Knocking sound when turning on cold water

[ProjectPete]

When I turn on my cold water I get a loud knocking sound which I believe is to do with pressure and air pockets. Hoping someone has some tips to avoid having to get the pricey plumbers out.

I've tried turning off mains, expelling all water from the lowest point and leaving it open then turning the water back on (or something to that effect (sorry I did it a couple of months back)) and it seemed to help but not resolve and the issue seems to be worsening again.

[MitchellMc]

@redracer01,

I'm not sure about Pete's situation but I know @Ankitannex was certainly looking for a solution for the same issue recently.

This could solve their issue, many thanks for sharing.

Mitchell

[ProjectPete]

It became more intermittent over time so I ignored it then sold the house to my mother-in-law so I never actually solved it.

She ended up having a burst pipe in the roof space and when the plumber came to fix that she mentioned the knocking to which he blamed the high water pressure from the mains. He turned that pressure down and she hasn't had a problem since.

[micheleBB]

Had the same issue as well, had a plumber came and did install a water pressure valve in the main meter, still has not much changes. It did reduce the noise when running wash machine and that is it. Still noise when turning on some cold taps, flushing toilet, any advice or tips pls? 

[MitchellMc]

Welcome to the Bunnings Workshop community @micheleBB. It's wonderful that you've joined us and many thanks for your question.

You can try purging the excess water from your lines. The air in your lines provides a cushion that stops violent water hammering. Turn your mains water off and then flush the toilet and open all taps in the house to drain the water. Once this has been done, turn the taps off again. Turn your mains supply on slowly so it fills your pipes. Alternatively, you can try the opposite. Turn the mains off, open all taps and allow the water to flow out. Turn mains back on slowly until the taps all run freely, then turn them off.

You can also check to see if any loose pipes need to be fixed more securely or try changing the tap valve washers out to a different style. I managed to reduce some noise by installing Kinetic 12mm PVC Anti-Hammer Tap Valve - 2 Pack

Please let us know if that makes any difference.

Mitchell

[micheleBB]

Hi, Mitchell, thanks for tips and appreciated. Just a quick question: do I just need to run the cold tap water Or run both cold and hot tap water to remove the water hammer?  As I only have issues with cold taps. And any tips to check the loose pipes as they are in the wall? Thanks heaps! 

[MitchellMc]

Hi @micheleBB,

If you only experience the water hammer on the cold line, try purging with just the cold taps. If there are loose pipes within the wall, installing a pressure reducer or water hammer arrestor might be a less costly operation.

Water hammer is caused when a pressure surge or wave of water is suddenly halted by shutting off a valve. Air is compressible and acts as a shock absorber within the system. When the air leaks out, and only water is present, it slams into your valves when you shut them off and creates the knocking or water hammer. In older houses, vertical sections of pipe were added to the system to act purely as air chambers. Newer houses use water hammer arrestors which are cylinders that hold air and act as a shock-absorbing piston. 

Mitchell

[micheleBB]

Thanks, we had a plumber coming and installed a pressure reducer at front meter a couple week ago. It did reduce the noise when running the machine but that was pretty much. I will try to purge highest and lowest tap water out but still no changes; will do all cold water out this weekend and also to check the water hammer arrestors at Bunnings this weekend. Thanks!