boiler losses pressure from 3 bars to nil every 5 hours,cannot find any leaks on central heating pipes or radiators,
Have to top up every 5 hours.
I have checked the expansion pipe and their is no obvious sign of water coming out .I have pressured the system up with the central heating on and we have not ran any hot water from taps the pressure still falls?

Ian Ferguson, March 2008

there is a leak somewhere, water has to be leaving the system.

leave a bucket in place under the vent pipe, see does it catch any. filling to 3bar is way too high, 1bar max, cause pressure rises as water heats up. If it is rising from 1bar to 3bar when heating is on, safety valve is surely liftling letting water out vent pipe (as it is designed to do at and over 3bar) and a sure sign that your expansion vessel is knackered.

T, March 2008