Loft shower controls hot water for house?!

7 Oct 2022
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United Kingdom
Hello. So I’ve recently moved into a 1900’s terraced house with a loft conversion from 2001.

The shower in the loft is either scorching hot or Luke warm. It also had problems with being jammed stuck until I loosened it with WD40. However, as it’s the loft, it’s not a priority (I imagine a problem with the mixer?).

The weird thing is that when left luke warm position (and not in use at all), the main 1st floor bathroom (bath and shower) will only get Luke warm also. When left hot, the water is fine as the main bathroom mixers appear to work.

Any ideas why this would be the case and how this would be fixed?! Any help appreciated, mind boggling!
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Water is passing between the hot and cold via the mixer shower.

Installing non-return valves in the supply to it is a possible fix. A better one is replacing it with a new mixer that has those built in.
Do you have a combi boiler or stored hot water? What type of shower is it?

It does sound like crossflow, how do the other hot taps behave? If the supplies are balanced then there shouldn't really be cross flow between the supplies but can depend on shower type.
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Thanks all. It’s a combi boiler on the first floor (loft is the second floor). Shower is a thermostatic mixing valve.

Think the problem is to all taps and water, and potentially even more pronounced in the ground floor kitchen (water is barely Luke warm).

So am I hearing the solution is fixing a new shower and mixer in the loft? That should stop the cross flow if has a built in non-return valve. Are there any likely side effects to this? E.g lower pressure on the floor 1 bathroom

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