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I have a 2 year old Worster Greenstar Combi at the back of my flat and a thermostatic shower at the front of the flat. Last winter and now again this winter when I have a shower, the water is nice and hot, but then goes cold, and then gets hot again while I am showering. There are no taps being used or the central heating when the water temperature fluctuates. The boiler and shower work fine the rest of the year. The expansion pipe is not frozen ( it was 6 deg C outside) Does anyone have any suggestions? Its not much fun dipping in and out of the shower first thing in the morning.

katiesdad,
Suggest you replace the thermostatic cartridge in the shower and eliminate this as a possible cause.
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A friend suggested that it might be a problem with the ‘modulation’ of the burner on the boiler not reducing the flame, and therefore the heat output. So we tried running the hot tap slightly on the sink next to the shower, working on the theory that as there is more flow, so the boiler would not need to modulate and would stay alight. This worked, and a normal shower was had!!
Unfortunately the same procedure the next morning had no effect.
We tried just running a normal shower that evening, and there was no problem at all.
But the next morning, even with the sink hot tap running a bit, the shower ran cold, then normal, then cold again.
We are in the London area and there are always heavy lime scale deposits. The boiler is fitted with an inline anti-limescale device, but the shower spray head shows signs of limescale. Do you think that the thermostat at the boiler or the shower could be effected by this?

katiesdad,
Limescale is a problem in thermostatic valves and will cause the problems you have described. There may also be issues with the temperature of the cold supply to the shower valve.
You could try de-scaling the valve but this does imply taking the shower valve apart and acid cleaning it. My preferred action would be to replace the thermostatic cartridge - you then know you have all new components.
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You need to find out what is the problem, the shower or the boiler. If all other taps, and the bath can be ran and maintain a steady temperature then as stoneyboy says the shower is at fault.

If the water temperature to the taps fluctuate then it's the boiler