I did a bit of research and thought this would be the right one for my system but evidently not.
The hot is provided by a brand new combi boiler. The cold I am less sure about. I live in a fourth floor 1930s flat and there seem to be two downpipes providing cold water: one which serves the WC and kitchen, where both cold and hot pressure and temperature mixing are fine, and the other which serves the bathroom (bathroom and WC are separate). In here the cold water pressure is low, and I don't know whether the supply is likely to be tank or mains supply.
As a result, the shower mixer is totally ineffective. The water heats up to an unusable temperature, and the cold does absolutely nothing to balance it out, even on max. By slowly turning down the hot you can eventually find a very small sweet spot temperature-wise, but by that point the pressure is piddling.
I'd rather not have to attack a newly-installed bathroom but the current situation is not really fit for purpose. A plumber I got out to check over the quality of the original installation quoted £500 for installation of a thermostatic mixer.
Very grateful for anyone's ideas on why the downpipes might carry cold water at different pressures (if this is indeed the case) and what the options are for resolving this.
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