Banging noise from water pipes
 

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 cock up the
combi boiler?

TIA
Petek

In article .com,
petek writes
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 cock up the
combi boiler?

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).
--
Tim Mitchell

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 cock up the
combi boiler?

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 http://www.bes.ltd.uk/.

Christian.

Thanks Christian, and Tim also.

I'll try turning off the main stop cock 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.

"petek" wrote in message roups.com...
Thanks Christian, and Tim also.

I'll try turning off the main stop cock 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.

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.

In article ,
writes
(Winged Cat) wrote in message
. com...
"petek" wrote in message
egroups.com...


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

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.

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.
--
Tim Mitchell