Doctor D wrote:
I'll watch out for Dr Evil.

From your previous visits, you might recall a frequent poster called
IMM. IMM is now Dr. Evil.

You can set up a hot water storage cylinder to operate from the heating
circuit of a combi boiler. I've done it and it's not very hard.
However, it is unusual, wiring diagrams aren't published and is likely
to cause much head scratching by the 'always done it like that' type
plumber.

Also, the disadvantage with electric showers is that they're limited
to 9 or 12kW. If you do the maths, this doesn't convert to much of a
flow rate when you calculate the gpm heated from 10 to 40 degC. The old
shower may be past it, but a new one will still have a similar kW
rating and produce a similarly inadequate flow rate. Electric water
heating is expensive; I disagree with the suggestion to change washing
machines to cold fill.

The way to a decent flow rate is a storage system, which will
accumulate the required kW of hot water, or a combi oversized for the
heating load, say 30 or 40kW. I don't like combis, but they do have
their uses.