With the use of chemicals and water pumped around the system at high velocity (much faster than your central heating pump) the cleaning process is used for each individual unit (each radiator, and boiler) from a central point, cleansing is only stopped when the ph in the system is neutral and the total dissolved solids are within 10% of the tap water, then the system is treated with a protector to prevent further corrosion, this is guaranteed effective for two years. If you have cold spots in your radiators, a noisy boiler, cold radiators at the bottom, or get a lot of air in the radiators, you should consider having this done.