Banging noise from water pipes

[petek]

This is my first attempt at posting a question so apologies in advance if I make a mistake or offend anyone!

My son has recently purchased a small Victorian terraced house. He has had the central heating overhauled, and a condensing combi fitted by a reputable local plumbing firm. While they were doing the job we asked them to disconnect the bathroom washbasin and fit 2 tails with isolating valves so that the washbasin could be removed and replaced later after we had the wall behind replastered and tiled. This was done and we got on with the replastering and tiling. When we came to refit the basin, we decided to move it along the wall about 18 inches as this would give more space in the bathroom. I turned the water off at the main stopcock in the cellar and extended the pipework using plastic

15mm pipe, push on fittings and flexible tap connectors with isolating valves. We then turned the water back on and all was well, no leaks! Unfortunately we now get a banging noise whenever we turn off either the hot or cold tap. I understand this is some sort of pressure wave travelling back down the pipes which are probably poorly fixed at a location somewhere under the floor, and are probably caused by the relatively higher pressures in the combi system (but I don't remember the banging occurring after the combi was installed and before we had refitted the basin) . To avoid taking up all the floorboards and trying to trace the poor pipe fixing, can we just reduce the pressure by turning the main stopcock off a bit? If we do this will it c*ck up the combi boiler?

TIA Petek

[Tim Mitchell]

In article , petek writes

You could try it, it won't hurt anything (although it will reduce your water flow rate if you turn it off too much), and try playing with the isolation valve settings on the taps as well (if it's just those taps that do it).

[Christian McArdle]

Turning down the stopcock will not reduce the pressure, only the flow rate. Most importantly, it does it for every tap, even those not causing problems.

The problem occurs because modern taps on a mains pressure system can very suddenly stop the fast, highly pressurised "slug" of water travelling down the pipe. This slug then compresses into itself and bounces back, sending pressure waves through the system.

There are a number of solutions.

1. Don't use flexible tails. They can resonate.
2. Turn down the isolating valves to the taps. This reduces the flow rate and, hence, the maximum speed (and kinetic energy) of the slug of water that you're trying to stop.
3. Install water hammer arrestors near the taps.

The last solution is the "correct" one. It is basically a tiny pressure vessel, usually spring loaded, rather than air balanced, that simply absorbs the pressure wave from the slug of water over a longer period, making the shock much longer in duration, but, more importantly, much lower in magnitude. It needs to be close to the tap for maximum effectiveness.

See part 11355 at

[petek]

Thanks Christian, and Tim also.

I'll try turning off the main stop c*ck a bit first (forget to mention that the banging comes from all taps not just the washbasin), then if that doesn't work I'll try the isolating valves on the basin taps. Don't fancy the arrestors as they cost 10 quid each and presumably you would need one near to every tap in the system. I don't think resonance is the problem but I'm reluctant to change the flex tails in any case

- we've got the kitchen to do next job and I will probably used them on the kitchen sink also as thay make the plumbing so much easier for an amateur! I'll let you know how we get on.

[Winged Cat]

If it occurs on all taps it sounds like recent plumbing work, perhaps involving a drain-down, may have introduced a "Deadleg" into the system, i.e. a length of pipe going nowhere with air trapped in it. Then whenever taps are turned off, air gets compressed in the dead leg, causing some movement and hence the banging noise. Might be worth finding deadlegs, or indeed opening all rarely used taps in the house (garden hose?) to eradicate air.

I do not think turning down the main stopcock is a sensible solution.

[pknw29683]

Can you explain why this is not a sensible solution? I was only going to try half a turn at atime until the banging stops.

[Tim Mitchell]

In article , [e-mail] writes

Because you might have to reduce your flow rate unacceptably before it has any effect.

On the other hand, in my old house just a small adjustment fixed the problem. It is worth a try. If you have to wind it right down then obviously you will have to look for another solution.

[Winged Cat]

Well if you want a proper solution to the problem you really want to work out what is directly causing the banging and then rectify that. Turning the stopcock down MAY reduce or disguise the problem, but you might open yourself up to other annoying plumbing phenomena, such as not being able to have a shower whilst someone else is running the tap at the kitchen sink without being burnt alive or starved of water.

[pknw29683]

Thnaks for all your replies and just to let you know how we got on (and for the benefit of others with the same pronblem). Delay in replying is that my son lives in Sheffield and I live in Lancashire so have only just visited him again. I turned down the main stopcock about 1/2 turn and Bingo! - no banging when we turn off the taps. It might not be the "proper solution" but it works and so far no knock on effects. Thanks again to everybody for their help.