Then your central heating system needs powerflushing. Basically, your system has large amounts of black sludge blocking the radiators, pipework, and boiler. This makes it ineffective and puts a lot of strain on the whole system. If the water is not circulating effectively within the system, the heat isn’t either. The boiler has to work harder to get your home up to the right heat, and you are wasting an awful lot of gas costing much more money than a clean system would.