A magnetic filter on the return to the boiler captures magnetite before it reaches the heat exchanger, and a separation chamber catches non-magnetic debris. It must be cleaned at every service. It protects but does not clean an already sludged system.
How they work
The filter sits in the return pipe, as close to the boiler as practical, so it is the last thing system water passes through before reaching the heat exchanger.
Inside, two mechanisms operate at once:
- A magnet, usually a rod in a sleeve down the centre. Magnetite is magnetic, so as water passes, the particles are attracted to the magnet and cling to the sleeve. This catches the great majority of what a heating system produces.
- A separation chamber, where the flow slows and changes direction so heavier non-magnetic particles drop out – jointing compound, flux residue, copper swarf, scale fragments, bits of old radiator.
When the filter is serviced, the magnet is withdrawn from its sleeve, which releases everything clinging to it into the body, and the body is drained into a container.
What they catch
| Material | Caught by | Where it came from |
|---|---|---|
| Magnetite (black iron oxide) | The magnet – very effectively | Corrosion of steel radiators |
| Jointing compound and flux residue | The separation chamber | Installation debris |
| Copper swarf and filings | The separation chamber | Pipe cutting during installation or alterations |
| Scale fragments | Partially | Deposits breaking away, particularly after a clean |
| Fine silt and general debris | Partially | Age and system history |
| Dissolved substances | Not at all | Chemistry, not particulate – inhibitor’s job |
| Air | Not at all | Air vents’ job |
The key limitation: a filter removes particles from circulation. It does not remove deposits that have already settled in the bottom of radiators, because those are not circulating. A filter on a sludged system will catch what is moving and leave what has settled – which is why cleaning comes first and the filter goes on afterwards.
Servicing, which is the whole point
Once the magnet sleeve is coated in magnetite, its effective attraction is reduced and the collected material simply sits there. From that point the filter is a decorative brass fitting. We have opened filters that had clearly never been touched since installation – the system had been unprotected for years while everyone assumed it was not.
- Close both isolation valvesEvery decent filter has them built in. The system stays full and pressurised – only the filter body is isolated.
- Drain the bodyInto a container. What comes out is black and it stains everything.
- Withdraw and wipe the magnetThis is where the magnetite actually is, clinging to the sleeve as a thick sludge.
- Rinse and inspect the bodyCheck the seals while it is apart.
- Reassemble and reopenBoth valves.
- Check system pressureTop up if needed – the filter body held some water.
- Re-dose inhibitor if significant water was lostMinor for a filter clean, but worth noting.
It is a five-minute job for someone doing it routinely, and it should happen at every annual service without being asked for.
What the contents tell you
The material that comes out of a filter is a free annual diagnostic on the whole system, and it is worth asking to see.
- Nearly clean after a year – the system is in good order, properly inhibited, and not taking in oxygen. Nothing to do.
- A modest amount of black sludge – normal on an older system that is under control. Keep going.
- Heavy black sludge every year – the system is still actively corroding. Something is letting oxygen in. Check the inhibitor concentration and look for a leak being topped up.
- Bright metallic particles – copper swarf, usually recent installation debris. Should reduce after the first year.
- Gritty grey material – possibly scale, which on a heating circuit suggests fresh hard water is entering. Look for a leak.
- Very little, on a system you know is dirty – the filter may be bypassing, wrongly orientated, or fitted on the wrong pipe.
Fitting one to an existing system
Worth doing, and the sequence matters.
- Assess the system first. Magnet test. If the system is already dirty, clean it before fitting the filter, or you will be emptying the filter weekly for months.
- Fit it on the return, close to the boiler, in the manufacturer’s specified orientation.
- Make sure it is accessible. A filter behind the boiler in a cupboard with a shelf over it will not be serviced, whatever anyone intends.
- Check the isolation valves work before you need them.
- Dose inhibitor at the same time.
- Check whether your boiler warranty requires one – several manufacturers now do, and fitting one retrospectively may be a condition of an ongoing warranty.
See magnetic system filters for the service itself.
What a filter does not do
- It does not replace inhibitor. Inhibitor stops corrosion happening; the filter catches the product of corrosion. A system with a filter and no inhibitor is still corroding – the filter is just collecting the evidence.
- It does not clean a sludged system. Settled deposits are not circulating.
- It does not remove air. That is what air vents are for.
- It does not fix a leak, and a system being repeatedly topped up will keep producing magnetite faster than any filter can help with.
- It does not balance the system or fix a cold far radiator.
- It does nothing at all if nobody empties it.
Have a filter that has never been emptied? That is the cheapest improvement available on your heating system, and it takes five minutes.
Questions we get asked about this
Do I need a magnetic filter?
Most boiler manufacturers now require one as a warranty condition, and the practical case is strong: 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.
Can I clean it myself?
Yes, if the isolation valves are accessible. Close both, drain into a container, wipe the magnet, reassemble, reopen, check the pressure. Have old towels ready – what comes out stains.
Will it clean my existing sludge?
No. It catches what is circulating, not what has settled in the bottom of radiators. Clean the system first, then fit the filter to keep it clean.
Are expensive ones better?
The differences that matter are magnet strength, separation chamber volume, and how easy it is to clean. The last one is underrated – a filter that is awkward to service will not be serviced.
Does it replace inhibitor?
No, they do different jobs. Inhibitor prevents corrosion; the filter catches its product. You need both.
Need this doing?
Call and describe the problem — you will speak to an engineer, not a call centre. Written quotes, no estimates over the phone.