combi is emptying water out of overflow when topped up?

the pressure releif valve is not seating fully. it will almost cirtainly need replacing, most important that you also check the pre charged pressure is correct in the expansion vessel. this needs to be done with the boiler drained. if the air pressure in the expansion vessel is too low the system pressure will rise as the system heats up. this will lift the pressure releif vave normally set at 3bar and not a cheap part to replace.an expensive vicious circle if your not carefull.

ray
April 2010

Expansion vessel is flat or the PRV's seating is trapped.

The expansion vessel takes up the expanding water in a sealed system when heated, if the vessel is flat the water has no where to expand, except to the 3.0 bar safety release valve on the PRV, dripping outside.

Firstly release system pressure and keep open, pump up to around 0.5 - 1.0 bar on the vessel (red tank, bike tyre connection) close vent and fill water back to 1.2 bar when heating is cold, observe dial.

If pressure still rises when your heating is on and PRV drips, seating on PRV not sealing or blockage in the system, probably connection from return pipework to expansion vessel.

Steven
April 2010