pcb for universal application cost more that the heater.
I believe the the units fitted to Rover 75 models work, as do the ones fitted to PSA models, but I'm not 100% sure on the latter one. (There is a post on YouTube where some guys have a Thermo Top c from a Peugeot 807 they got to work.)
https://www.youtube.com/watch?v=qcfPMQw3toUIf you find one thats been refurbished from a boat then they will be ok.
If you are thinking about doing a diy one you must understand how the heating program cycle works. A heater that reaches temperature takes 180 seconds to go through its shutdown cycle and then uses a fair bit of current to get going again when starting up.
The other issue with the Thermo Top is that if you start cleaning windows a few moments after switching the heater on , you will always seem to have cool water. The heater really needs to get up to speed before you start drawing heat from it. This is why its recommended that you start the heater whilst on the way to your first customer.
The 8lpm gas heaters that were popular with some cleaners delivered 16kw and they gave instant heat. Diesel heaters don't deliver anywhere near that amount. A Thermo Top C is 5.2kw and the ST90 is 9.1 kw.
Making up your own heater may seem pretty easy but it is more complex than it looks. Where do you get the diesel from? Just putting a 'standpipe' into the diesel tank is a major operation.
We don't do much mileage and I use a tank of fuel every 4 or 5 weeks. A diesel heater drawing fuel would probably take another tank full a month, and possibly a bit more.
