There's something wrong with the plumbing, because the shower temperature simply will not stay constant. It gradually changes to either scalding hot, or freezing cold while it's running.
The temperature dial is over sensitive – the slightest touch sends the temperature to one of the extremes. A plumber mentioned to me that the water pressure of hot and cold water coming into the shower might be uneven, and the shower unit cannot balance it.
How do I balance the pressure myself? I had a look at the pipes under the boiler but couldn't see anything to change the pressure.
Our flat is in London, England in a house that's been converted into four flats. We have a combination boiler and a 'normal' shower (not a power shower).
P.S. I've noticed that if I turn on the central heating then the shower temperature stays constant. Also, the radiators heat up when the shower is turned on hot, even though the central heating is off.
Best Answer
Is the shower mixer a thermostatic control?
We have a similar set up. The one shower with a non-thermostatic mixer is a bit temperamental, but the other shower with a thermostatic mixer keeps a steady temperature. The former has a single control - left and right for temperature (though mixing), in and out for flow rate. The latter has two controls - one for flow rate and the other for temperature.
The problem with the radiators heating up implies that you haven't got a valve on the radiator feed that prevents the hot water entering the system. I'd get a plumber to check that the system is set up correctly.