Webasto thermotop water heater. Disassembled, cleaned and serviced kit swapped out. Now pissing out coolant. What is the problem?

[Falling_Penguin]

As the title says this thermotop C unit has been carefully stripped down, cleaned, service kit applied with new seals etc, now finding coolant just pissing out.

Service pack (no printed guidance, Webasto? Why?) had a large green gasket as well as a rubber one for the main seal. Fitted the rubber one as metal was still in good order.

Any ideas?

 

[Boater Sam]

Hole in heat exchanger? Something not fitted properly?

 

[Alex_Blackwood]

Hole in heat exchanger? Something not fitted properly?

Most likely