What is a magnetic filter and do I need one?
What the black sludge in a heating system is, what a magnetic filter does about it, and whether it is worth fitting to yours.
- Difficulty
- beginner
- Time
- 10 min
- Read
- 2 min
- Safety
- caution
Short answer
A magnetic filter sits on the pipework near the boiler and catches the iron oxide sludge that circulating water strips off radiators. It is worth having on any system with steel radiators, and most boiler warranties now require one.
Central heating water slowly corrodes the inside of steel radiators, producing a fine black iron oxide sludge. That sludge collects in the coldest, slowest parts of the system and eventually blocks heat exchangers — which is the expensive part.
Safety
Step by step
- Understand where the sludge comes from.Steel radiators corrode from the inside where water and any dissolved oxygen meet them. The product is magnetite — a fine black iron oxide that stays suspended and circulates.
- Know what it damages.It settles in radiator bottoms, causing cold patches, and it abrades and eventually blocks the boiler's heat exchanger and pump. Heat exchanger replacement is one of the more expensive boiler repairs.
- See what the filter actually does.It is a canister on the return pipe with a powerful magnet inside. Circulating water passes through and the magnetite sticks to the magnet instead of reaching the boiler. Non-magnetic debris settles in the body.
- Check whether you already have one.Look at the pipework near the boiler for a cylindrical canister, usually with a brand name and isolating valves either side. Most systems installed in the last decade have one.
- Check your boiler warranty terms.Many manufacturers now require a system filter and correct inhibitor levels as a warranty condition. A claim can be refused if the system is full of sludge and no filter was fitted.
- Have it emptied at each service.The magnet is removed and the collected sludge flushed out. It takes a few minutes and is normally part of an annual service — worth confirming it was actually done.
- Understand it is not a substitute for inhibitor.Chemical inhibitor slows the corrosion that produces sludge in the first place. The filter catches what is produced anyway. A system needs both, and inhibitor is topped up whenever the system is drained.
- Weigh it against a power flush.A filter is preventative and inexpensive. A power flush is remedial and costs several times more. If radiators are already cold at the bottom, the flush comes first and the filter keeps it that way.
Tips
- If a heating engineer opens your filter and it is full of black sludge, that is a sign the inhibitor has been depleted rather than that the filter is failing.
- Fitting a filter is straightforward for a plumber and usually costs less than a single emergency call-out.
- Cold spots at the bottom of radiators, with the top hot, is the classic sludge symptom and is worth acting on early.
Questions people ask
Do I need a magnetic filter on my heating system?
On any system with steel radiators it is worth having, and many boiler warranties now require one. It catches the corrosion sludge that would otherwise block the boiler's heat exchanger.
How often should a magnetic filter be cleaned?
Annually, as part of the boiler service. It takes a few minutes for an engineer and is worth checking has actually been done rather than assumed.