Megaflow bubble lost every three months

[Mr_Duck]

I am trying to find out why my Megaflow CL210 loses its bubble regularly after three months and would appreciate any comments on why this might be. The current position is:

1. The unit was professionally installed in 1999 and seems in good condition.

2. There is secondary circulation system plumbed in but he pump is not currently operational.

3. I know how to regenerate the bubble and have done so three times now since moving into this property. Each time the bubble is lost after three months and overflowing occurs. At my previous property in another county the bubble lasted over a year.

4. I have managed to get a decent pressure gauge onto the hot supply from the Megaflow and found that overflow only occurs when the pressure approaches 7.5 bar (with a 8 bar release valve). The test I used when I observed overflowing was to run quite a bit of hot water with the boiler off (pressure is then 3 bar as expected due to the PRV) and then to run the boiler to heat the water back up. With no further water being drawn the pressure quite quickly ramps up to 7.5 bar and overflowing then starts (which seem reasonable). Clearly at this point the bubble has been almost entirely lost. The question is why?

5. If I regenerate the bubble the pressure increase in the same test is from 3 bar to about 3.5 bar which seems OK. As the bubble is gradually lost the top pressure presumably creeps up until the PRV operates.

6. I have read about defective shower mixers etc. causing loss of bubble but mine seem in good condition and the rest pressure in the cylinder is a solid 3 bar except when being heated by the boiler with no water drawn. So I don't believe there is any back-feed from mains cold water ramping the pressure up, and in any case my mains water pressure is 4.5bar max.

7. My local (Suffolk) water is extremely hard but has a powerful water softener on it. Is it possible that the artificially softened water might absorb the bubble? After all, 12 months seems to be the normal life of a bubble so it may be that some waters might be more prone to absorbing the bubble than others.

Any suggestions much appreciated. Fitting an external expansion vessel would fix the problem once and for all but that's a last resort and I would like to confident that it was justified. Is it possible the cylinder is defective? How might I prove this?

 

[kozak1968]

Unless it was serviced every year and the benchmark book filled out you will not get it fixed under warranty.

 

[Mr_Duck]

Thanks for the response. There is history of servicing by the original installer who has confirmed that bubble loss was a problem for the previous owners. I may need to check for more records.

But more to the point, how might the cylinder be defective? It all seems very simple. If it is defective I should maybe replace it, rather than faf around with repairs!

 

[Chalked]

I find large cylinders, like yours. With low occupancy ( how many people in the house?). They suffer from large volumes of water, that is not used enough. This causes more expansion problems.

 

[townfanjon]

I look after some old internal expansion unventeds , some loose the air bubbles some dont lol , if they loose the bubbles I fit external expansion vessels.

I will not tell you how to do it , and you need a qualified G 3 guy to do it .

 

[Best]

Makes you wonder if an air gap Megaflow is worth paying a lot extra for.
Personally, if I was the OP I wouldn't bother anymore about the air gap problem.
The OP should just get a G3 qualified plumber in to fit a suitable external potable expansion vessel and have the plumber service it and the unvented unit once a year.
Contact Megaflow to check with them that they are happy for the alteration and that warranty will remain.