After you refill the system and as it heats up watch the outlet pipe for the boiler this should be a metal pipe going outside. If a rush of hot water comes out of this pipe it means that the pressure inside the system was too high, the safety valve in the boiler opens and all of the water is "dumped". Some of the causes of this can be: 1. Overfilling the system - my system needs to be at 1 bar maximum when cold but check your boiler handbook for details. 2. Faulty or unpressurised expansion vessel: every system has an expansion vessel which absorbs the increase in pressure as the water in the system heats up. if the expansion vessel is faulty then you'll see the same problem as described above.
Nick Waller, December 2011
