I forgot to take an earlier picture but it will give you a general idea.
As my 'firebox condenser'
cannot be pulled apart (they wouldn't desolder easy), I have to be sure there are no leaks.
I thought I was extemely thorough but eventually found 5 leaks. I blocked one end of and put the
mains on. If you hold it up so the air inside gets pressurised by the water you'll hear the air hissing
out the leaks. It's much easier to find the holes that way. Once you think you've finished soldering
the condensers together, let them sit under pressure for a long time, just in case.