That is one big heat exchanger. I reckon you could boil the water for a cuppa. I had a shower kit exactly the same as yours and one day went to have a shower and the pump was seized. Pulled it apart and it was full of rust. There is nothing more annoying when you set it all up, looking forward to that nice hot shower.
I now have mounted under my bonnet a fixed pump. If I was going to stick with what you have, I would buy a second one and carry it with the other spares.
Dave.

