Pressure rising in central heating system?

I have a Ferroli combi boiler which has worked fine for the last 8 years. However it has recently begun to give me a problem. The pressure builds up slowly whilst it's running to bar 4. Each day I bleed off enough water to reduce the system pressure to 1 bar, but by the end of every day I need to bleed off more water to reduce the pressure. I am constantly taking water out of a 'closed system' which by definition is impossible... Can anyone help with this?

Peter Naylor
December 2007