Vibrating hot water return on indirect cylinder

[AntonyE]

Hi,

I have two indirect water cylinders (Gledhill) connected to a gas boiler (Vaillant). The system is setup/programmed so that the tanks heat up twice a day for about 2 hours. When the tanks start to heat, one of them makes a loud vibrating noise, sometimes for about 15-20 mins, sometimes for longer. The noise seems to be coming from the hot water return pipe (i.e. the pipe taking the hot water back to the boiler). If I hold that pipe (it has insulating foam) tightly close to the cylinder then the noise stops.

This has been happening since we moved into the property about 9 months ago. I was worried that it's getting louder, but my wife thinks it's the same as it always was.

I was wondering if anyone can tell me ...

• What is the likely cause?
• How much do I need to worry? We're booked for a service in a couple of months anyway so could just get our plumber to look at it then, but do we need to get him out sooner?

Many thanks,
Antony

 

[cross thread]

Do a search I think you will find it’s a issue that other have reported ,I believe gledhill have replaced some cylinders.

 

[AntonyE]

Thanks. I did find some similar issues, but no solutions unfortunately (I've tried the zone valve and it's not that).

 

[Roddy Young]

We had a similar issue with our new Vaillant boiler which was a direct replacement for an older boiler which developed a leak in the heat exchanger.
We also have a Gledhill tank. The noise was so bad I could feel it in the pipes and also on the boiler case.
I was reluctant to leave the house with this system running in case something failed.
Vaillant themselves were at a loss, suggesting before anything else that the system should be flushed. (Underfloor heating with entirely plastic pipes and no steel in the boiler itself....)
No one on this forum was able to suggest a solution.
After a few annoying (stabbing in the dark) visits from Vaillant under warranty, I contacted a local plumber who unscrewed the flame control valve by 1 turn. This raised the CO2 slightly from 8.4% to 9.1%. That's in the middle of Vaillant's spec.
Since then there has been no vibration. (6 months so far)
Mechanical Engineers will describe noises and vibrations as a form of resonanance generated in any system with flexible parts - the pipework, tank, flue and boiler itself are all slightly flexible. A resonance needs only a small input vibration to stimulate this much larger and irritating one.
I suspected the flue was being hit by wind from a particular direction.
It seems, however, that the flame in our case was fluttering very slightly and that set it off.