If your heating is struggling to reach the required temperature or energy bills are rising it may be due to sludge in the heating system. Here we explain why, what you can do to address it now and how to prevent future issues.

The presence of corrosion, limescale and sludge in a heating system can seriously impact its effectiveness and efficiency. Also, any damage or problems that arise as a result of sludge or corrosion are not covered under the warranties issued by boiler manufacturers. That is why keeping the system clean and maintaining the correct levels of chemical water treatment is so important.

What causes sludge in the system?

One of the key sources is the corrosion of the metal components within the system. Almost any heating system will contain a mix of different metals – steel radiators, copper pipework and aluminum heat exchangers to name just a few. The presence of these metals that then come into contact with the water flowing around the system can lead to corrosion and the creation of iron oxide. This settles throughout the system as well as coating internal components.

Limescale is also an issue, particularly in hard water areas. Just as with kettles, showers and taps, limescale will form on vital parts of the boiler and heating system, including the heat exchanger or pump.

What are the signs?

Often the first indications of contamination in the system are issues such as boiler noise, cold spots in radiators or heating that is slow to heat up or does not reach the intended temperature. For homes with a combi boiler, poor hot water temperatures may also be a sign. In addition, you may notice that your household energy usage is steadily rising as the boiler has to work harder and burn more fuel to generate heating and hot water. It is estimated that just a 1.6mm coating of limescale on a heating element can reduce boiler efficiency by up to 12%.

What is the solution?

New systems

When installed, a new heating system should be cleaned and flushed by the engineer to remove any installation debris. To prevent sludge and scale from forming it is important that a good quality chemical water treatment product is used.

Also known as inhibitor, this is a fluid that is added to the heating system water to prevent corrosion occurring and protect against the formation of limescale. It works by reacting with the metal in the system to form a barrier that inhibits the normal corrosion process. Good quality inhibitors will also include pH balancing properties that keep the water at a neutral pH to further protect the surfaces within the heating system.

British Standard BS7593 for the preparation, commissioning and maintenance of domestic central heating makes it compulsory to fit an inline filter to capture sludge and debris for safe removal during the annual service. The standard also states that the system cleanliness and inhibitor levels should be checked annually using an on-site test and topped up as required. Every 5 years the inhibitor should be fully re-dosed or a full water quality test completed to highlight any issues.

Existing systems

For existing systems where the inhibitor levels have been allowed to lapse, the solution will depend on the amount of contamination present. A heating engineer will be able to carry out a simple water test to determine the level of sludge and corrosion present and advise you on the best course of action.

For systems that are heavily sludged, flushing is usually required. This is typically carried out using a special unit that pushes water through the pipes at speed to dislodge and sweep away sludge and limescale build-up. A specially formulated chemical cleaner may also be employed to improve the effectiveness of the flush and remove as much of the build-up as possible. Although some homeowners are initially concerned about risks to the system, this is assessed on a case-by-case basis by the engineer attending, depending on the age and condition of the heating system.

For the installation of a new boiler on an existing system, it is recommended that a thorough clean is carried out to prevent any sludge that may have formed from damaging the new boiler and affecting its efficiency. Again, the engineer will be able to offer advice on what type of cleaning is required.

Key advice

Due to the potentially damaging effects of sludge, scale and other contamination, it is always better to carry out preventative maintenance rather than addressing the problem once it is already causing issues with your heating. This is especially true when you consider that cleaning and power flushing processes are time consuming and more expensive than simple servicing and maintenance would be. Keeping the system clean will ensure it operates at peak efficiency and heats your home while keeping energy costs to a minimum.


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