What a magnetic filter does
An in-line filter is plumbed into the return pipe just before the boiler. A magnetic core inside it catches the magnetite — the black iron oxide particles that make up most heating sludge — and a chamber catches non-magnetic debris such as jointing compound, flux residue and scale fragments. It is emptied by closing two valves and draining the canister, which is a few minutes' work at the annual service.
MagnaClean is the brand name most people know, which is why the old version of this site had a page for it. It is one of several; Adey, Fernox, Sentinel, Spirotech and others all make equivalent units, and what matters is that one is fitted and emptied, not whose badge is on it.
What BS 7593:2019 actually says
BS 7593:2019 is the British Standard code of practice for preparing, commissioning and maintaining domestic central heating systems. Its practical requirements are:
- The system is cleaned before a new boiler is installed, or when a new boiler has been isolated from the rest of the system.
- A permanent in-line filter is fitted, capable of capturing both magnetic and non-magnetic debris.
- The correct dose of corrosion inhibitor goes in, calculated for the system volume rather than by rule of thumb.
- Inhibitor levels are checked over the life of the system, by test or laboratory analysis of the system water.
- The filter is serviced as part of the annual boiler service.
This is a code of practice rather than a law, but it is the standard manufacturers reference when assessing warranty claims and the one a competent installer will work to. If you are quoted for a boiler and none of the above appears on the quote, ask why.
What it costs
- Filter supplied and fitted: roughly £150 to £250 depending on unit and how awkward the pipework is.
- Inhibitor dose: roughly £60 to £100 including the visit, less if done alongside other work.
- Inhibitor test: often free or nominal as part of a service, and the cheapest diagnostic available to you.
- Annual filter clean: usually included in a boiler service.
Against £350 to £800 for a flush every five or six years, a filter fitted once and emptied annually is straightforwardly good value.
What a filter will not do
Three honest caveats, because filters get oversold too:
- It does not clean an already sludged system. It catches what circulates past it. Deposits sitting in the bottom of a radiator are not circulating.
- It does not replace inhibitor. The filter catches corrosion product; the inhibitor stops it forming. You need both.
- It does nothing if nobody empties it. A full filter is not filtering. This is the most common failure, and it is why the annual service matters.
If you are having a new boiler fitted
The clean, the filter and the inhibitor should all be on the quote, itemised. A quote that bundles them invisibly into a headline price is one you cannot compare against another quote. More on what a boiler replacement quote should contain.
Common questions
Is a magnetic filter worth fitting?
On a system with steel radiators, yes. It costs roughly £150 to £250 fitted, catches the corrosion product that would otherwise reach the boiler heat exchanger, and is required under BS 7593:2019 when a new boiler is installed. It only works if it is emptied annually.
Does a magnetic filter mean I never need a power flush?
It substantially reduces the likelihood, but it does not clean a system that is already sludged, and it does not replace corrosion inhibitor. A filter fitted to a clean, inhibited system that gets emptied every year may well mean you never need another flush.
How often should a magnetic filter be emptied?
Annually, as part of the boiler service. BS 7593:2019 treats servicing the filter as part of the re-commissioning process. A filter that is never emptied stops doing its job.