Changing to a new combi boiler first involves removing the old boiler as well as the additional hot water cylinder/ cold water tank. Then, the central heating system will need to be drained via a chemical or power flush that will remove any build up of debris. Next, the pipework will need to be upgraded to 22mm diameter pipes before the new boiler can eventually be fitted. You may also want some additional upgrades such as a magnetic filter, new boiler flue or heating controls. Once it’s installed, your new boiler will be registered by your installer. Altogether, this will take 2-4 days to complete.