I had a leaking overflow pipe from my hot water cylinder. I had the pressure relief valve replaced(?), the valve that takes the mains pressure and reduces the pressure before feeding it into mains pressure cylinder.

It is still leaking, but is a hell of a lot better. Plumber has said that it is supposed to drip but not run. I did a rough measure and it filled up a small cup over night.

Can anyone advise?

EDIT 1 I have since had the pressure relief valve replaced, but still the same dripping.

Reading here

If the overflow is dripping this is most likely normal expansion, that occurs when a hot water cylinder is in a heating cycle. It is normal for a 135L cylinder to expand up to three and a half litres of water in a heating cycle.

So it could be normal expansion. The way I understand this is that if the cylinder is filled up with cold water, it come to a point where it is full and no more water will get in. But then as the water gradually heats up the pressure increases so the pressure relief vavle will kick in and slowly release water to reduce the pressure. This I think is what I am seeing with mine

The question is how do i confirm this? In theory if the HWC is filled up and heated completely, then if no hot taps are turned on over a period then the HWC should not drip.

If hot water taps are turned on then the HWC will fill up. the HWC will then have to heat this new cold water up again. So you might get some dripping in this example. I guess it depend how long the hot taps were run for or how much water the HWC released.

  • 1
    A properly working hot water tank is not supposed to drip. I'd get a second opinion from a different plumber.
    – Grant
    Oct 19, 2015 at 1:33
  • 1
    Not sure I would trust that site after reading "tempering valve be installed. This allows your hot water cylinder to be set between 60 - 65°C, but the water to be delivered anywhere from 35 - 70°C." How exactly do I get 70C water by mixing 65C+cold water?
    – Grant
    Nov 2, 2015 at 3:23

1 Answer 1


I have to say, similar to what @Grant said in comments, that an overflow valve is not supposed to have a normal drip. In normal usage your drip/overflow line should be bone dry.

The plumber that tried to tell you that a drip is normal is not to be believed. It is also likely that the plumber was far from professional if they just said that to cover up noting the drip right after the installation.

The pressure relief valve is a safety device designed to keep the hot water tank from exploding if the internal pressure builds up too high due to the heating source failing in an always on state.

  • tks, could it be then that the thermostat is broke? or is it the pressure release valvle(?), the valve that opens when the pressure gets too great in the clylinder to let water out through the overflow pipe?
    – HattrickNZ
    Oct 19, 2015 at 18:34

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.