Cold running Shower - is it the valve

[trev_t]

Hi looking for some advice - before I cut a hole in the wall to replace my shower valve.
The shower was fitted about 6 yrs ago and has worked fine. It is feed from cold water tank & hot water tank to twin impellar pump (separate hot & cold in and out).
Recently the shower has started to run cold midway through a shower, ie start of warm and goes cold after a while. There's still plenty of hot water - checked nearby taps.
Been in roofspace tonight and, after the shower started to run cold checked the output feeds from pump - still hot both in and out of the pump.

That leads me to think a faulty valve? Unfortunately it is buried in the wall so will need to cut away to replace/repair or even undo the connections :(

Only other bit that makes me wonder in it di something similar about a year ago and then seemed to correct itself.

Any ideas, recommendations or ways to test?

Many thanks

[stoneyboy]

trev_t,
Good chance the valve is scaled up causing it to stick. Turn the shower on and move the temperature control from fully cold to fully hot many times - this may free the internals. If this doesn't work you may be able to fit a new cartridge - depends on the make and model.
end