The water pressure in my Worcester 24 CDi combi boiler keeps rising to about 5 bar even though you have to insert the special plug to fill it up normally. Any ideas why?

Richard James, June 2010

Thanks I replaced the heat exchanger and the problem has now gone away. The pipe from the pressure relief still drips a bit but I think that's due to the valve not reseating properly

Richard James, July 2010

Yes,I hate to repeat this but assuming that the expansion tank within your boiler hasn't lost its initial charge of air pressure ,about 0.8 bar, there is only one way that the pressure can increase constantly.Water can get into your system via a leak on your hot water exchanger allowing the domestic cold feed to leak into the boiler side of the heat exchanger.Cost about £120 last time I bought one.Drop the pressure back to 1 bar run up the boiler but shut off the cold feed supply to the combi and check for only a small pressure rise,let this settle ,then turn back on the cold supply and put a hot water demand on the boiler from your tap, observe the pressure if it continues to rise your exchanger is knackered.

Guytawe, June 2010