Knocking pipework?

Shoxt3r

Hi there,

We're unsure if this is related to a pressure change within the area (we recently had Southern Water digging up the road and having to turn off the water for a short time to do some work) or something completely unrelated.

Within the last couple of months we've noticed that the pipework around the house will "clang" when turning off the mains water either via any of the taps or when using the washing machine -it's a sortof "knock, knock, knock" sound. The noise will continue for some time and then eventually settle down but it takes a long time to do so.

I've think I've narrowed it down to a particular pipe run which is situated next to some hardboard which someone decided to put in as a kindof false ceiling I presume as the boiler was moved from one of the bedroom cupboards into the roof at some point. When the taps/washing machine is run, my guess is that the pipe is knocking on this as there is a little bit of play in the pipe.

There are several joints to this pipe which runs:

• From the kitchen
• Into the cupboard under the stairs
• Up into the landing area
• Across the landing into a bedroom into a cupboard
• Up from here into the roof to connect via the cold water feed tank

I could of course just cut into the hardboard where the pipe is and it will stop at least some of the knocking but as far as I can tell it's also knocking further down the pipe in the cupboard under the stairs.

I asked the heating engineer who installed our boiler about a year ago to take a look when he did the boiler's annual service, but annoyingly we couldn't replicate the issue. We then managed to capture a video of the knocking and he recommended that we clip the pipes in place and also install a mini expansion vessel - he offered to do this for £160+VAT parts and labour but this doesn't seem to be a definite fix - just a "try and see".

Before I call in some plumbers to take a look and quote on any work I thought I'd make a post to see if anyone else has any ideas what may have caused this to happen and what possible fixes there are? As I say it's only been doing it in the last couple of months and there's been no change to our system since the boiler was installed.

Thanks in advance!

Ayr_Rage

Most likely air in a pipe, run all your taps and outlets for a while.

Turn down the main stop valve so that there isn't excessive pressure coming into you home.

twopenny

I tried this when one of the pipes started knocking on flushing the loo.
Eventually I turned the stop tap down just a tad and that did the trick. 
Think I increased the flow at the stop tap to run the garden tap better at some point and I'd increased the amount of water in the toilet cistern - it took a while to object.

Shoxt3r

Great thank you - I did wonder if it was air in the pipes. Will try running the taps and reducing the pressure via the stopcock and see if that helps.

If it doesn't I guess we need to get a plumber in to get a second opinion?

grumbler

Ayr_Rage said:

Turn down the main stop valve so that there isn't excessive pressure coming into you home.

It controls the flow, not the static pressure. it will remain hight when there is no flow. I can imagine some sound produced when flow starts and pressure drops.  If so, it makes more sense to open the valve fully to reduce the pressure change.

Shoxt3r

grumbler said:

Ayr_Rage said:

Turn down the main stop valve so that there isn't excessive pressure coming into you home.

It controls the flow, not the static pressure. it will remain hight when there is no flow. I can imagine some sound produced when flow starts and pressure drops.  If so, it makes more sense to open the valve fully to reduce the pressure change.

Opening the stopcock wouldn't increase the pressure by allowing more of it in then? Yes, the knocking/hammering only happens after the water has been shut off - either by the tap being turned off or the washing machine stopping its draw of water. It also doesn't always happen straightaway as there could be a few minutes after the water has stopped flowing before we hear a knock, sometimes several times over.

grumbler

Shoxt3r said:

grumbler said:

Ayr_Rage said:

Turn down the main stop valve so that there isn't excessive pressure coming into you home.

It controls the flow, not the static pressure. it will remain hight when there is no flow. I can imagine some sound produced when flow starts and pressure drops.  If so, it makes more sense to open the valve fully to reduce the pressure change.

Opening the stopcock wouldn't increase the pressure by allowing more of it in then?D

It controls the flow, not the pressure. Unless it's fully closed, the pressure in the house remains the same, high, and drops when flow starts. If the stop valve is almost closed and you open all taps on the ground floor, the water might never reach the first floor because the pressure gets too low.

Yes, the knocking/hammering only happens after the water has been shut off - either by the tap being turned off or the washing machine stopping its draw of water. 

Correct. Water hammer happens when flow stops sharply.  Pressure doesn't have to be high.

It also doesn't always happen straightaway as there could be a few minutes after the water has stopped flowing before we hear a knock, sometimes several times over.

This I can't explain. Hot water pipes make some sounds, not loud, when expanding and contracting

stuart45

I used to get it in the roof space after the water had been cut off for work in the road. It used to start when the valve was starting to close, and lasted a few minutes. Sounded like a low flying plane.
Changed to an unvented system now, so nice and quiet up there.