HW problem, weird

[tabbypurr]

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

[harry]

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.

[Harry Bloomfield]

[e-mail] formulated on Sunday :

NRV's or the design of the outlets should prevent back feeding from either H to C or C to H, so as to avoid contamination of the CW mains supply.

[tabbypurr]

then I guess there aren't 2 NRVs. Preventing hot going into cold is the one that matters.

NT

[harry]

Cold going into hot water can cause floods. So that matters too.

[Harry Bloomfield]

Our mixer taps have internal concentric pipes, so the mix only occurs at the end of the spout, so as to prevent any reverse flow.