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Whats Inside Your Boiler | Sludge & Scale.

This heat only boiler was experiencing difficulties in heating up the central heating system. Are you surprised?

This is a common site in the plumbing and heating industry and can be fatal to our new high efficiency boilers that are being installed across the country. You could have the best boiler on the market, but if your system is full of this dirty water it's not going to last the recommend life span. So what is sludge and scale?


Sludge is a semi-solid slurry that is produced in the system due to the corrosion process. Its the product from the mixture of water, air and loose corrosive metals. The abrasive nature of sludge can lead to premature pump and motorised valve failure. When the amount of sludge exceeds the ability of the boiler water to carry it in suspension, it settles on boiler surfaces. A film of sludge deposited on a heat exchanger acts as a heat insulator and reduces the transfer of heat to water, taking longer for the boiler to reach its set temperature.

Baked or hardened sludge is very difficult to remove and requires the use of mechanical machines and chemicals to break down the slurry and flushed out of the system.


Scale is a hard mineral coating and corrosion deposits made up of solids and sediments that form from calcium and magnesium when heated. The amount of dissolved solids in our water determines how hard our water is. Hard water is high in dissolved minerals, both calcium and magnesium.

Scale is a metal salt and is a good conductor of electricity. When it builds up on metal surfaces such as inside boilers or central heating pumps, it acts as an anode, with the metal becoming a cathode. This anode-cathode setup allows electrons to flow freely between the scale and the metal, allowing corrosion to take place. As the scale is not uniform, the corrosion becomes localized rather than spread evenly over the surface of the metal.

Sludge and scale can greatly affect heat transfer performance. 1 millimeter thick scale or sludge can add 7.5% to energy costs, while 1.5mm adds 15% and 7mm can increase costs by over 70%.

We have an understanding of all central heating systems, what the best method of reducing heating bills, keeping families warm when it matters most and maintaining the efficiency of the boiler.

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