Slow leak

scottiecb19 · May 18, 2015

Hi All,

I'm having a major panic about a leak on the HW side of my CH system.

We recently had some work carried out by a plumber to replace the pressure reducing valve and pressure relief valve on the cold water feed to the HW system.

Over the weekend I noticed a wet patch appear on my kitchen ceiling underneath the HW cylinder. On closer examination, water was running through the HW run-off, as I could see water flowing through the tundish. Due to corrosion, water was not flowing through the tundish but out of it onto my floor. I understand this needs to be replaced and have come up with a temporary fix using a water bottle to stop further water damage.

However, I am concerned that water is coming through the pressure relief valve. Surely if the pressure reducing valve is working properly, the pressure relief valve should only open if there is an excessive pressure build-up in the HW cylinder. However, even with the cold water supply isolated and HW taps open, the valve continues to leak until, presumably, the water pressure reduces to nothing - then the leak stops.

Big problem is the plumber won't respond to my calls. I don't hold out much hope of assistance, seen as he has failed to turn up to another job he was supposed to do.

I'm just wondering whether it is a fault on the system or just a faulty valve.

Any thoughts/comments would be appreciated!

steadyon · May 18, 2015

Unvented cylinders require either an internal air reservoir or / and and external expansion vessel. As the water heats and pressure rises, the "bubble" or vessel accommodates the expansion by allowing the air inside to compress, water of course being virtually incompressible.

If you have the type (e.g. Heatrae Sadia Megaflow) with an internal air bubble, these tend to lose the air over time. The bubble can be simply re-formed, and the MegaFlow type has the instruction printed on the side of the cylinder.

If it is one with an external expansion vessel (a white or blue metallic tank around the size of a football) the vessel might have failed, or might need re-charging on the air side. Probably best to get someone in to do this if needed.

Other than re-forming the air bubble, any maintenance work on these cylinders must (legally) be carried out by a qualified plumber holding a current "G3" certificate. Because they are pressurised, all safety devices must be in good working order to avoid catastrophic, and potentially lethal, failure.

scottiecb19 · May 18, 2015

Hi,

Thanks for the reply. I have checked to see whether the pressure relief valve on the tank is the cause of the leak, but I do not believe it is as the water flowing through the tundish is cool.

I do note that with the cold water inlet feed isolated, the pipework from the HW cylinder (I do have a Megaflow system btw), becomes warm, suggesting a backflow of warm water from the cylinder to the suspect offending valve.