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.