Willis Heating System

[JohnW2]

Is your complaint/issue with the system that the heat exchanger is above the tank?

Sort-of. My 'issue' relates to the fact that the feed from the Willis heater is connected above the top of the 'tank'.

Water heated by the Willis heater will rise up into the expansion pipe. When the water in that pipe eventually cools it will fall ('convection') back down the expansion pipe (back towards the heater) and/or into the main cylinder IF the water below it is hotter - that will eventually be the case in terms of movement back down towards the heater, but I don't see how it would ever be the case in terms of movement down into the main cylinder.

Does this lead you to believe that the water in the hx would just heat up and stay there, and the tank wouldn't warm at all?

Again, essentially yes. Assuming no hot water ws 'drawn off' (by taps), it seems to me that the heated water would just stay in the heater and expansion pipe, and that no heated water would get into the 'tank' other than by conduction (which I imagine would be trivial). As I wrote ...

... I can see how the Willis system would work if the feed from the Willis heater fed into some point below the top of the main cylinder - BUT I can't see that that would be essentially any different from (or superior to) having an immersion at the same point (as the 'connection') in the main cylinder.