Shower mixer tap often becomes cold, but hot water comes out of all the taps

[Potatan]

I have a combi boiler working fine for central heating and hot water to all upstairs and downstairs taps. The shower uses what I think is a Mira mixer tap maybe 15 years old, running from the boiler and mains cold. Since I've lived here (10 years) the shower has taken about 90 seconds to come up to temperature but is then generally okay.

When using the shower it doesn't always stay at the temperature you've set, drifting either up or down between not quite scalding and tepid, and lately (the last week or so) it has occasionally lost all hot water and just comes out cold. Putting the bath tap on produces hot water, and sometimes this can "trick" the shower into delivering hot again.

Some other things:

* the mixer tap feels sometimes a bit "loose" or clicky, like there is a washer inside with some sort of stiction, or something else has happened in the mechanism
* it can stay at a temperature for a few minutes before drifting off, almost as if some component is warming up and expanding
* occasionally (possibly unrelated) there is a distinct hum from the pipe, like a long continuous tone. I'd estimate this as a B major...

Does this sound like an easy fix? How tricky is it to take the shower mixer apart to diagnose?

 

[ericmark]

I will guess down to flow rate, and boiler turning off, had it with late mothers house, a kink in the pipe to sink taps reduced output to below that at which boiler would cut in, still a fair flow, but not enough.

The boiler had two options Eco on and Eco off, with Eco off even a dribble is hot, but the shower would go from cold to hot, then cold again as reservoir run out, then hot again and stay hot, with Eco on the flow must exceed a set rate.

Cleaning out shower head may help.