Pressurised hot water system - top up ?

I've got what I hope is a straightforward central heating question:
My house has a pressurised hot water system, based on a Heatrae Megaflo unit. I've only lived with vented systems before, so I'm not familiar with how closed systems work.
Above the Megaflo, connected at the top of the boiler/radiators/tank hot water circuit, there's an expansion vessel. This has a pressure gauge on the bottom that's currently reading approx 1 bar. There are two other connections to this expansion vessel. One is to an overflow pipe, and the other is to a flexible hose that's connected to a cold water pipe with a tap on it. The tap is currently closed.
So it looks to me like this tap is for topping up the system. What I'm not sure about is under what circumstances I'd need to do that ? Also, what is the pressure gauge on the expansion vessel for ?
Any advice gratefully received,
--
Trevor

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On Sun, 23 Nov 2003 21:36:39 +0000, me wrote:

The expansion vessel etc will be for topping up the primary heating circuit - see FAQ below. 1 bar is fine and will not need to be topped up for now.
The Megaflow contains its own expansion space for the HW - this is in the form of a trapped air volume inside the tank. The air has a habit of dissolving into the water over a period (typically a year or so). There is likely instructions on how to renew the air pocket on the side of the cylinder.
--
Ed Sirett - Property maintainer and registered gas fitter.
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The parts you describe are unrelated to the Megaflo. They would exist on any modern wet central heating system, even with a vented cylinder or no cylinder at all. On a combi or system boiler, they would likely be within the boiler casing.
They are used to top up the primary circulating water.

Look at the gauge regularly when the system is cold and the system turned off (heat in the system or the pump being on will give false readings). If it drops below 1 bar, then:
1. connect up the flexible hose 2. turn the tap on until the pressure rises to 1 bar. 3. turn the tap off 4. disconnect the flexible hose 5. bleed all the radiators 6. next day, bleed all the radiators again 7. check pressure again, particularly if much air was bled
If you need to top up regularly you may have a leak, either to the outside world, or within the hot water cylinder heat exchanger. (But not in your case, as your cylinder should be at higher pressure than the central heating, so you are more likely to get rising pressure, not falling).
Christian.
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