Megaflo airgap - recharging the bubble?

[QuackingPlums]

Hi,

Please excuse me if this is answered already, but I have tried a search and looked it up on the wiki but I still have a few questions!

My overflow is dripping, and the sticker on the side of the cylinder says I should turn off the cold mains, run the lowest hot tap, and hold open the PRV until the gurgling stops.

Well I stood there for 10 minutes with the cloakroom hot tap on and holding open the PRV, yet the water didn't stop running! Is the purpose of this to use up all of my hot water? If so, how long should that take? I guess I shouldn't do this if I intend to take a shower later?

Also, how can I tell if the mains supply has been properly closed off? There's only one big pipe that is labelled "Cold Mains Supply" and it leads up to a big red tank via two small valves (the two that I'm supposed to use to set the input pressure to 1.5bar every month or so). There wouldn't be more than one would there?

Thanks in advance for any advice!

 

[Joe4542]

You may not have closed off the mains for the Megaflow.

Follow the cold mains pipe into the megaflow back along, there will normally be a valve within the vicinity of the Cylinder. The cold inlet is at the lower left of cylinder, with a blue collar around the pipe.
You should find the 'combination valve' on this pipe, it looks like
View media item 6707
Turn the black lever 90 degrees, so it lies across the pipe, instead of inline with it - this will shut off the supply to the megaflow.

Then try recharging the gap like you did before - it should only gurgle for a few minutes normally.

 

[gavinda]

in also find it easier if i open all the hot taps , other wise it can take a hell of a long time to empty.

 

[Softus]

Well I stood there for 10 minutes with the cloakroom hot tap on and holding open the PRV, yet the water didn't stop running!

You didn't do it for long enough.

Is the purpose of this to use up all of my hot water?

Is the purpose of you to be silly?

how long should that take?

Anywhere between 30 and 60 minutes.

I guess I shouldn't do this if I intend to take a shower later?

You guess wrong.

Most guesses that people make turn out to be wrong.

Also, how can I tell if the mains supply has been properly closed off?

If you close all hot taps and open the kitchen cold tap, then water should stop coming out.

There's only one big pipe that is labelled "Cold Mains Supply" and it leads up to a big red tank via two small valves (the two that I'm supposed to use to set the input pressure to 1.5bar every month or so). There wouldn't be more than one would there?

There could be.

 

[QuackingPlums]

You may not have closed off the mains for the Megaflow.

Follow the cold mains pipe into the megaflow back along, there will normally be a valve within the vicinity of the Cylinder. The cold inlet is at the lower left of cylinder, with a blue collar around the pipe.
You should find the 'combination valve' on this pipe, it looks like
View media item 6707
Turn the black lever 90 degrees, so it lies across the pipe, instead of inline with it - this will shut off the supply to the megaflow.

Then try recharging the gap like you did before - it should only gurgle for a few minutes normally.

Thanks Joe - that's really helpful! I found the combination valve I think, but I'm a little confused with the pipes I've got so I took some pics:

This is where all the "magic" takes place...

Untitled

• QuackingPlums
• 7 Dec 2008
• 1

...and this one I've included just for some context as to where all those pipes go!

Untitled

• QuackingPlums
• 7 Dec 2008
• 2

In the first pic, what is the purpose of the pipe labelled "cold water supply"? Do I need to turn that off also?

If I follow the pipes from the combination valve back to the blue-collared cylinder inlet, there's another black valve that looks similar to the PRV up by the label. Do I just leave that alone?

If I close the combination valve, am I shutting off water to the entire house or just the cylinder?

Is the purpose of this to use up all of my hot water?

Is the purpose of you to be silly?

This was a genuine question - the valve I hold open gets hot in my hand, and the water flowing down into the black overflow "cup" is hot. I know that there are separate reservoirs of hot water in the system for heating, showers etc so which body of hot water am I discharging?

how long should that take?

Anywhere between 30 and 60 minutes.

Joe says it should only take a few minutes - that's quite a large variation?
I don't mind standing there for an hour as long as I know I'm doing the right thing - there's no point wasting all of that water if I haven't closed off the right valves!

 

[WDIK]

how many showers do you have? multiple balanced cws taken off, the htg press vessel is confusing too.

isolate incoming below combination valve.

 

[Joe4542]

Looking at your pic, and if the direction of the combi valve is correct, it seems that the combi vale does all, if not most of the cold supply. Turning off the black plastic lever will isolate the megaflow, and prob most of the bathroom taps.

Don't need to touch the other black knob further along, thats the expansion valve.

The time taken to drain the cylinder will depend on how big the cyl is, how many taps you open and hence how quickly the water drains. You'll know when its done cos the water will stop draining thru the tundish and the gurgling will stop.

 

[xr4x4]

::bump::

im sorry but thats just stupid, do i read that as any work to the hot water side of things need to drain the whole cylinder of hot water?

 

[Joe4542]

No, the OP wasn't asking about working on his DHW system, they simply wanted to know how to 'recharge' the air bubble in a Megaflow cyl.

Since the system is a mains pressure water store, turning off the inlet and opening an outlet, first relieves the pressure stored. Then water can only drain down to the level of the outlet on the vessel - in this case - the level of the T&P valve, usually no more than ~300mm from the top of the megaflow.

No-one said that the entire Hot water cyl needed to be drained.

 

[WhydidIstart]

I have one of these tanks - it takes about 4 mins for the water to stop flowing out of the tap and then about 6 mins of holding that T&P valve on the cylinder open.

My understanding of how the cylinder works is that when you open the T&P valve nothing should come out (be seen in the tundish) - air should be going in and water should be coming out the tap(s). If water is coming out the tundish for any length of time then something is wrong!

i.e. the cold water is not off, or somehow being forced back up the hot pipes from somewhere. (A shower mixer with cold supply still on but broken check valve perhaps?)

 

[WhydidIstart]

Just noticed the OP was in 2008, lol.

 

[QuackingPlums]

I know this is an OLD thread, but I was the OP and the relevant photos/discussion is all in this thread so I hope this is ok.

First, an update to the above issue: I found out that if my boiler control panel was heating my hot water at the same time as I was trying to recharge the air gap, then the hot water would never run out and the gurgling would go on way beyond 10 minutes (and even way beyond an hour, as suggested it might take by one of the replies above).
I discovered that if I turn OFF the call for hot water before starting the recharge procedure then the hot water tap would stop running within a minute or two, and I could then recharge the air gap within a few minutes. The gurgling would STOP in less than five minutes. I've been successfully recharging my air gap ever since.

Fast-forward to today. I've had an extension built recently and my boiler was 'upgraded' as part of these works. It's a Worcester-Bosch unit and the hot water control is incorporated into the new Nest thermostat. I've just tried to recharge my air gap because there's a constant drip into the tundish, but even with the call for hot water showing as 'OFF' in my Nest app, the hot water tap doesn't stop running. And if I open the PRV then the gurgling continues for way beyond 10 minutes again. Since I know that it can recharge in far less than the half-hour suggested above, would it be valid to infer that the Nest water control isn't closing off my hot water supply as my previous control panel did?