How to cure Water Hammer (on combi system)?

Discussion in 'Plumbers' Talk' started by dan1, Jan 27, 2012.

  1. dan1

    dan1 New Member

    Hi - recently moved into an old house.

    When we first moved in we could hear a bad water hammer (clanging) on the pipes upstairs whenever we turned off the taps on the sink downstairs - it was louder the quicker you turned off the taps - it didn't occur anywhere else.

    However, we had a combi boiler fitted soon after, and this seemed to co-incide with the problem disappearing. It has all been quiet for a year - until now.

    We have just replaced the bathroom suite. Now when turning off any of the new taps on the shower (lever type), bath (1/4 turn), sink, flushing the toilet (when it finishes refilling) and when using the washing machine, the water hammer is back with avengance.

    We have secured the pipework with clips in the bathroom floor, and, to the extent that is possible, under the boards upstairs in the hallway. The clanging is so bad that we could even feel the pipework move right back to the front door by the stopcock where it enters the house.

    So, my question(s).

    I've read a bit about Water Hammer (shock) Arrestors

         1 - Can they be fitted with a combi boiler system?
         2 - Can anyone recommend one? - might this work for example

    I have also read that changing the main stop cock to a lever type can help - why would this be?

    Thanks in advance for your help!
  2. Hi Dan.

    Hammer is caused by a moving part shuddering under the flow of water - that is usually a washer-operated device such as found in a mains stopcock. Often the overriding cause is high mains water pressure - any idea what your water mains pressure is like - does the water come out with considerable force?

    Anyways, the cure is down to the cause. Sometimes it can be a matter of securing the vibrating pipework - if happens that that was all that was allowing it to happen. Better is to find the vibrating component - such as the offending washer. So if you fancy trying to swap your mains stopcock with a ball-lever type then that could well cure it.

    (From your description, it seems clear that it isn't ONE single thing that can trigger it, so we can - I think - discount it being caused by one loose washer or part in a tap or toilet in the house - it must be down to an independent part that's common to all the water system - so the mains cock is again likely.)

    Will a shock arrester fix it? Very likely. Can it be used with a combi system? I believe so - I can''t see why not.

    If the 'cause' is very high mains pressure, then another solution could be to fit a PRV (pressure reducing valve) to lower the mains pressure to, say, 3 to 4 bar.

    Things to try - turn the mains stopcock off, and then open it up just, say, 2 full turns. See it that makes a difference. Or, get your system vibrating nicely, and then slowly shut off the mains cock - see what it feels like as you do this; can you feel the vibe? Try and get a sense of whether it's this stopcock that's at fault.
  3. graceland

    graceland New Member

    yes i thought a shock arrester almost HAD to be put in with a combi ?
  4. HOTDOG ø

    HOTDOG ø Active Member

    Twaddle! It wont hurt to fit one but it is only masking the cause.
  5. graceland

    graceland New Member

    I didnt say it was causing it *?

    Message was edited by: Screwfix Moderator
  6. Supra

    Supra Member

    We've also got a problem with water hammer, the vibration actually travels through the shared water main into the house next door.

    These quarter turn taps aren't good as they shut of the water so quickly, the same goes for these compact plastic float valves in the cistern.

    Obviously if you turn off the quarter turn taps slowly you won't get the water hammer.

    I replaced the cistern float valve with the old fashion brass valve and arm as they're much better in reducing the hammering . You can get them for high and low pressure (low pressure if you have a cold water tank).

    We tried a shock arrestor like the one in your link, it did work a little but the noice was still there. We bought it at Toolstation, not stocked anymore but they're selling a different version which works on the same principles. It's called a "Mini-Rester". The bottom one is made by Sioux Chief, not sure about the top one.


    You could install as many as you like on the cold water pipe (close to the offending taps/toilet etc as possible).


    I made a similar device from a vertical copper pipe which traps the air so it cushions the vibration but it has to be drained every so often unlike the  "Mini-Rester" which is maintenance free.


    Message was edited by: Screwfix Moderator
  7. It's because the pipe work isn't clipped correctly
  8. petertheplumber

    petertheplumber New Member

    All pies should be supported at regular intervals 15mm every meter of length, but few plumbers do it since it takes longer to do the job and diy are the worst offenders, clip the pipes or live with it.
  9. diymostthings

    diymostthings Well-Known Member

    ".....and diy are the worst offenders,...." I resent that statement Peter!! a lot of DIYers take care and pride in there work and try to keep to the latest regs. They havn't got time/money restraints like a pro, who needs to rush to make a living! Obviously there ARE DIYers who are careless but - I would argue that there are a lot of pros who are too. A quick look at the plumbing in new build might convince you.

  10. petertheplumber

    petertheplumber New Member

    May convince me. We are all entitled to our opinion and I have seen enough DIY mess up's to stand by what I said. End of.
  11. Crowsfoot

    Crowsfoot Well-Known Member

    100% agreement here Peter - did I tell you about the bath that was fitted over an old shower tray?
    The DIYer was too idle to knock out a new waste hole and on pulling out the bath plug it discharged into the shower tray beneath it !!
    All was well until the shower tray waste got blocked up with hair and flooded the bathroom !
  12. petertheplumber

    petertheplumber New Member

    That sounds about right Crowsfoot.
  13. pra

    pra New Member

    I don't have water hammer ( just a little 'flutter' in the thermostatic valves ). Anyway I had a flood when the plastic pipe coming into the house came away from the internal stoptap immediately above it. My plumber sorted it, suggesting that it was due to a badly installed olive in the connection. That may be, but I wondered if it finally gave way during or just after running the washing machine or toilet. We have high water pressure, and in the washing machine the water input goes on/off/on/off etc., which must cause some variation in pressure ( but not quite water hammer ).
    Might my solution here be to lower the pressure in the quarter turn outside stopcock, or leave the kitchen tap dripping to damp down the pressure extremes ?
  14. Dr Bodgit

    Dr Bodgit Well-Known Member

    I don't think that will have much effect if any as closing the stopcock doesn't reduce pressure, it reduces flow (albeit pressure will reduce more substantially as flow increases).
  15. pra

    pra New Member

    Thanks, I think that I can see your point. Would putting in a pressure regulator achieve the desired effect ? These seem to have some sort of valve controlling what comes out ?
  16. Dr Bodgit

    Dr Bodgit Well-Known Member

  17. My Gas Fitter

    My Gas Fitter New Member

    it is advisable to install a shock arrestor/water hammer on the incoming cold water supply.
  18. pra

    pra New Member

    Yes, its going to be difficult - I'll have to carefully dismantle the nice tiled panel hiding the pipes. But that's my problem.
    I wonder if anyone else has had a problem where a plastic pipe goes into ( in my case ) a brass fitting. I'm old-fashioned, but brass and copper have lived together for decades. Is the surface of plastic just that little bit 'slippy', and the pipe just that little bit compressible, even though it fits snuggly ? I have read elsewhere of inserting a pipe stiffener. I'll have to ask my plumber if there is one there.
  19. rogerk101

    rogerk101 Well-Known Member

    I've cured water hammer in the past with a drink-water-safe expansion vessel. I've also had success by just installing a capped vertical pipe anywhere in the water system. It must be capped while empty, and then when you turn the water back on a column of air gets trapped in the vertical pipe and acts like an air cushion, which allows the shock of 1/4 turn taps to be absorbed by the cushion of air rather than juddering the pipes. The expansion vessel is probably best if you can find somewhere to hide it, but the vertical pipe of air also works.
  20. pra

    pra New Member

    Thanks - another good idea - that last one may do something like my idea of leaving a tap dripping.
    But if I can find space to fit it in, I may be best with the shock arrestor

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