Refilling the air gap on a megaflo.. please help!

[Johntheo5]

When the cylinder is only drained down to the T&PRV level which on the Megaflo seems to be in line with the bottom of the dip tube which is ~ 25/30% below the top of the cylinder, say 25%, and assuming a 200L cylindere, then there will be a 50L, 25%, air bubble at 0 bar (atmospheric) pressure, when the cylinder is refilled to, normally, 3bar pressure, then the bubble will be compressed to 1/4 of its original volume of 50L,
50*(0+1)/(3+1), 12.5L, If the HW Tap(s) are closed on a empty cylinder before refilling then there is a 200L (100%) air bubble at 0bar pressure, after refilling to 3.0bar pressure then the bubble will be 200*1/4, 50L, 50/12.5, 4 times the vol of the other bubble so you mightn't have to recharge for 3 years or so, don't tell anyone.

 

[aveatry]

When the cylinder is only drained down to the T&PRV level which on the Megaflo seems to be in line with the bottom of the dip tube which is ~ 25/30% below the top of the cylinder, say 25%, and assuming a 200L cylindere, then there will be a 50L, 25%, air bubble at 0 bar (atmospheric) pressure, when the cylinder is refilled to, normally, 3bar pressure, then the bubble will be compressed to 1/4 of its original volume of 50L,
50*(0+1)/(3+1), 12.5L, If the HW Tap(s) are closed on a empty cylinder before refilling then there is a 200L (100%) air bubble at 0bar pressure, after refilling to 3.0bar pressure then the bubble will be 200*1/4, 50L, 50/12.5, 4 times the vol of the other bubble so you mightn't have to recharge for 3 years or so, don't tell anyone.

Not pretending to fully understand but wouldn't that mean you end up with a smaller Vol of HW in your tank.
Also

4 times the vol of the other bubble so you mightn't have to recharge for 3 years or so, don't tell anyone.

Why not tell anyone - do people usually pay an engineer to come out and do the recharge

 

[Johntheo5]

Not pretending to fully understand but wouldn't that mean you end up with a smaller Vol of HW in your tank.
Also

Why not tell anyone - do people usually pay an engineer to come out and do the recharge

Yes, you will end up with a smaller vol, there will be 190L in the cylinder with the smaller bubble after a full reheat and 152L in the cylinder with the bigger bubble but, when you have drawn off 150L (200*0.75)) of HW then cold water will enter the dip tube and leave the remaining 40L or so of HW sitting above the bottom of the dip tube, the bigger bubble cylinder HW vol will be slightly above the dip tube bottom but will still supply ~ 150L of HW.

No, you don't require a engineer to redo the bubble but might be tempted to overlook the yearly service if the bubble lasts for years!.

 

[Madrab]

Its actually the minimum bubble

When the cylinder is only drained down to the T&PRV level which on the Megaflo seems to be in line with the bottom of the dip tube

That's exactly right - the T&PRV is level with the dip tube - and as the bubble cant be any greater than the dip tube then it has to be the maximum?? With the HE's, they use a baffle to hold the bubble and it rides down the dip tube until it's about 30mm from the bottom where it hits the stop flange on the tube, avoids the baffle from dropping below the level of the tube and misaligning.

Unless I'm missing something glaringly obvious??

 

[Johntheo5]

That's exactly right - the T&PRV is level with the dip tube - and as the bubble cant be any greater than the dip tube then it has to be the maximum?? With the HE's, they use a baffle to hold the bubble and it rides down the dip tube until it's about 30mm from the bottom where it hits the stop flange on the tube, avoids the baffle from dropping below the level of the tube and misaligning.

Unless I'm missing something glaringly obvious??

So, you are saying that the baffle is a tight fit in the cylinder like a piston ring and is air tight?
Even if it is, and you start filling a empty cylinder with HW taps and T&PRV closed, you are still effectively compressing a cylinder full of air?
I'd be amazed if this baffle is a very tight fit either in the cylinder or where it slides up and down on the dip tube.

 

[Madrab]

No it isn't airtight at all, it is quite loose but no matter what, even if the cylinder is fully emptied and it's full of air when refilling the cylinder, the excess air is supposed to escape up the dip tube, I think this may be where the disparity is.

As part of the Unvented course work, when filling or refilling any unvented it is advise to always keep one HW outlet open until the water starts to run and any excess air is bled out, then the tap is closed. With the megaflo - if I remember rightly - that is part of their bubble re-charge procedures too, always have one outlet open during re-charging/filling.