2nd new hot water tank knocking sounds

MrLogical

Hi all, we moved into a new build about 18 months ago. We very quickly noticed that our hot water tank (megaflo) was making a knocking sound every once in a while as hot water was was being used or it was heating up. This may or may not be related but a few months later, there was water coming through the closest tundish. We had the plumbers over. They replaced the pressure relief valve which solved the leak in the tundish, but called baxi in to look at the tank. Baxi decided to replace the tank under warranty. All was well for about 4/5 months. Then the knocking gradually began again. It’s now as bad as it was originally and recently, I noticed water coming through the tundish again. I have called the plumbers back in but wondered if anyone else has experienced anything like this or if anyone who knows about these systems has their own theories on what could be going on. Many thanks in advance.

ComicGeek

Has anyone checked the expansion vessel for the hot water cylinder? 

coffeehound

Megaflos have an internal air 'bubble' instead of an EV.  The bubble needs to be regenerated from time to time -- the manual gives instructions for this

ComicGeek

coffeehound said:

Megaflos have an internal air 'bubble' instead of an EV.  The bubble needs to be regenerated from time to time -- the manual gives instructions for this

Yeah, my mistake - just used to people stating 'Megaflo' when they actually mean a generic unvented cylinder. 

Rodders53

My Oso cylinder also has an air bubble.

AIUI the Megaflos have a 'floating baffle' that moves up and down with expansion/contraction (my oil service tech told me).  Could that cause such a noise?  Perhaps even be 'normal'?

https://megaflo-unvented.co.uk/megaflo-description/

MrLogical

Thanks for the replies guys. I did regenerate the air bubble and whilst this does appear to have stopped the leak in the tundish, the knocking continues and has not improved. It’s a pretty loud knock that happens once every 30 seconds or so. Especially when the timer kicks in and it begins heating up. Please remember this tank isn’t even 6 months old and it’s the second one we now have that’s doing this. Any other ideas? Thanks again.

ComicGeek

Is the central heating pump installed close to the cylinder? If the pipework to the cylinder is short, then this can cause noise as the control valve is opening. Ours does this, but I can't turn the pump down as otherwise not all the downstairs rads get hot. You could try and turn down the pump and see if this helps.

Bendy_House

Adjusting the pump to test this should be fine, and it can always be returned to its previous setting if it doesn't help. Beyond that, Mr Log, you are obviously limited in what you can safely/legally do with an unvented cylinder.
There's another thread on here with someone having 'bangs' from an unvented system. I don't think that's fully resolved, but was improved after a plumber adjusted what was a manual by-pass valve. I'd be surprised if it was this in your case, tho'.
Bottom line, there would appear to be an inherent and ongoing issue with your system, if the prv has blown twice in 18 months. If 'Baxi' can't sort it, then you'll likely need someone else who can - a fresh G3 approach. And ideally get the installer to pay for this.

MrLogical

Thanks you for your advice. The installers are coming out to look at this on Wednesday. I am still within warranty on everything until September and won’t let this go until it’s fully resolved.