I have just fitted a new mono bloc mixer in the sink and a new mixer in the bath and the hot pressure is a lot lower than it was with the old taps. There is a 22mm hot feed into the bath and a 15mm feed to the sink. I have used flexible tails to connect to the tap. They are both 1/4 turn ceramic taps. I have got a gravity fed, electrically heated hot water tank.
I assume your cold feed is mains pressure as you report slow hot feed only.
I would guess the problem is with your flexy connections. Standard flexys are only 10mm bore, regardless of the size of end fittings. This does tend to restrict flow somewhat! You can get larger bore 13mm, which is an improvement. Another problem can be with monobloc mixers, most of which have 12mm connections at the end of 15mm tails. If you have mains cold feed, with mixers, you should fit check valves to prevent possible back flow contaminating the mains water. (See water regs) Also check the spec of your new tap fittings - a minimum 'head' should be specified. Does this exceed your available head?
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