Methven NEFA NRV 76 pressure relief valve, noise and gush when turning off tap

[oligeandlexp]

A few days ago we had a pressure release valve installed in place of the open vent on our hot water cylinder as we no longer have a wetback and I want to remove the vent pipe as it tends to track water in through the roof. However, turning off a hot tap will result in a woodwind tone and associated gush from the overflow as pressure is released. There is a mix of mixers with independent hot and cold supply from the same or co-located header tanks, at least one that takes cold from the inlet pipe for the hot water cylinder, as well as standalone hot taps. I contacted Methven, and this was their tech's response.

I assume there is no longer a wetback installed. Wetbacks (uncontrolled heat source) must be open vented.

As for the woodwind section tooting a tune... if they have mains pressure to the sink mixer, it is possible they are experiencing back pressure from the cold water. This may be over-pressurising the cylinder as it back feeds down the hot line, in which case the valve is doing exactly what is should - relieving excess pressure. This will be evident by a gush of water out of the drain pipe, particularly when using the sink mixer. it would have been happening before, just the gush would be out on to the roof. Resolution - install a non-return valve on the cold supply to the sink mixer.

If it is just thermal expansion...no it shouldn't be whistling Dixie. depending on hot water usage, they might expect between 2-4 litres per day of thermal expansion. This is not wasted water per say, just the change in volume when water is heated.

As this happens on taps that aren't mixers, I'm not sure if non-return valves on the other lines would help - and I'm not sure how a non-return valve on the cold line will stop back-feeding into the hot? You'd think that would require a non-return on the hot line? Any thoughts, similar experiences or accounts of resolution of similar problems?

 

[Madrab]

Unfortunately this site is primarily based in the UK with UK specs and regs. You would not be allowed to just cap the vent of an open vented HW cylinder with a Pressure Relief Valve (PRV), therefore your issue wouldn't be seen here.

If you are getting pressure spikes on the HW circuit then there has to be a mechanism whereby there is a hydraulic shock being created sending a pressure spike back up the HW circuit. Are these taps 1/4 turn ceramic and what is the limit on the PRV?