Water hammer whenever any cold tap is turned off

[michael123123]

I'm trying to find a solution for a water hammer in my cold pipes. I get a bang every time a cold tap is turned off quickly (washing machine included). If the tap is turned off very slowly, I don't get the hammer.

Suggestions I get being when there is nothing running at all.

Interestingly(?), the hammer disappears fully for about a day whenever a fully drain and refill the system.

I measure a pressure of 76 PSI on my garden tap.

Does installing a pressure reducing valve after the stop cock have a role here?

Edit: now reading 82 PSI

 

[stem]

This is a result of a hydraulic shock when something is turned off quickly. Washing machines have solenoids that stop the flow of water immediately. When this happens, the shock wave resonates back down the pipe and creates the bang.

I had a similar problem and my first port of call was to find out specifically where the noise was coming from. I had a kitchen extension built 17 years ago and it was plumbed in using plastic pipes unsecured and resting on the ceiling. The shock made one of them bounce off the plasterboard and land back on it amplifying the noise. It was in an inaccessible position between walls and under the hot water cylinder so I slid some pipe insulation along it. This prevents the pipe moving and no more noise since.

Interestingly(?), the hammer disappears fully for about a day whenever a fully drain and refill the system.

You are probably introducing small amounts of air into the pipes that can be compressed and act as mini shock absorbers, one the air has all gone water doesn't compress.

I measure a pressure of 76 PSI on my garden tap.

That does seem a bit on the high side. I think from memory, mine was around 50 PSI, so it might be a contributing factor, but unfortunately I can't say for certain. Hopefully someone who can will be along shortly......it certainly won't help.

 

[oldbuffer]

1. 76 psi is around 5 bar, which is a fairly high pressure for a cold water main, although not vastly excessive.
2. I'd suggest fitting a pressure reducing valve (PRV) (e.g. Honeywell D04) after the main incoming stop cock.
3. If you do fit a PRV,:
3.1 Read the installation instructions before purchasing. For example, the D04 must be installed in horizontal pipework.
3.2 Make sure the stop cock is working, particularly if fitting a PRV with a pressure gauge. In my experience the gauges eventually leak and need to be replaced, so you must be able to turn the water off. If in doubt, fit a quality lever valve (e.g. Pegler) between the stop cock and the PRV.
3.3 I'd suggest setting it to an output pressure of 3.0 bar.

 

[michael123123]

Thanks for the very helpful replies.

Reading a bit more, I see that you can get dynamic (i.e. D04) and drop tight/static (i.e. D05) PRVs, whereby the dynamic valves will drift back to the full mains pressure when all taps are closed.

My house has an open vented F+E system - does it matter which type of PRV I select?

 

[michael123123]

Update: I installed a Reliance compact 312 PRV from Screwfix (this model is drop tight), just managing to fit it in the short space between the stop cock and the tee to the kitchen appliances.

I set it to 3 bar and I've been keeping an eye on it for the past few days. We've had no bangs in the pipes at all since installation. Success!