Plumbing problem - concealed valve shower

I have a concealed valve shower, pump in the attic. Gas boiler heats the water. Over the summer the radiators were turned off. Since I turned back on the radiators the water in the shower is not hot - its hot for 30 seconds when the shower is turned on but then goes cold. Hot water in all other taps. Had the same problem last winter but water not as cold. The water in the shower has never been as hot as taps but is ok once radiators not turned on. No luck getting a plumber last year. Don't suppose anyone can suggest a simple solution to save me trying to track down a plumber?

Do you have a hot water cylinder?
How old is the shower valve?

Yes cylinder in hot press - it has a thermosat thing on it which I have tried turning up/down. The shower is 4 years old.

If the water to the shower was hot when the radiators were off
it means the heating system needs to be balanced.

A.P said:

If the water to the shower was hot when the radiators were off
it means the heating system needs to be balanced.

Click to expand...

Sounds like cylinder is not heating allright, but OP did say hot water at the taps ok. OP should double check that there is plenty of hot water at the taps. If there is plenty of hot water at the taps and still very little hot water at the shower, there is something wrong with the pump or the shower valve.

Your thermostatic shower valve will always give you a safe level of hot water for a shower and that is the reason the taps are hotter than the shower itself. Could be that your shower valve needs a service. Four years in a hard water area may have led to a build up of scale inside the fitting. You can buy replacement parts for the valves easily enough but sometimes fitting them is sometimes an awkard job.

Thanks guys, at least now i know is not something simple I could fix myself, will restart the search for a plumber.