A magnetic filter sits on the return to the boiler and captures magnetite before it reaches the heat exchanger. It is inexpensive, most manufacturers now require one for the warranty, and it must be cleaned at every service or it stops doing anything.
What it catches, and why that matters
A sealed heating system corrodes slowly from the inside, producing magnetite – fine black iron oxide from the steel radiators. It is magnetic, abrasive and heavy, and it does three specific kinds of damage.
- It blocks heat exchangers. A modern condensing boiler’s heat exchanger has very narrow waterways, precisely because narrow waterways transfer heat well. That also makes them exactly the wrong shape for tolerating particulate. A partially blocked exchanger overheats locally, kettles, and eventually fails.
- It destroys pumps. Magnetite is abrasive and it grinds bearings. A pump failing every few years on the same system is telling you something.
- It settles in radiators, where flow is slowest – the bottom – and produces the classic cold-at-the-bottom radiator.
A magnetic filter takes it out of circulation. Magnets inside the body attract the magnetite as water passes, it collects on the magnet assembly, and it is removed when the filter is cleaned. Non-magnetic debris – jointing compound, flux residue, copper swarf, scale – is caught by the filter’s separation chamber.
Where it goes and why
On the return pipe to the boiler, as close to the boiler as is practical, so it catches debris on the last stretch before the heat exchanger. Orientation matters – most filters can be fitted in several positions but the manufacturer specifies which, and getting it wrong reduces the separation effect.
It also needs isolation valves either side, which every decent filter includes as part of the assembly, so it can be cleaned without draining the system. A filter fitted without isolation is a filter nobody will ever clean.
Access matters more than it sounds. A filter tucked behind a boiler in a cupboard with a shelf over it will not be serviced, whatever anyone intends. Where the obvious position is inaccessible, it is worth spending slightly more effort putting it somewhere a person can actually kneel down in front of.
Servicing it, which is the entire point
A magnetic filter that is never cleaned is a decorative brass object. Once the magnet assembly is coated, it captures nothing further, and the collected magnetite simply sits there.
- IsolateBoth valves closed. The system stays full and pressurised.
- Drain the filter bodyInto a container. What comes out tells you a great deal about the system’s condition.
- Remove and wipe the magnetThis is where the magnetite actually is – a thick black sludge clinging to the magnet sleeve.
- Inspect and reassembleSeals checked, body rinsed, magnet replaced.
- Reopen and check pressureTop up if needed, and check for leaks.
A filter producing a heavy black sludge every year is a system that is still actively corroding, which usually means the inhibitor has depleted or there is a leak letting oxygen in. A filter that comes out nearly clean after a year is a system in good order. It is a free annual health check on the whole system, and it is worth asking to see.
Inhibitor, which works alongside it
A filter catches magnetite. Inhibitor stops it forming. They are complementary rather than alternatives, and a system with a filter but no inhibitor is still corroding – the filter is just collecting the evidence.
Inhibitor depletes over time and is lost with every drain-down and every leak. BS 7593 recommends testing its concentration periodically rather than assuming it is still there, and a test is a quick thing to do at a service. Topping up after any work that drained part of the system is basic practice and frequently forgotten.
If your system needs repressurising regularly, the inhibitor is being diluted continuously by fresh, oxygenated water, and no amount of filtering or dosing will keep ahead of it. Find the leak – see leak detection.
Worth fitting to an existing system?
Yes, and it is one of the better value jobs available on an older heating system. It is a short visit, the part is inexpensive, and it protects the two components most likely to cost real money.
If the system is already dirty, the sensible sequence is to clean it first – chemical clean or power flush depending on how bad it is – then fit the filter and dose with inhibitor, so the filter starts from a clean system rather than immediately clogging. Fitting a filter to a heavily sludged system without cleaning first means emptying it repeatedly for months. See power flushing.
Boiler under warranty? Check whether a filter is a condition of it. Several manufacturers now require one.
Questions we get asked about this
Do I really need a magnetic filter?
Most boiler manufacturers now require one as a warranty condition, and the practical case is strong regardless: it protects the heat exchanger and the pump, which are the two most expensive things likely to fail.
How often should it be cleaned?
At every annual service as a minimum. On a system that is still actively corroding, more often initially until it settles down.
Will it clean my system?
No. It stops new debris reaching the boiler and gradually removes what is circulating, but it will not lift sludge that has already settled in the bottom of radiators. That needs flushing.
Can I clean it myself?
It is not difficult if the isolation valves are accessible – close both, drain into a container, wipe the magnet, reassemble, reopen, check pressure. Have a container and old towels ready, because what comes out is black and it stains.
Are the expensive ones better?
The differences that matter are magnet strength, the volume of the separation chamber, and how easy it is to clean. The last one is underrated: a filter that is awkward to service is a filter that will not be serviced.
Does it slow the system down?
A correctly sized filter has a negligible effect on flow. A filter that has been left full of sludge for years does restrict flow, which is another reason to clean it.
Regulations and standards referenced on this page
- BS 7593: Code of practice for the preparation, commissioning and maintenance of domestic central heating systems — BSI. Not free to read; available from BSI.
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