jonathan wrote:
Our power shower has just started misbehaving after 2 years of use.
The temperature is OK when the pump is not on, however, when the pump
is switched on the temperature drops to just cold.
Is it the impellor on the hot side not working, which sounds OK, or,
is it the mixer? My money is on the pump.
Any suggestions before I rip them apart.

Thanks
Jonathan

Turn the shower mixer to hot only... what happens next?
No water at all = knackered pump!
If, on the other hand the pump also powers the bath taps - as ours does -
hot water at the bath hot tap = blocked shower feed pipe!
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