Gas Power Shower

[paddyk]

Our power shower has started playing up. You have to run it for about 4 minutes before the water gets up to temperature (it runs cold until then)
I don't think it's a fault on the Gas Combi boiler as the hot water from the taps works fine (it's an almost new Worcester 28 kw boiler). Has the power shower had its day ? (it's around 10 years old). Does anyone know where the problem may be ?

[The Builder]

I guess it's thermostatic? The cartridge has become faulty.

If it's a make such as Mira or Triton, then replacements are available, Paddy.