Central heating - Water pressure falling

2 Jan 2010
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United Kingdom
I have a problem with loss of water from the radiator circuit, which has to be topped up every few days. There are no visible leaks of course.

I was suspecting the expansion vessel, however ...

Am I right in assuming that, when it is running normally, the air pressure that I measure in the expansion vessel is the same as the water pressure in the radiator circuit?

(If not why not?)

If so, then when I measure 1 bar air pressure in the expansion vessel, and 0.4 bar water pressure in the radiator circuit does that mean that the expansion vessel is empty of water and entirely occupied with air?

If so then presumably I am not losing water to the expansion vessel?

Is it plausible that there could be a leak in the heat exchanger itself, with the water escaping with the condensation water (that seems the only route left)?

Suggestions gratefully accepted.

Many thanks.
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There is no leak from the pressure relief valve.
Water does exit the boiler from the condensation pipe, and at one point there were signs of radiator anti-freeze in this water.

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