We have recently moved into a new house with a Worcester 30cdi Combi Boiler installed 2006. The heating and hot water appear to be working fine. We had a Nest thermostat installed and the installer said the boiler pressure was high, about 2.5 bar nearly in the red. As we have taken out a maintenance contract with nPower we contacted them and they sent an engineer who was baffled. He ordered a new plate heat exchange and pressure gauge. Second engineer arrived to fit parts and was also baffled but fitted the parts anyway. The pressure remains high. Pressing the dump valve handle under boiler at the back empties and reduces the pressure via the overflow but the boiler immediately refills to 2.5 bar. The filler key is removed. Turning the mains water off stops the boiler refilling. Turning the mains water partially off allows the pressure level at which the boiler fills to to be reduced, but that is no good as it also means little pressure out of the other taps in the house. Running either hot or cold tap whilst releasing the pressure works until you turn the tap off at which point it immediately rises again. I have been told either my hot and cold taps are on the wrong way around! My shower mixer valve has failed! Someone has made a link from the mains water to a radiator pipe in order to fix a leak! I have searched for said link and can not find it yet. Has anyone got anymore ideas before I rip up anymore floorboards? Or do I just ignore the problem as all seems to work ok despite the high pressure, but I don't want to blow the boiler up down the line.