Pipe shock - banging when turn off taps

W/Ms are bad for this as they shut off very quickly, whereas taps are usually shut off progressively - so not such a shock.

Anyhoo, turning down your mains stopcock doesn't affect the static water pressure, only the dynamic flow. But that's another story...

As you suspect, it's a case of a PRV to reduce the pressure properly (which you don't want) or shock arrestors. Or, locating the miscreant pipe(s) and clipping them further).

Not sure about the SAs - I think it's as you say, located as close to the cause as poss (eg W/M) but I doubt they are are critical.

Someone will advise on that, or it'll be on the 'net somewhere.

(If your mains pressure is really high - say 5 bar - then a PRV might be a good idea anyway as this can cause quite a bit of stress on all you water bits. You could tweak it down to, say, a compromise 4bar which would still be higher than most houses, and should be pretty decent.)

 

Pressure and flow are related - connected - but are different things. If you were to turn down your mains stopcock a significant amount, then it almost certainly would prevent the 'shocks/bangs' - but by then your flow would have been reduced by an unacceptable amount as a result.

And, when every tap has been turned off again - when the water has stopped flowing - the pressure in your house would go back up to whatever it is now. In other words, you'd have reduced the flow but not the 'stopped' pressure. Yes, when you then open a tap, the pressure on the house side of the partially-closed stopcock will drop, because it cannot be maintained by the reduced flow. So, with a partially-closed mains stopcock, as you open and close a tap in the house, the pressure in the house's pipe will alter dramatically, from very low with a tap fully open, to right up to full mains pressure when shut. If you open two taps simultaneously, the pressure on the house side will drop will be affected even more dramatically, and flow reduced as a result.

This is why partially-closing off a mains stopcock in order to control 'pressure problems' is a very crude and unsatisfactory method.

The whole idea of a PRV, on the other hand, is that it will deliver a full flow as required, but never let the pressure at which it's being delivered rise above a set value, usually around 3bar-ish. So if you open a tap a wee bit, you'll get great flow and it'll be at 3 bar. If you open it up fully - or open a second tap at the same time - you should still get a great flow, and still delivered at 3 bar (in practice, the pressure will drop a bit).

So, the correct solution to problems caused by excessively high mains pressure is to fit a PRV.

And you measure mains pressure using a... pressure gauge

Ok, I think you are right - you will also be able to sort some of your shock issues by 'isolating' the individual causes - you say there are isolating valves on the main protagonists - the toilet (which I guess has a Fluidmaster-type valve that shuts off 'instantly'?) and the W/M (which we know does this.) So, yes, if you turn these isol valves down enough to reduce the water flow in to these appliances, then the shock effect of them shutting off should be reduced as well.

(The reason for this is, the water on both sides of the isol valve will be at exactly the same pressure when the appliance (toilet or W/M) is 'off'. However, when you operate the appliance, that end of the water pipe is 'opened' allowing the water out. The pressure on the appliance-side of the isol valve will therefore fall dramatically as it's not being 'held back' any more, and the pressure can no longer be supported from the mains as you've reduced the flow to it by that partly-closed isol valve. Then, when the appliance is 'full' and they shut off, the pressure in that pipe between the isol valve and the appliance doesn't 'snap' up instantly to full mains pressure any more (causing that shock) because you've partially closed that isol valve. Instead it'll take a fraction of a second to do so - to balance out either side of the isol valve - and this will act as a 'dampener'.)

You aren't still reading this, are you...?

So, by all means try your trick; close off the isol valves until you can notice a difference - flush the loo and start to tweak off that isol valve until you can hear the flow reduce a wee bit. Then see if it still 'knocks' when it finally shuts off. If it does, try a further tweak closed...

You can try ditto with the W/M.

But bear in mind that actual taps will still need shutting off slowly by hand if you don't want the shocks caused by them - unless they have isol valves too?!

This is a compromise 'solution' tho' - you will have reduced flow to these items as a result. Whether that's a problem will, I guess, come down to which items are involved.

It's hard to tell what the best proper solution in your case is, PRV or shock arrestor. I mean, if your mains pressure is stupidly high (5 bar+) then I'd say you definitely need a PRV for the sake of your system. If, however, your mains pressure is very good but not stupid (~4bar) and the 'shock' is actually mainly caused by loose or not-well-clamped pipes, then a shock arrestor might be best ('cos you don't really want to reduce your pressure down from 4 that bad...)

By the way, I really need to get a life.

 

Good lecture there DA.

In short, measure the water pressure, if it is high fit a PRV.
You do need to sort the knocking noise though. Eventually it will lead to some form of failure which means a potential flood.

So either a prv or get the floors up, find where the pipes are knocking and re-secure them.
PRV it is then.

 

that'll be 1/4 turn then

 

Wow, that's high indeed.

Keep us posted.

If 3 bar sorts your issue, you could always tweak it up a bit more if you feel you have lost flow, especially when two taps are opened simultaneously. But if flow seems ok at 3 bar, then keep it there.

If the shock hasn't disappeared completely, then you may also need a shock arrestor. The PRV won't have been a waste, tho', as 5+ bar is really too high and will be putting unnecessary stress on yer bits. You'll probably even find that ceramic quarter-turn taps are more stiff in operation due to the pressure pushing the discs tightly together, and they'll almost certainly suffer problems sooner than they should.