Magnetic boiler filter — what it does and why you need one
Check the steps below first — if you're not confident, get it fixed safely today.
Post a job — we'll find you an engineer →Not sure if this matches your problem?
Use our interactive tool — answer a few questions and get a personalised diagnosis.
Most likely cause & what to check
A magnetic filter (also called a system filter or magnetic separator) is a device fitted to the central heating pipework — typically on the return pipe just before the boiler. It contains a powerful magnet that captures ferrous particles (black magnetite sludge) as they circulate in the system water.
Why this matters: all steel radiators corrode slowly over time. The resulting black sludge (magnetite) circulates through the system and deposits in the boiler heat exchanger, pump, and narrow pipework — reducing efficiency, causing kettling, and shortening boiler life. An MCS-approved boiler installation now typically requires a filter.
Popular UK brands include Fernox TF1, Adey MagnaClean, and Sentinel Eliminator. All work on the same principle and achieve similar results — pricing varies from £60–£150 for the filter unit. Most boiler manufacturers (Worcester Bosch, Vaillant, Baxi, Ideal) now require or recommend a compatible filter as a condition of warranty.
The filter is best fitted by a heating engineer as it requires draining down the relevant section of pipework to insert the filter body between two pipe cuts. Cost installed is typically £100–£200 including the filter unit.
Once fitted, the filter needs cleaning periodically — typically annually, coinciding with the boiler service. Cleaning is simple: close the isolation valves on the filter, unscrew the canister, and slide the cartridge off the magnet. Wipe the magnet clean (the black sludge adheres strongly) and rinse the canister.
If a significant amount of sludge is captured in the first few months after fitting, this indicates the system was already heavily contaminated and a power flush may be needed to remove the bulk of the sludge from the pipework and radiators.
Not confident doing this yourself?
Post the job and we'll match you with vetted local engineers. Free, no obligation.
Was this guide helpful?
Frequently asked questions
Do I need to fit a magnetic filter if my system is new?
Yes — even new systems generate small amounts of iron oxide as metal components bed in. Fitting a magnetic filter from the outset prevents any particles from accumulating in the boiler heat exchanger or pump. Most boiler manufacturers now recommend or require a filter to maintain warranty validity.
How often should a magnetic filter be cleaned?
Clean annually at your boiler service — the engineer should include this in a standard service. On systems with significant sludge, check the filter every 3–4 months initially until the water runs clean. A heavily loaded filter (thick black sludge on the magnet) is evidence that a full powerflush may be needed.
Which magnetic filter should I buy?
The most widely fitted brands are Adey MagnaClean (Pro2 for domestic), Fernox TF1 Filter, and Spirotech SpiroTrap. All perform similarly. Buy one that matches the return pipe diameter (usually 22mm) and is compatible with your boiler — some boiler manufacturers specify a particular brand in their warranty terms.