Something seems odd about the pressure behaviour of the central
heating system in my 3-bed house, powered by a Britony 80 combi
boiler. Over the past 3 days, with the boiler running, I've been
carefully recording the water pressure levels (according to the
boiler's pressure guage) at various intervals. Before doing this, I
rigged a pipe going from the pressure relief valve into a bowl, so I
can see what gets expelled from the valve. Here is my record:

Wednesday
4:30pm: I top up the pressure to 1.75-bar. The bowl is empty.
5.30pm: The guage reads 2-bar. Bowl is still empty.
10.45pm: Guage reads 1-bar. Bowl now contains 1/4 pint of water.

Thursday
6.00am: I empty the bowl and top up the pressure to 1.75-bar.
7.20am: guage reads 2.65-bar and there is 1/4 pint in the bowl.
8.42am: guage reads 1.4-bar. Another 1/8 pint has gone into the bowl.
8:56am: guage reads 2.1 bar. No further expellationinto the bowl.
10.00am: guage reads 1.3-bar. No further expellation into the bowl.
10.30am: guage reads 1.8 bar. No further expellation into the bowl.
2.00pm: guage reads 0.8 bar. No further expellation into the bowl.
6.20pm guage reads 1.5-bar. No further expellation into the bowl.
11.20pm guage reads 1.0 bar. No further expellation into the bowl.

Friday
10.04am, guage reads only 0.2bar! No further expellation into the
bowl.

Anyone tell me what's going on? Expansion vessel problem perhaps?
If so, what? What's the procedure to cure it? The whole system was
installed about 4 or 5 years ago.

Thank you for any help.

Dave L