Cold water pipes banging... with a twist

[jsmills]

Hopefully you guys can help me as i'm puzzled. Heres how it goes..

Only happens on the cold water system, which is a direct feed with no storage tanks.
When any cold water appliance (tap, toilet etc) is opened a period of about 5 seconds occurs then a drop in pressure seems to be heard (change in the water noise pitch) followed by a loud hum / hammer which can last from 1 to 10 seconds.
The noise only occurs when the tap is opened and doesnt bang on opening or closing. It doesnt always occur (2 or 3 times a day), seems to be worse when the water isn't used for a long period and doesn't seem to change when the main stop cock is used to reduce pressure.

I dont think it is a loose pipe issue as it is happening at every tap even the sink which is right next to the main inlet (less than 1m of pipe). Could i have air trapped? if so how can i flush it? also under the sink is a black unit which is connected to the mains after the stop cock which has a vent pipe which goes outside. I can manually use the knob on the top of this which vents water out the pipe outside. What is this?

Please help its driving me mad!

 

[WelshMan]

Can you post a pic of this " black unit " ?.

 

[Space cat]

The symptoms certainly suggest that you have a lot of air getting trapped somewhere. The pressure drops as the air expands and then something starts to sound off as the pressure across it changes. Mains water has a lot of air dissolved in it so anything that can trap air will keep filling up.

The five second delay is not as relevant as the amount of water that runs out in that time. Is it pints or gallons? If you turn off the water while it's sounding off and wait a while does it happen all over again or will it not happen for several hours? Does any air come out with the water before or during the noise?

 

[jsmills]

Ok here is the black thing/



1, water rate im not sure but with a litre jug with a fully open tap takes about 2 seconds to fill

2, Generally if you turn off a tap while its noisy and turn it on again it will continue to sound. If left a while it will take time to sound again.

3, I dont believe any air comes out before or after the sound.


thanks , jason

 

[Slugbabydotcom]

The valve is a composite valve that you would expect to find on an installation that has an unvented cylinder. It comprises a pressure relief valve, a filter and a non return valve. Some include an isolation valve as well
It is possible the NRV is faulty and the filter could do with a clean. This should be done by someone who is qualified to work on unvented cylinders.

Next thing to try is fitting a shock arrestor on the cold water supply before any valve that solely isolates this one.

 

[jsmills]

sounds right as i have 2 cylinders in the airing cupboard, one red with a pressure gauge (i think this is for the central heating) and one grey (dont know what this does).

 

[Slugbabydotcom]

Grey one is for your domestic hot water and you were right about the red one.
A shock arrestor is a mini pressure vessel that absorbs the shock of water being turned off or causing resonance in the pipes as it travels through.

 

[jsmills]

so
1, if i get someone in to check the composite valve first
2, fit a shock arrestor

is it possible there is air in the system? can this be drained? Could i try that before i get someone in costing me ££.

Thanks for your help...

 

[Slugbabydotcom]

1 Yes
2 Yes

The problem could be cured by 1, 2 or both 1 and 2

Air in a system under mains pressure always clears itself in my experience.
You have 30 litres per min flow rate so theres no way that air is your problem. [If it was you would bleed it rather than drain it BTW]

I suspect that your non return valve is not functioning correctly and letting the pressurised water from you UV cylinder back into the system. That NRV is the source of resonance until the pressure is equal on both sides of the valve. Its best to renew it.

Officially I must say that UV cylinders are potentially dangerous and any DIY you do on them or their associated pipework is at your own risk. Also they should be serviced every year along with your boiler.

Now that my disclaimer is out of the way, heres what happens on planet earth

If you felt confident enough you could change the valve yourself if you read the instructions for the unvented cylinder. You wont be altering any pipework so I will say that it is a low risk operation as far as UV cyls go.
Theres really only 2 mistakes that you can make. One will stop the valve from working [fitting it the wrong way round] and the other [incorrect fitting] will cause a leak. Make sure you get an exact replacement then you wont need to alter pipes.

A shock arrestor is nearly always a way out for otherwise incurable water hammer. You must eliminate the dodgy NRV first as the shock arrestor will only mask something that needs attending to.

 

[jsmills]

thankyou very much for the detailed description. Suppose thats my saturday gone! I will try items 1 + 2 and post the outcome for reference.

cheers, jason.

 

[Slugbabydotcom]

Just covering all the angles for you here.

When you turned the cold water off and opened all the cold taps, de-pressurise the UV cylinder by opening a hot tap and closing it again after the water stops coming out.
This will stop a gallon or 2 of water coming back from the cylinder.

 

[jsmills]

heres an update..

went to take off the nrv to get a replacement, so first i depressured the cylinder as described above but had to go out before i got any further. So i just turned the water back on. When i got back the hot + cold taps all spluttered air with first use but then ran fine. Since then (Thursday) i have not had a single noise from the pipes. Hopefully it will stay that way.

Thanks for all your help, and fingers crossed its sorted.