Are your radiators giving off a bad smell of eggs or similar when you bleed them of air? (the removal of air via the air vent) Then this could probably be a build-up of a chemical called Hydrogen Sulphide in the water. Hydrogen Sulphide which can build up in central heating systems not only causes a bad smell but will cause corrosion sludge and magnetite within your central heating system over time and will require removing. A few well known brands of central heating cleaner

This gas boiler was struggling to heat the central heating system. Surprised? This issue is common in the plumbing and heating industry...

To achieve optimum performance and reliability for our customers domestic boilers & central heating systems LHS Plumbing Services use th