Honestly I was expecting
@Pattree to be the one to mention a giant Kelly Kettle
I’ve not involved my self in this discussion because it isn’t a place for theory and I have no knowledge or experience. The solution is very probably commercially available.
Wood fired systems are very efficient where very low volumes of fuel and water are in close proximity. Even a device with an efficient heat exchanger is going to use and awful lot of wood if it is to supply a shower.
Far more important than its efficiency, a shower connected to a boiler must have a fail-safe thermostatic control.
I haven’t read the whole of this thread so this point might have been made already:
A Solar pe-heater would seem appropriate. My simple black plastic shower bags can get too hot on a bright summer day. Solar panel water heaters that I’ve seen demonstrated look wonderfully effective.
