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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.

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If I don't want to be at the mercy of a plumber, how can I learn how to fix this myself? –  Will Sheppard Jul 22 '11 at 10:37
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up vote 2 down vote accepted

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.

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I believe it is a thermostatic control. It must be walled up behind the tiles as nothing is visible except the tap and shower cable. Oh, and it's a single tap that you move towards you to increase pressure, and move left or right to change temperature. –  Will Sheppard Jul 22 '11 at 10:36
@Will: Why do you believe it's a thermostatic and not a non-thermostatic control? –  BMitch Jul 22 '11 at 11:41
@BMitch actually I suppose I was just guessing, I'm not sure what type it is. How can I tell? –  Will Sheppard Jul 22 '11 at 11:53
@Will - without either documentation or getting a physical look I wouldn't know how to tell. –  ChrisF Jul 22 '11 at 12:02
@Will: If it is thermostatic, then it's broken and you should replace it. If it's not thermostatic, it sounds like you want one, and so you should replace it. –  BMitch Jul 22 '11 at 12:53
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