Could shower mixer tap be leaking cold into hot flow?

3 Feb 2014
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United Kingdom
Hello all. We have a non-condensing combi boiler that provides very hot water very quickly to our kitchen tap.
However, the shower, even when the mixer tap is turned right over to 'full hot' is nowhere near as hot, and takes far longer for the hot water to come through.
The boiler is in the kitchen, and the shower is next to the kitchen - on the same level - though much further away from the boiler than the kitchen tap.
We don’t need the tap water to be so hot but we have the boiler turned up to get the shower water hot enough – which obviously seems a bit of a waste.
Could it be that the mixer tap on the shower is not fully blocking off cold water even when turned to the full-hot position? If so how do I check that (as a complete DIY novice).
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hello sorry, still no picture, can't find USB cable!
What is 'the cartridge'? Is that a component in a thermostatically controlled shower mixer? How do I tell if I have one of those and not just a basic mixer?
When I take the tap fascia - which is a circular dial - off the mixer, behind it is something that I would describe as a 'cartridge': a plastic cylinder about the size of a large battery sticking out perpendicular to the wall, with a space around it.
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The mixer tap looks a bit like this:


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