Recharging Mego flo air gap ?

1 May 2008
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United Kingdom

I've followed instructions (from various sources, all pretty similar).

When I open the pressure release valve (after turning off main stopcock and draining tank with lowest hot tap in the house), it hisses for about 15 secs but no water drains through the tundish.

Is this normal or do I have a problem ? Am I doing something wrong ?
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You shouldn't be able to drain past the Temp and pressure relese valve (T&PRV) by just opening the hot tap, all that should do is release the built up water pressure. Have you been having problems with the PRV or T&PRV passing and that's why your are regenerating the bubble?
Our hot water pressure has dropped a little so I thought recharging the air bubble might help. This is what I did:

- shut off main stop cock to house
- turned on cellar hot water tap (this took about 3 mins to stop flowing
- went to top floor (where mega flo resides) and opened release valve (above tundish)
- this hissed for around 15s then nothing else happened
- closed the valve
- returned to cellar, turned off hot water tap and reopened main stop cock

Did I do something wrong ? I was expecting the stage when pressure release valve opened to last considerably longer (according to videos I have seen).

PS I am not a plumber (in case u hadn't guessed !)
Nope doesn't sound like you're doing anything wrong.

Usually opening any of the hot water taps would just release the HW that's being held under mains pressure, once that's released then your left with a full cylinder of water without any pressure. When you open the valve it should then drain down through the valve to the level of the valve, keeping the HW tap open lets air into the cylinder and allows it to drain through the release valve. Why yours isn't doing this is a bit of a mystery, the hissing from the valve almost sounds as if air is being sucked back in rather than released. I presume the hot water supply pipe comes out of the top of the cylinder?

BTW - Your HW pressure is governed by the pressure reducing valve on the combi valve on the cold water supply and the mains water pressure itself. The bubble shouldn't alter that pressure to any major degree.
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If, when you re-ressurise the cylinder and when the water is warmed back up, there isn't any discharge through the tundish then all should be well. Only thing I would say is if it was me, a qualified UVHW(G3) chap, that once all re-pressurised I would test both valves to ensure they are working ok.

Trouble with that is if one of them doesn't re-seat properly then they may need replaced and that's for a qualified engineer to do.
Thanks - there is no flow through tundish.

However I've never seen much flow through the tundish, even just prior to air bubble recharging.

I'm sure I was told that if hot water pressure drops this can be a sign that the bubble needs recharging..

Guess best to get mego flo and assoc plumbing checked then.
If things aren't right and/or you're not too sure then get a qualified un-vented (G3/UVHW) engineer in to service it. It should be serviced once a year, same time as the boiler.

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