Have a 9 litre hot water tank in my caravan, was on a manual foot pump. Have fitted a pump 14psi on 1/2 pipe. Cold is fine, hot has a non adjustable PRV on the inlet, but on running hot taps water pours out the overflow

Let's see if I understand your problem. You replaced a manual foot pump with a electric one that cuts off at 14PSI. Correct? Please explain what you mean by a non adjustable PRV , And why in the world would you need a Pressure Reducing Valve on a system with only 14PSI. What am I missing here?

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on running hot taps water pours out the overflow

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What overflow? The sink? Or do you mean the T&P valve on the heater? Can you repst and explain some more? Thanks Tom