Vibrating hot pipe when cold tap turned on.

[domll]

Hi,

I currently have issue that when cold water tap turned on in kitchen and Bathroom, the hot pipe from Combi Bolier (Worcester Greenstar 8000 Life) vibrates. This also happens when shower turned on or when toilet cistern fills.
Call British Gas Homecare as Boiler is under warrenty (fitted January 2020), engineer spent sometime looking for cause but could not find it; was supprise hot pipe vibrating as cold was turn on (doesn't happen when hot turned on). After 2 and half hrshHe fitted none return valve on hot pipe which vibrates incase there was some back syphoning when cold turned on but, did explain Boiler is prtected against this anyway. Unfortunatly this did not work.
His final solution was to turn water supply stop cock down too quarter turn open. Intially this worked but very shortly after a few test runs of turning cold tap on the hot pipe started vibrating again.
This is how it has been left and is frustrating, especially at 1am when visiting the bathroom and you get rude awakening (especially the neihbours) when cistern fills (i live in a ground floor flat).

I have heard tap washers can cause issues so, have indipendantly isolated taps and turned cold water to test, didn't work. Also done same with cistern and shower.
additionally, i have checked that no pipes are loose or touching each other which they are not.

Currently i'm stumpped, cannot understand why hot pipe from boiler that supplies hot taps and shower vibrates when cold water turned on.

This issue only started after firing up central heating system few weeks ago but, engineer discounts this as a case but coincidence.

Has anyone exsperienced this issue and, maybe have idea of cause??

appreciate any advise.
Regards,
Dominic.

 

[domll]

Okay, I found the problem and nothing to do with boiler:

Noticed on kitchen mixer tap there was a drip from tap, every 30min or so.

After isolating both taps independently, discovered it was hot tap dripping. With hot tap isolated, turned cold tap on... no noise. Soon as isolation opened on hot tap, noise starts.

Additionally, the hot water pipe feed to the tap vibrated when isolation open and stopped when closed.

Removed the cartridge, no obvious sign off damage so, using cotton bud, dabbed some Vaseline on ceramic washer discs.

Couple of turns on/off to spread Vaseline and I replaced cartridge in tap.

He presto, no leak and no noise I.e. vibrating pipe.

Appears that when cold tap turned on, as they both share same exit for mixing water, cold water was causing some kind of ‘back syphon’ on hot tap cartridge, the noise probably air being drawn into cartridge and, given they are hard ceramic washers, cause vibration in tap which in turn travelled down pipe back to boiler (only 7ft away)..

Would also now make sense why it happened when other cold taps, cistern and shower turned on as they all share same feed; when turned on cause back pressure within cold system and enough to cause issue experienced.