Expansion vessel relation to unvented system pressure loss

2 Jun 2003
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United Kingdom

I have continuing problem with pressure loss with my Megaflo system. I have not found any leaks with the accessible pipeworks, and no sign of water leakage on the walls & ceilings.

From reading the various forum messages, the problem could be with the expansion vessel.

However, can somebody please explain to me how the expansion vessel can be a related fault for pressure loss in the primary heating circuit? From my understanding, the expansion vessel is part of the DH circuit and separate to the primary heating circuit.
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You dan't say which boiler you have but it's most likely the expansion vessel inside it which you should be checking.

How high does the system pressure rise when the heating has been on for a while? If it goes up to around 3bar you could be losing water through the boiler Pressure Relief Valve.
If you let your megaflow go cold. The switch the hot water on.
There is an pressue relief pipe that should be near the megaflow that probably goes out through the wall.
Check this for it dripping as the hot tank heats up.
The megaflow as I remember uses an air bubble at the top of the tank that has to periodically be drained to re-establish the bubble at the top of the tank to allow for expansion as the tank heats up.
Some unvented cylinders have a separate expansion vessel and some have them hidden at the top of the tank under a cover. These need to be refilled with a foot pump.
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Just check the tundish by the cylinder. If there's sign of limescale or if it's wet then you probably need to recharge the air gap in the cylinder. There should be instructions on the side of the tank telling you how to do this. It's quite simple. Plenty of links to the instruction manual online.

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