The shower in my master bathroom is no longer staying hot. It's
hot enough for me if the knob is all the way over to the hot side,
but not hot enough for my wife. I used to set the handle to more
or less the center, so something has changed. The downstairs
shower and all the rest of the faucets are fine so I don't think
it's the boiler (domestic coil).

I've replaced the cartridge in the Moen mixing/diverter valve (the
Moen site said either a 1220 or 1225 - I used a 1225) but the
problem remains. I've been told that a hot-cold transfer is
possible at any faucet so I've tried shutting off the supply to
them; nope. I would have thought this would affect all faucets
anyway. Any ideas?

--
Keith