A magnetic system filter is a small device fitted to the pipework of a wet central heating system that captures iron oxide sludge — known as magnetite — before it can circulate and cause damage. Paired with a chemical inhibitor, a liquid added to the heating water that slows corrosion, the two keep the system's water clean and the boiler running efficiently.
What a magnetic filter does
Inside most magnetic filters sits a strong magnet, usually housed in a canister plumbed into the return pipe near the boiler. As heating water flows through, the magnet attracts and holds magnetite — the fine black iron particles produced when steel radiators and components corrode over time.
Magnetite is the most common debris in a heating system. Left to circulate, it settles in radiators as cold spots, blocks narrow waterways, and clogs the heat exchanger and pump. By trapping it in one place, the filter stops that debris from reaching the parts most likely to fail.
Many filters also catch non-magnetic dirt and scale, which falls to the bottom of the canister and is removed when the unit is drained. Most modern filters are designed so the magnetite can be flushed out without removing the whole device, and an isolating valve on either side lets it be serviced without draining the system.
How inhibitor protects the system
Paired with a chemical inhibitor, a liquid added to the heating water that slows corrosion, the two keep the system's water clean and the boiler running efficiently.
Chemical inhibitor works at the source of the problem rather than dealing with debris after it forms. Heating water naturally reacts with the metals it touches — steel, copper and aluminium — and that reaction produces corrosion and magnetite. Inhibitor coats internal surfaces and adjusts the water chemistry to slow these reactions to a crawl.
A typical inhibitor dose addresses several issues at once:
- Reduces corrosion of steel radiators and the boiler heat exchanger.
- Limits the build-up of magnetite sludge in the first place.
- Helps control limescale formation in hard water areas.
- Can protect against bacterial growth in some formulations.
Inhibitor is added to the system water, usually through a radiator, a dosing point, or the filling loop, in a quantity matched to the system's volume. It does not get "used up" quickly, but its protective strength fades over time and after any work that drains water from the system. That is why the concentration needs checking and topping up.
Why both work together
Neither measure does the whole job alone. Inhibitor slows corrosion but cannot stop it entirely, so some magnetite still forms — and the filter is there to catch it. The filter removes debris but does nothing to prevent it being created, so without inhibitor it would simply fill faster and the corrosion would carry on unchecked.
Used together, they form a two-part defence. The inhibitor keeps the rate of corrosion low, which means less magnetite is produced. The filter removes the small amount that still forms before it can settle in the system. The result is cleaner water, better heat distribution, and less strain on the boiler and pump.
This pairing matters most after a system has been chemically cleaned or "power flushed" — a process that circulates cleaning fluid at high flow to clear out existing sludge. Once a system is clean, fresh inhibitor and a working filter are what keep it that way. Installing one without the other tends to give shorter-lived results.
Servicing and topping up
A magnetic filter is not fit-and-forget. It needs cleaning to remove the magnetite it has trapped, otherwise it fills up and stops working. Most manufacturers suggest the filter is checked and emptied at least once a year, which fits neatly with an annual boiler service.
The cleaning process involves isolating the filter using its built-in valves, then removing the captured sludge — either by draining the canister or wiping the magnet, depending on the design. A heating engineer will normally do this as part of a service, though some homeowners with accessible filters choose to clean them more often.
Inhibitor levels should be checked at the same time. The concentration can be tested with a simple kit, and the engineer will top up or re-dose if it has dropped below the recommended level. Topping up is particularly important after any of the following:
- Draining the system to replace a radiator or valve.
- Fitting a new boiler or other major component.
- A power flush or other chemical clean.
- Repeated bleeding that has let in fresh, untreated water.
It is worth keeping a note of when inhibitor was last added and at what dose, as this makes future top-ups straightforward. Some installers also fit a small dosing vessel or chemical pot to make adding inhibitor easier without draining radiators.
Signs that the protection may be slipping include radiators that are cold at the bottom but warm at the top, discoloured water when bleeding, frequent pump noise, or a boiler that seems to lose efficiency. Any of these can point to magnetite build-up or low inhibitor levels, and are worth raising at the next service. Catching them early is far cheaper than dealing with a blocked heat exchanger or a seized pump later on.