I have a thermostatic bath / shower mixer tap running off a combi boiler, I have had It few years now, the temperature is set to 38 It runs fine nice steady temperature for having a shower, but I can't make It run colder, I can get It hotter but not colder, I can press the red button down and turn temperature control anti clock wise to make It hotter, but when try to turn It clock wise It will only move a little, It won't turn all the way to make colder.
Sounds very much to me as though the hot and cold pipes on the thermostatic valve are fitted the wrong way around. It's normally anti-clockwise for cooler and the red button is a safety button so you cannot accidentally turn the temperature above 38 degrees, it's certainly not to make the water cooler.
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