brief knocking sound from hot water taps on unvented system

sunil1234 · 30 Dec 2010

Hi All

The set up is as follows

Vailliant thermocompact conventional boiler
Ultrasteel 210l indirect tank
Zilmet 18l expansion vessel

Following some recent work covered by insurance where the boiler prv was repalced, the engineer was kind enough to replace (i supplied part) the expansion vessel as it was dripping in the tundish.

After a day, i noticed the stopcock in the airing cupboard dripping very slowly that shuts off the water to the taps, toilets etc. Called insurance co and they sent someone round to tighten it.

All seemed good but at somepoint following prv etc replacement, i now have a brief knocking noise when opening the hot taps fast. Seems to be worse if the hot water timer is actually on at the time although this could be tosh lol.

I have bled all radiators thoroughly from lowest floor upwards.

No pressure loss in boiler anymore or dripping in tundish.

Cant think what else, please could anyone advise.

thank you[/b]

sunil1234 · 1 Jan 2011

am i allowed to bump this please?

doitall · 1 Jan 2011

What expansion vessel did he change, the boiler or cylinder.

If you open quarter turn taps on a pressure system really fast, there's a good chance something will rattle, especially if the pipes are not bolted down properly.
Sounds to me like you changed the white vessel and not set the pre-charge pressure.

sunil1234 · 1 Jan 2011

Hi

thanks for the useful feedback.

Yes it was the zilmet 18l external expansion vessel that was changed.

Probably the pipes are not bolted down properly as this darn dodgy builders in this 2003 house have cocked all sorts up.

However as the knocking was not there before the prv and expansion vessel were changed, it is probanly something as you say to do with the vessel.

I think it was just taken out of the box and swapped over although shouldnt the pre-charge have been set when it was made.

How can i, as a layman, not engineer please set this pressure correctly now and i assume it isnt too late to adjust this?

thank you

Sunil

doitall · 1 Jan 2011

The pre-charge is set before it leaves the factory, however it has to be adjust for the individual system, if you look in the MI for the cylinder it will tell you what it should be.

sunil1234 · 1 Jan 2011

thank you, how do i go about adjusting this.

is it

turn off mains stopcock in kicthecn cupboard
open lowest level taps until no water flow
pump/adjust schrader valve
close taps,
turn on stopcock slowly

it says set to 3.0 bar on the vessel, will try to dig out instructions or fine online

regards

doitall · 1 Jan 2011

Unless it's a real bore I wouldn't touch it.

But you would need an very accurate digital type gauge.

The instruction are in the manual.

sunil1234 · 1 Jan 2011

thanks, the manual says 3.0bar

i will leave it alone and ask for it to be looked at when the tank is serviced (hint simond!)

however one thing i noticed just now, the vessel is wall mounted on a metal bracket with braided hose down to piepwork.

Whilst the bracket screwed into the wall is tight, the vessel is strapped onto it but it can move around quite freely if i use one hand to wobble it (despite the metal strap looking tight around the lip in the middle of the vessel).

Should this be the case?

thanks

doitall · 1 Jan 2011

It shouldn't be loose, or it may fall off the bracket

sunil1234 · 11 Feb 2011

Guys and Gals

Sorry but i have to revive this

Right, the pressure on the expansion vessel was checked and slightly reduced so now spot on 3 bar with an accurate gauge.

The knocking sound is still there but more often occurs when

1) the hot water heating is on at the time
2) If the hot water heating is not on then the knock will only occur the first time hot water is used.
3) The braided hose running to the expansion vessel is the one that knocks when i tried to put my hands on as many hot water related pipes as possible in the airing cupboard and got my wife to turn on the bath tap fast to instigate the knocking.

I have spoken to the supplier who, in turn, has liased with the manifacturer. They are adimant that it is not a faulty vessel but maybe a pressure reducing valve?

Could anyone please provide some help

thank you

Sunil

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brief knocking sound from hot water taps on unvented system

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