How do I know if I've got sludge in my central heating system?

The common signs of sludge in the central heating system are:

• Radiators are hot at the top, but cold at the bottom
• Radiators require bleeding more frequently
• Pipework to the radiators get hot, but the radiators themselves do not
• Some radiators do not heat up to the required temperature, even when the temperature is turned up
• The boiler pump leaks or fails
• When boiler parts are removed or replaced, they are full of dirt
• The water pipe coming out of the boiler (the flow) is hot but the water pipe coming back to the boiler (the return) is lukewarm or cold
• The system is noisy – it gurgles and ‘kettles’ (the term kettling means exactly what it sounds like – your system whistles, pops and bubbles, just like a kettle does when it is boiling)

Sludge forms when the water in the system reacts with the metal in radiators and pipes. Because sludge is heavier than water, it can settle in radiators and pipework and, if left to build up, it can damage the boiler and block the system or radiators leading to cold spots and inefficiencies.

When your installer carries out your annual boiler service, they should check the water in the system to make sure it is not full of sludge, and that there is the correct level of chemicals in the system to slow down corrosion and prevent wear and tear, which in turn helps to reduce sludge.