I have a 5 year old Worcester/Bosch 24CDi combi boiler, in the last 18 months I have had to replace the water to water heat exchanger because the domestic hot water feed is leaking into the central heating water and causing the pressure to rise continually. Each time Worcester have said that the problem must be the heat exchanger and each time the part has been replace under warranty. Each time the problem is resolved for approximately 4-5 months when the symptoms return. Worcester always maintain that there is no known problem with the heat exchanger - I wondered if anyone else has come across a similar problem???