Central heating filters or magnetic filters all work pretty much in the same way. The return flow passes through the filter body, generally fitted just before the boiler and passes through a series of chambers or vanes to create turbulence, whilst a powerful magnet situated in the middle of the filter attracts the magnetic particles or sludge to a central core. The sludge is held there by the magnet until either the filter is dismantled and cleaned, or the magnet is removed and the filter flushed.

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This much sludge after 24 hours! I kid you not, this system needs power flushing.

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Sludge rinsed off. Constant removal of radiators, topping up and leaks caused dilution of inhibitor


There are slight variations, such as the MagnaClean Twintech which has a separate filter to trap non magnetic solids, but basically they all work in the same way.

The better ones have a lower chamber which collects the non magnetic particles and can be flushed away. A simple concept, with no moving parts, that works continuously to remove sludge and particles.

What is Sludge?

There are two types of sludge, black and brown, these can be present separately or together depending on the type of system. Brown sludge is bacterial in nature and usually found in older open vented systems where the header tank in the loft has no lid or a badly fitting lid and organic material forms in the water. When the central heating water is replaced, brown sludge is drawn into the system.

Black sludge, sometimes referred to as “Magnetite” is caused by corrosion, usually of steel radiators and cast iron heat exchangers. Corrosion occurs when there is insufficient or no corrosion inhibitor in the system. The insides of the radiators rust and create iron oxides, in extreme circumstances the corrosion goes all the way through, causing leaks, then requiring topping up (again with fresh water) and the process repeats.

A video showing one of my preferred filters is available here. The brands I recommend are Adey, Fernox & Sentinel.

What are the effects of sludge

These can vary from cold spots on a radiator to, in extreme circumstances, a ruined boiler or central heating system. Sludge can build up in a radiator or pipe to reduce water flow, In an open vented system this could lead to pressure building up and hot water being vented into the header tank, mixing with brown sludge present in the tank, the warm water causing increased bacterial activity and being drawn back into the system as it cools where the process repeats until the system is blocked.

In a sealed system or a combi boiler, black sludge can block the narrow pathways in the heat exchanger, leading to poor hot water flow and eventual exchanger replacement or block radiators and pipes, leading to increased pressure causing the pressure relief valve to operate thus requiring topping up (with fresh water) and exacerbating the process, or because Magnetite is magnetic, sludge is attracted to the magnetic field of the pump and causes failure.

Two of the main repairs on combi boilers are plate heat exchangers and pumps. There is a reason why and it’s not because these components are badly manufactured.

So all the problems I’ve outlined above are generally caused by the heating system not being sufficiently treated with corrosion inhibitor and not being correctly maintained. All these problems and expense, for a few pounds a year!

Corrosion in wet central heating systems is a constant process. If radiators are removed for decorating, or there are leaks on the system and radiators are bled, fresh water is added which is oxygenated and dilutes the inhibitor and keeps the corrosion process going.

How To Deal With Sludge

The first thing to do is to get rid of the existing contamination. With an open vented system the loft tank must be thoroughly cleaned or replaced. The next step applies to both open and closed systems.

The whole system needs to be flushed out, this includes pipework, radiators, cylinder heating coil if fitted and the boiler. The most effective way to achieve this is by Power flushing. See separate post for further details.

Once the system is cleaned, a magnetic filter should fitted and the water suitably dosed with an inhibitor.

For new heating systems The Compliance Guide to Part L of the UK Building Regulations for England and Wales requires a central heating system to be cleaned and flushed before a corrosion and scale inhibitor chemical is added during commissioning.

Most boiler manufacturers now also make the warranty dependent on the system being flushed in accordance with BS7593, Benchmark and Part L.

As an alternative, each radiator can be removed, taken outside and flushed with a hosepipe, this carries many potential risks such as spillage, damage to decor or radiators and doesn’t clean the pipework or boiler.

The magnetic filter now continuously removes any residual contaminants within the system, but, and I cannot stress this enough, the inhibitor level must be maintained, simple to do, once a year, when the filter is cleaned, a bottle of inhibitor is poured into the empty filter case, then filter is refitted and that’s it. This can be done either by me or by yourself.

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