Are such crossflow heat exchangers easily available? I've searched
via
Google but could only find air-venting ones.

I'm not familiar with anyone selling them yet, it would likely be a diy
job. But its not very difficult.

You'd need 2-3 ft of copper drain sized pipe, some microbore, and
solder. Wrap microbore round the big copper pipe, but rather than one
piece, use 4 in parallel. Solder them in place. Use a 4 way manifold at
each end of the microbores, so the cold water flow goes thru the 4 in
parallel.

The cold supply for the shower goes into the microbores at the drain
end of the thing, and comes out at the plughole end, going to the
shower.

It cuts right back on energy use, and makes your tank of HW last
substantialy longer. Payback varies according to use, but maybe a year
or so, that sort of region.


NT