In article ,
Robert Inder writes:

All the mixer showers I have looked at expect roughly even
pressure on hot and cold. So it looks like we might be forced back to
an electric, because it will run of mains alone.

Thermostatic showers designed for Multipoint/Combi use expect
high pressure cold and anything from low to high pressure hot.
However, they might not be designed for a situation where
backfeeding the hot is possible, so you will probably need to
check installation instructions carefully before selecting one.

--
Andrew Gabriel
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