Stu, I think you are missing a bit here. At no time would you need to have any hot air being pushed by the car's movement, through the vents. You'd have your air con on nice and cold. Engine would be boiling hot. You park up, move the slider to meduim hot, start the engine pump which would sent water round the heater matrix with no fan on at all anywhere. This heats the heat exchanger. You switch on the submersible pump and hot water pours out of the tap. The only thing you could not do would be run the show AND have the air con on at the same time. But car parked, engine off there is tonnes of hot water for at least an hour or more. In fact Ii think with my idea, more hot water than with the original. With the set up as sold, the water goes from the engine to the exchanger at a fixed temp. Bloody hot. This is actually a waste of heat because you only want it warm. So you have to mix in cold water to cool it at the shower end. If you use the temp slide to control the temp from the engine to the exchanger, then you actually take less heat from the engine, in fact the temp going into the unit would be only marginally more than the water coming out of the tap (there will be some losses). That way, you could actually heat much more water than before.
Clive all of that capacity stuff has been done. One hot engine, switched off, will heat buckets full of water. There is a proven system sitting in a box waiting for delivery - at a cost. I just think that between us, we're cleverer than that and can do this more cheaply and efficiently. Why waste all that engine heat, heating water to more than you need to have to cool it back down again?
You guys do get the principle that there is an additional water pump that pushes the coolant around the block - yes? I didn't get that at first and couldn't see how the exchanger got fed.
Grant, with you on the 'get the same stuff but cheaper elsewhere' Absolutely.
Yes the rear heater is fed from the main heater pipes. This is why corroding rear heater pipes is a big issue. If they go then you loose the whole engine coolant. Pretty sure that the rear heater can only deliver what comes out of the main vents. But not in relation to aircon though.