So I ordered the C-Warm
calorifier and it turned out not to fit

Apprently nothing is made in this size anymore

Although my
boat is not that old (2000).
So I took out the old one, and it occurred to me that -- a radiator shop would be the right place to take it. A few hours and 48 pounds later, my
calorifier is back in usable condition. They soldered it and pressure tested it and declared it fit for use, but of course without guarantees that other
parts of the seam might not fail later. The new one is on its way back to the dealer -- saving me about $1300.
It's a lot of
work detaching and reattaching seven pipes (cold
water in, hot
water out, two central
heating connections, two
engine connections, one pressure relief outlet) and so forth. These plastic push-fit
plumbing connections are not so completely perfect to use.
But anyway, the
repair seems to have worked. I'm about to take a hot
shower on board!
Think -- radiator shop.