Megaflo Air Bubble not taking expansion

[fezster]

I noticed today that the pressure gauge on my incoming water supply was showing over 5 bar and creeping up fairly rapidly until it went past 6 (that's the max it goes to) and continued. The Megaflo pressure reducing valve should be 3 bar, and this is normally what the gauge shows. The only thing that was heating at the time was my Megaflo cylinder. It got to about 7 bar and then stopped. There was no water dripping from the tundish on the TPRV. However, did notice the tundish on the pressure relief valve (8 bar I believe) was passing water.

This said to me the air bubble is depleted, so recharged it as per the instructions, and it stopped all it's gurgling noises and dripping from the hot tap after about 2 hours (!). Then turned on the heating to the HW again. After 10 mins or so, came back and the pressure gauge is showing 4 bar. So much better, but still creeping up - slowly this time. It got to about 5.5 bar before the HW zone valve closed.

This implies to me the air bubble is not taking the full expansion of the hot water. I would guess that most people don't realise as the PRV passes water and all is well. I happened to have a pressure gauge fitted at some point, but most wouldn't.

But is this normal? I'd have expected the air bubble to take the full expansion? Should I look to have an external potable expansion vessel fitted at some point, as recharging the air bubble every year or so is quite annoying.

TLDR - Is the Megaflo air bubble not sufficient to take the full expansion of hot water?

 

[Best]

Sounds like it is nothing whatsoever to do with the air bubble, although it can also be failing.
You can't do any work yourself on unvented cylinders, except on a Megaflo type unit to get the air in the bubble.
You need a G3 qualified plumber to service and repair the unit. They can also recommend any future modifications (if needed) that would still be acceptable to the unit manufacturers.
Unvented units require a service once a year.

 

[Harvest Fields]

Time to get a G3 engineer in by the sounds of it. As stated above.

 

[fezster]

As the megaflo is on the ground floor, opening the lowest tap as per the instructions was not regenerating the air bubble. Solution was to drain the cylinder sufficiently to allow air into the cylinder.

All sorted now and pressure does not increase and PRV does not open. Hopefully this will be useful to somebody else.

Will definitely have an expansion vessel added at the next service, as this air bubble malarkey is damn right annoying.

 

[Best]

As the megaflo is on the ground floor, opening the lowest tap as per the instructions was not regenerating the air bubble. Solution was to drain the cylinder sufficiently to allow air into the cylinder.

All sorted now and pressure does not increase and PRV does not open. Hopefully this will be useful to somebody else.

Will definitely have an expansion vessel added at the next service, as this air bubble malarkey is damn right annoying.

Opening a hot tap will not drain the unit as the hot draw off comes from top of unit. Needs the water drawn from bottom of unit, as you later did, to get air into the top.
Just note that an expansion vessel added to the unit will still require that expansion vessel to be checked and serviced once a year for a suitable air charge.
Nothing is totally maintainence free unfortunately

 

[fezster]

Opening a hot tap will not drain the unit as the hot draw off comes from top of unit. Needs the water drawn from bottom of unit, as you later did, to get air into the top.

The instructions do say this is the way to do it, though. Apparently if the tap is lower than the megaflo, this will allow air to enter the top of the cylinder (even though, as you say, the water draw off is at the top of the cylinder). Aside from wasting ~30L of hot water, drawing from the bottom didn't take too long, so this is the method I'll use in future.

Just note that an expansion vessel added to the unit will still require that expansion vessel to be checked and serviced once a year for a suitable air charge.
Nothing is totally maintainence free unfortunately

Thanks and understood. I actually do have it serviced once a year (even though, if Im being honest, I've considered not bothering!), but it seems even the G3 qualified technicians have been recharging incorrectly, and hopefully explains why I've been needing to recharge it more than once a year. The EV just seems like a cleaner solution, as at least the pressure can be measured to see if it needs recharging or not. Cheers.