Air in pipework/water hammer

[terry.shitcrumbs]

The plumbing at a friend's house produces a loud, rhythmic knocking noise, of diminishing amplitude, over 2 or 3 seconds, each time a tap is turned of f. Is this "water hammer"?

My first thought was that there was air trapped in the system and that when the pipework goes from mains pressure (all outlets closed) to sub-mains pr essure (one or more outlets open), then back again, any trapped air would b e subject to considerable expansion and contraction in volume, with a high degree of elasticity on the final, abrupt return to mains pressure. It seem ed to me that this was the most probable cause of the noise, with slugs of air likely to be trapped in the upper reaches of the system, where there ar e dead end pipes dating from replacement of gravity-fed CH with combi and o ther assorted works.

Reading a little about water hammer, it seems that contrary to my suppositi on described above, air can be used to suppress water hammer.

Am I using the wrong term to describe the phenomenon at my friend's house, but with sound reasoning as to the cause, or am I using the right term, but wrongly suspecting trapped air as the cause?

Cheers.

Terry.

[Chris Hogg]

You are using the right phrase, and you're wrong to think it's due to trapped air. Quite the opposite, in fact. Trapped air, in the form of a specially designed air reservoir, is often suggested as a cure for water hammer. The air 'bubble' absorbs the sudden shock of the water flow being instantaneously brought to a halt. Several suggestions for your friend, in no particular order: (1) Turn taps off more slowly, so that the water flowing in the pipes doesn't come to a sudden halt. (2) Fit a hammer suppressor to the piping system. (3) Turn down the main stop-tap to limit the velocity of the flow of water in the pipes (in my own case, I actually found that opening up the stop-tap cured the hammer; paradoxical, and I offer no explanation!).

See

and the ads for hammer arrestors on the RHS of that page. Also, the discussion yesterday on this NG 'will water hammer be an issue'.

[Chris Hogg]

I should have added, that sometimes the hammer can be caused or made worse by the ball float valve in the cold water tank in the loft, bobbing up and down for a moment or two as a result of the shock of suddenly turning off a tap. Replacing the rubber washer in the valve can help, or possibly fitting some sort of damper to the float.

[Andy Burns]

Ads?

I need to do something about water hammer, but it's not coming from within my own house, a couple of years ago neighbours (who have converted to combis) started complaining about low pressure showers during the morning "rush" and eventually the water board upped the pressure.

But now I get a banging like a shithouse door through the plumbing (actually when it first started happening I thought it *was* my garage door slammimg in the wind) presumably from neighbours flushing loos and the valves on washing machines and dishwashers etc.

My supply isn't on a shared pipe, separate stop-c*ck in street about 30' from the one in the kitchen, given the shockwave can travel at least twice that distance from a neighbour, what chances for a little stub of

3/4" air filled pipe suppressing it?

[Chris Hogg]

Perhaps you've got an ad blocker, but on clicking on that tinyurl link, they come up across the top of the page for me, but they were down the side on the original page I viewed.

In case you don't see them, here are a few tinyurl direct links. Don't know if any of them will work in your situation though.

Have you tried simply turning down your main stopcock?

[fred]

In article , Andy Burns writes

The likelihood is that the air in the stub will be re-absorbed into the pressurised water (although some pressurised HW cyls do use unprotected bubbles for expansion space).

My suggestion would be one of these placed just after the stopcock and turn the stopcock down a tad if possible.

Item 11355 about a third of the way down. Item 20923 is a different design claiming to do the same thing.

is a new one to me, American design and claims not to just be an inverted pipe.

All under 20quid so worth a go. Location at the source of the problem (your incoming main) is important. The action is the equivalent of a transmission line parallel terminator.

[Brian Gaff]

Interestingly, I only encountered this when a new washing machine was fitted, which assumedly had a solenoid valve from the cold main. The turning down of the stopcock a little cured it completely. Now however I cannot move the stop c*ck.. Life is just not fair. Brian

[Andy Burns]

I think I'll give the "piston" one a go ...

[fred]

Please let us know how it goes. In a way, the piston one could be superior as the friction of the piston may provide a degree of damping whereas the ball ones just have the friction of the water in the pipework plus the 'capacitor' of the air diaphragm. I can't remember whether the ball one I fitted to a CH system had any restriction on the inlet flow that could be considered an extra friction component.

[Andy Burns]

OK, that was this morning's little job, nothing tricky apart from opening the pipes far enough to insert the 'T' as the top pipework is constrained where it passes through a wall, out of shot.

Previously I could only generate a small thump by judicious closing of my own taps, after fitting the mini-rester I can't even generate that, so it it working to some extent.

But the really annoying clangs that made me decide to fit it were coming from somewhere outside my house, so I'm at their mercy, and me noticing them (or hopefully not) over the next few days ...

[Andy Burns]

Just to wrap-up, this has been a complete success, gone from several annoying clangs per day to none at all in a week.

[Geo]

Thanks for the update - was going to ask.

We have the same problem here since nextdoor had new downstairs toilet fitted (and possibly washing machine) I suspect off the mins supply. We have independent stopcocks on the pavement 30 feet away. I have added extra pipe clamps to the incoming main up to the loft and ensured not touching any joists etc.

I bought one of those devices but have not fitted it yet as:-

1. our internal stopcock will not shut down completely
2. the pavement stopcok will not turn (crutch key bending)
3. our pipework above the stopcock is enclosed in plasterboard trunking
4. it is 3/4" stainless tube.

Wondering if it would work if fitted further away from the entry point e.g. in the loft where I have some 15mm copper to the CH expansion tank. Alternative would be on the 15mm copper to the outside tap.