On Sunday, 18 December 2016 23:15:33 UTC, wrote:
Lots of hot comes out the sink taps, but the shower is stone cold. The HW pipe feeding the shower goes from lukewarm to cold-water-incomer-cold as the shower runs. It worked fine before, and they're adamant no plumbing has been done.

The system: Immersion heated HW cylinder, gravity feed. Old thermostatic mixer shower past its best. Mayonnaise pressure cold.

I suspect the shower's cold feed is backfeeding the HW supply pipe. I don't see what else could be happening. Plan to take a look tomorrow.


NT

You'll probably find there are check valves (non-return) in the shower mixer.
One has likely jammed open.
Cold water is transferring to the HW system
Just needs cleaning/freeing off inside mixing valve.

Happens in limescale areas.