Which I am afraid tends to suggest your plumber doesnt know what cross over is, it only needs to happen at one thermostatic mixer and all taps are affected.his is a boiler issue and testing at all outlets indicate this as chances of all taps being broken is massively high.
You have been given the correct way to test it, close the boiler for cold water and open a hot tap if water continues to run even a small amount you have cross over simple
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