"Sealed" system ejecting water

My sealed system has been losing pressure very rapidly, and I've finally found out why. When it heats up, the pressure on the gauge rises dramatically (3bar!) and water is ejected through the pressure relief valve. All is then well until the heating goes off, at which time the pressure drops right down to zero.
I'm assuming that there must be air in the system to make this happen (after all, water doesn't expand that much), but where is it? All the radiators are hot top and bottom, and none of them will bleed any air. The system runs quietly, with no bubbly sounds. Can it be that the big red pressure reservoir thing above the boiler (forgive my technical language) is full of air? It's at the highest point of the system, more or less, so that's where the air should end up. If so, why does water get vented when the pressure relief vave opens, and not air?
Puzzled.
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The expansion vessel has lost it's gas charge. Either repressurize it or replace it if the diaphragm has failed.
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"Peter Robinson" wrote | .. Can it be that the big red pressure reservoir thing above the | boiler (forgive my technical language) is full of air?
The problem is that the BRPRTATB is empty of air. It needs to squish the air in it to make room for the expanding water.
Owain
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It should have "compressed air" one side of a rubber diaphragm thingy, and water on the other. The "air" side will have a tyre valve on it which you can check with a normal gauge. It should read 3 bar or so, the right number should be on a sticker somewhere on it. If water comes out when you poke the valve, the diaphragm is gone and you need a new one (www.bes.ltd). Relatively easy to change provided you can get at it. If the pressure is zero, just connect up a tyre pump and recharge it.
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Oh yes it does.

This device is an expansion vessel. Its purpose in life is to absorb the expanding water then the system heats up so it doesn't shoot out the pressure relief valve.

There is probably no air in the system. After all, it is sealed, so the air shouldn't be able to get in.
Now, given the symptoms and the purpose of the big red pressure reservoir thingie, it should be clear that it is no longer serving its purpose. Inside, the vessel consists of two compartments separated by a stretchy rubber like diaphrapm. On one side it is connected to the central heating system. The other side is full of air. Air compresses easily, so when the hot water expands, it pushes againsts the air, compressing it, until the pressures match. This means that the pressure rises slowly with temperature.
There are two common failure modes to this vessel. Firstly, the air can all leak out. When this happens, there is no air to compress and the system can't allow expansion. The other failure mode is that the diaphragm gets punctured, allowing the air and water to mix. The air then escapes and isn't useful for its purpose.
To test which type of failure you have, press down on the refill valve on the vessel when the system is full. If any water comes out, the diaphragm is shot and the entire vessel needs replacing. If no water comes out, and particularly if no air comes out instead, then the vessel is probably empty of air. To refill it you need to drain down the central heating and open a bleed valve so the system is at atmospheric pressure. Then attach a bicycle pump with gauge to the vessel. Pump it up to the precharge pressure (usually 0.75 bar). If it won't hold the pressure, then it is shot and needs replacing. Then refill, remembering to replenish your corrosion inhibitor. You may wish to test it first using clean water only, until you are sure it is working.
Christian.
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