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

[Jim8]

So I’m trying to refill the air gap as per the inscructions on the tank but I’m struggling to locate the mains cold water supply valve/knob. Can someone please explain this to me like I’m a complete idiot lol. Our megaflo is on the first floor of the house.

Which of these in the picture do I turn to shut off the cold water supply?

 

[cdbe]

 

[cross thread]

Looks like a blue tap on the left

 

[oldbuffer]

1. It the valve with the black handle highlighted with a red arrow in cdbe's response.
2. They are often very stiff to turn. If this is the case you can always turn the water off at the main stop-cock. If doing so, switch the boiler off as well.

 

[Jim8]

Thanks for the replies guys. So I figured it was that valve on the left but I just wanted to make sure

So with that valve shut off, is there supposed to be cold water coming out of the taps still? Because there is

 

[dilalio]

So with that valve shut off, is there supposed to be cold water coming out of the taps still? Because there is

Your cold isn't balanced with the hot, ie: both hot and cold after the prv.

So your cold is teed off before the shut off valve and won't stop when closing it.

You only need to isolate and draw off the hot to regenerate the air gap/bubble.

That discharge pipework isn't right either.

 

[Jim8]

Your cold isn't balanced with the hot, ie: both hot and cold after the prv.

So your cold is teed off before the shut off valve and won't stop when closing it.

You only need to isolate and draw off the hot to regenerate the air gap/bubble.

That discharge pipework isn't right either.

Can you elaborate further on the pipe work please? What’s wrong with it?

 

[aveatry]

I always just turn off the main incoming stop tap - good idea to keep turning it every now and then as well. Then open a couple of hot taps a small amount of water come out the tap then stops. I monitor the tundish and if I see drips then its time to recharge the air gap. It seems to be once every 6 months that it needs ding - could be longer. I then go around running the taps to get the air bubbles out as it spits and splodges until the air is out the pipes and can otherwise catch you out soaking you.

 

[Johntheo5]

I recharged one of these a good few years ago for someone but didn't follow the labelled instructions which basically say that you only give ~ 25% air bubble, just above the bottom of the dip tube and then repressurize.
I shut the cold inlet, opened a hot tap and then drained down the complete cylinder through its drain cock, (I did not open the T&PR valve), when the cylinder was empty, I shut the hot water tap and repressurised the cylinder by opening the the cold inlet, I then cracked open the hot tap to puge any air from the system.
I reckon this gives a five fold increase in the air bubble, on a 210L cylinder, from 10L to 50L, after a full reheat, it obviously depends on the length of the dip tube but this, by my calcs, has to extend down to at least 25% of the cylinder vol to give a reasonable final pressure, if it is 25% then assuming a cylinder pressure of 3.0bar will result in a final pressure of 4.3bar on a full reheat but only 3.2bar if the cylinder is fully drained first.
The boiler and electric immersion have to be switched off before doing this.

OSO UV cylinders seem to use this method but have a separate "commissioning" drain valve.

 

[Madrab]

With the older Megaflop HE's, to recharge the bubble it should be drained down to the level of the T&PRV, that maximises the bubble that the cylinder can hold.
It's also part of the servicing procedure and using the T&PRV to do that allows it to be tested for functionality and flushes the seal. I know some say don't as they say it may not seat back down properly. I think I have only ever had that problem once and that was on a 10yr old cylinder that hadn't ever been serviced. Rarely have that issue on a cylinder that only ever uses clean water, yes on CH where the system can be full of crud/debris and chemicals etc but not on HW cylinders.

Even then all PRV's are supposed to be tested as part of a servicing procedure, on any appliance that uses them, even on CH but of course we all know they don't get touched.

 

[dilalio]

Can you elaborate further on the pipe work please? What’s wrong with it?

Looks like D1 is longer than 600mm and several bends.

The pipe from the Tprv to the tundish.

 

[Johntheo5]

With the older Megaflop HE's, to recharge the bubble it should be drained down to the level of the T&PRV, that maximises the bubble that the cylinder can hold.

Its actually the minimum bubble, but I suppose Megaflo have calculated that this minimum bubble + a baffle will last for a year when the UV is supposed to be serviced, if the bubble only lasts 6 months then the user only has to drain off ~ 25% of the cylinder to remake the bubble which will then see the year out. OSO, on the other hand, look for a empty cylinder before refilling so just wondering if these cylinders do in fact have a baffle, since a air bubble 5 times the volume of the Megaflo bubble might last equally as long without a baffle, will try and contact them.

 

[aveatry]

Its actually the minimum bubble, but I suppose Megaflo have calculated that this minimum bubble + a baffle will last for a year when the UV is supposed to be serviced, if the bubble only lasts 6 months then the user only has to drain off ~ 25% of the cylinder to remake the bubble which will then see the year out. OSO, on the other hand, look for a empty cylinder before refilling so just wondering if these cylinders do in fact have a baffle, since a air bubble 5 times the volume of the Megaflo bubble might last equally as long without a baffle, will try and contact them.

Perhaps I should of stated that mine is an OSO and maybe the procedure is different. I used to open the downstairs tap but found it makes no difference if I open couple of upstairs taps instead . My instructions also say to open the commissioning tap but nothing ever has come out of it yet. And the tank doesn't completely empty - judging by the amount that comes out of the hot tap.
I have found that I can carefully balance the PRV at the open point so I do not have to sit there holding it. The glugging stops after about 30-40 mins.

 

[Johntheo5]

Perhaps I should of stated that mine is an OSO and maybe the procedure is different. I used to open the downstairs tap but found it makes no difference if I open couple of upstairs taps instead . My instructions also say to open the commissioning tap but nothing ever has come out of it yet. And the tank doesn't completely empty - judging by the amount that comes out of the hot tap.
I have found that I can carefully balance the PRV at the open point so I do not have to sit there holding it. The glugging stops after about 30-40 mins.

You will only get ~ 25% max of the cylinder vol by draining through the hot taps because the dip tube only extends down this far. I belive those commissioning cocks are allways blocked , but there is normally a drain cock on the cold feed close to the cylinder, you can drain through that.

How often do you have to remake the bubble?

 

[aveatry]

You will only get ~ 25% max of the cylinder vol by draining through the hot taps because the dip tube only extends down this far. I belive those commissioning cocks are allways blocked , but there is normally a drain cock on the cold feed close to the cylinder, you can drain through that.

How often do you have to remake the bubble?

I used to keep a record and that shows about 10 months, its always long enough that I cant remember the last time. It goes for quite a few months before I start making a conscious effort to start checking the tundish for a drip when the hotwater is on.
If the commissioning cock is blocked then it has been so from its new fitting as it has never let out anything. Not sure about the 25% thing - how big is the bubble ? no more than 25% is it ?