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Break the sludge before it breaks the boiler

Richard Harvey, category director for heating and plumbing at Wolseley investigates the options available to combat system spoiling sludge.

At some point in their lives, most homeowners have bled their radiator and been surprised at the colour of the liquid that greets them from the bleed valve. As an installer, you’ve likely been asked questions about why the water is dirty, what causes it, whether it’s harmful and what can be done to combat it.

What is sludge?

Sludge is largely caused by corrosion of ferrous metals that make up the system with more than 98% being magnetite. When oxygen in the water reacts with metals such as the steel inside a customer’s radiator, the build-up — if left untreated — can be costly. Because magnetite is a mineral oxide of iron, any additional oxygen in the system that may be caused by leaks or a simple radiator change can speed up the reaction of corrosion and increase the risk of inefficiency or total boiler breakdown.

In fact, 80% of boiler breakdowns that occur within the first five years of a boiler being installed are caused by magnetite build up in the system.

Detecting sludge build-up

The most common tell-tale sign of sludge build-up is cold patches on radiators, usually starting at the bottom of the unit. The connecting pipework is hot but the radiator remains cold to the touch. Sometimes, customers may also complain about kettling, the need to bleed their radiators more regularly than normal, or that the house is taking longer to heat up than usual. To detect sludge build-up quickly, many installers now use thermal imaging equipment. With leading brands such as Testo and FLIR offering both standalone equipment or options that plug into a smartphone, thermal imaging cameras are a useful way of visually demonstrating the problem to customers, and also showing them the results after the problem has been resolved.

Flushing the problem

An effective way for customers to tackle sludge in a radiator is to remove it from the wall and give it a hose through in the garden, although this clearly is not the recommended course of action. As an installer, the onus is to educate customers on the benefits of having a professional flush the system. There are three ways installers can do this: mains flushing, power flushing or gravity flushing.

Power flushing, with its origins in the maintenance of commercial systems, uses specialist equipment to rid the system of corrosion debris and decrease the risk of it happening again and while this method provides exceptional results, it can be a time-consuming task that many customers are unwilling to pay for.

Mains flushing uses cold water from the mains to flush contamination out of the system. Highly effective, less fiddly and more affordable devices such as Thoroughflush even enable installers to change the flow direction, disturbing contamination within the radiators even further, ensuring a more efficient flush.

Gravity flushing is the least effective method therefore most installers will use one of the other two options.

Chemical inhibitors

Once a system has been thoroughly cleaned, as outlined in the Compliance Guide to Part L of the UK Building Regulations for England and Wales, a chemical inhibitor and, where appropriate, a scale reducer can keep the system trouble-free for longer. However, Part L does require the system to be flushed before inhibitor chemicals are added.

The key here is to educate your customers about the importance of their annual system service and inform them that one of the checks involved will be to undertake an inhibitor concentration test to ensure that it remains at the recommended levels to ensure sludge build-up doesn’t return. A useful education piece is to explain that up to 30% of the system’s fuel consumption is wasted without correct water treatment measures.

The use of an inhibitor is also a requirement for nearly all boiler manufacturers in the UK so ignore chemical inhibitors at your peril as the boiler manufacturer’s warranties can become invalid without following the correct procedures.


The water’s pH level and conductivity also need to be considered when preventing corrosion and some installers are now taking steps to remove minerals and salts within the water.

While Part L recommends the use of inhibitors, some installers are now also filling the heating system with demineralised water to prevent salts and other minerals increasing the water’s conductivity and therefore reducing the risk of corrosion further. The reason for this is that this method is considered more environmentally friendly than adding chemicals to the system.

In certain areas of the UK where water hardness is quite low, demineralised water could have a positive impact. While not currently outlined in the UK’s building regulations as a standard for reducing the probability of corrosion, demineralisation is a key part of Germany’s VDI 2035 which supports Europe’s EN12828 standard and outlines the pH of water in the system should be between 8.2 and 10. It’s worth noting however that demineralisation is not a substitute to using chemical inhibitors.

Deaerating the system

Another way to reduce the possibility of corrosion from occurring in the first place is to fit a deaerator to the system. Air that is dissolved into the system’s water is released as a highly corrosive gas when the water is heated. Increasing in popularity within the installer community, deaeration devices remove these microbubbles and vent them safely to the atmosphere before the gas has the chance to have an effect on the system’s components.

Attracting results

In addition to system flushing, the addition of inhibitors or demineralisation, installers should also ensure that sludge doesn’t enter the boiler’s heat exchanger by fitting an inline filter in all closed loop systems.

Magnetite is the most magnetic of all the naturally occurring minerals on the planet which is why it’s advisable to fit a magnetic filter when possible. Usually installed at the point of boiler installation but able to be retrofitted following cleaning or maintenance, magnetic filters are engineered to actively trap virtually all circulating ferrous debris within the system. And while these filters cannot prevent sludge build-up, they can remove any mobile magnetic particles from the water and minimise the damage it causes to the system.

In an independent test, ADEY’s MagnaClean was shown to reduce boiler emissions by up to 7% and save customers up to £80 a year on their annual gas bill. The real saving to customers (and benefit to installers!) though, lies in reducing the hassle of emergency call outs and potential boiler replacements. In return, BS 7593 and BENCHMARK require that these filters are cleaned annually to remove the debris they have caught. Installers can add more value by servicing the magnetic filter alongside the rest of the heating system during annual checks.

Bear in mind that inefficiencies can be caused by non-magnetic debris too like copper and aluminium which aren’t caught always caught by a magnetic filter.

While this debris doesn’t form in significant amounts, it can still be detrimental to system performance and cause localised blockages. To tackle this issue, filters including a gravity filter to remove these particles would be required.

Every system is different and installers must choose the correct course of action to suit their customer’s needs and budget while complying with building regulations BS 7593 and BENCHMARK. However, it is essential that customers understand the causes of sludge build up, as well as the solutions to reduce the risk.

Armed with the knowledge of how sludge is formed, and how additional solutions can save them money in the long run, customers should make a decision that is not only best for the ongoing performance of their system but also one that increases an installer’s bottom line.

5 August 2020


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