I am having problems with the hot water mixer and shower in the bathroom. The pressure on said lines is very low. The cold water taps have normal, good pressure. The hand basin hot water tap has normal pressure. This problem is only with the mixer (hot) and now the shower. The hot water pressure in the kitchen and elsewhere is fine.

Most likely there is dirt in the mixing valve port. Turn off the water, remove the cartridge, clean the ports and flush the line. Then reassemble.

Did it just start happening ? Is it a new shower valve ?

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Milo