If the central heating system is going to be updated by a new installation of a boiler, then the best thing to do, prior to the fitting is to carry out a full system clean and flush through. This should ensure that the new boiler is going to pump the water through good clean pipework. Debris if left in the system can cause corrosion or damage to the valves and more importantly the pump. South East London Plumbers carry out this work as standard everyday. To protect the circulating pump whilst the cleaning routine is done, it is necessary to remove it, and keep the continuity of the system with a length of hose in its place if you are doing the system one radiator at a time or if you are power flushing this is where the machine will connect. A full descaling and cleansing regime can be employed if your system is old or badly corroded. Use of a strong cleaner could expose minor leaks sealed by corrosion so a mild cleanser is a better option which should be introduced into the system via the feed-and expansion tank or into a radiator through the bleed valve. Once this is complete you can try and turn the pump impeller to make sure that it is free to turn before it is re-connected, it can be much easier to turn the pump impeller with a screwdriver before running the system after cleaning and flushing. Of course at any point if you are not confident, call a professional plumber like a South East London Plumber for a professional service. Note: If your boiler makes loud hissing or banging noises, then, you can treat it and the pipework with a powerful descaler. Please follow the instructions carefully.