"IMM" wrote in message
...


snip


You can make your own using two large bore copper pipes. One inside the
other.

....................that bit I understand ...

The mains water is in the gap between the two.

....................................I can even grasp this idea ....

Using two reducing
elbows and filing out the pipe stops inside the the fittings so the inner
pipe can slip you can easily make one yourself. Once filed out you solder
it up with a powerful blowtorch and wrap with insulation.

...... I'm afraid that I get lost about here ....
Can you provide a pointer to a sketch ?

--

Brian