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I Have a pumped pressurized system on both hot and cold pipes. Because there is a considerable distance from the hot water tank to the kitchen and the amount of water drawn before hot water reaches the tap, I have fitted a Santon vented undersink water heater together with a Heatrae Sadia monoblock vented mixer tap as per the instructions. The problem is that I cannot get hot water out of the hot side of the tap. Hot water will come out of the cold side mixed with cold water when the hot tap is opened together with the cold tap. What am I doing wrong? Also, to what do I connect the vent pipe from the tap. Thanks

Now I could be misunderstanding the description here, but the first comment I can think of is with "I Have a pumped pressurized system on both hot and cold pipes". Actually, it is not possible, can be dangerous and very probably a health hazard to pump a pressurised system. Pumps must only be used to increase the flow in pipework fed from a reserve of water such as a header tank - in effect, a low pressure system.

You don't describe what type of pump/s is/are used. Twin impeller pumps for example are fine for showers where simultaneous flows of hot and cold are required, but won't work when feeding taps where only hot or cold may be chosen. This may explain why you have no hot?

Success will also depend on where and how any pump is installed. Really more detailed information is required.

Vent pipes would normally be connected to a drain via a tun dish.