In article , Grimly
Curmudgeon writes
We were somewhere around Barstow, on the edge of the desert, when the
drugs began to take hold. I remember "rburr49"
saying something like:

Is it possible that the tank itself (copper with that dry foam
insulation stuck all over it and about 20 years old I imagine) has some
kind split/hole in it?

Old tanks sometimes develop leaks from the bottom where the metal is
stretched thin during the stamping process.

Is there anything we could use like radweld (?) to seal it up? The
thought of having to replace the tank (meaning getting a plumber in to
do it) is a bit of a nightmare cost-wise!

Not much. I've tried this before (solder or fibreglass) and quite
frankly it's not worth the hassle - it always leaks again.
Replace with new.

Take my advice and bite the bullet on this one!. I've had the same
situation as you with regard to the hot water cylinder in my daughters
flat. The immersion heather gave up the ghost, changed that the next one
lasted 8 months and slaughtered the heating element, something about the
water there!.

Course doing up and undoing the immersion heather that developed a crack
that you couldn't quite solder and decided to take it out to solder it
better and discovered that in places its wafer thin and when doing the
top output connector, that tore off cos the metal was so thin.

When it was all drained it weighed a ton because of all the limescale in
the thing as it was run too hot!.

All in all new 900x450 cyl from Screwfix for 94 odd plus the VAT end of
story all fixed and fine)

Take a bit of time and care and its well within the DIY scope. Just use
PTFE tape and a dab of sealant on the connectors and its difficult to
make it leak!...
--
Tony Sayer