What a magnetic filter does
A magnetic filter traps magnetite, the fine black sludge produced as the water in a central heating system slowly corrodes steel radiators and iron parts. As the water flows through the filter, a magnet inside a sleeve pulls the magnetite out of circulation and holds it until the filter is cleaned. Many filters also have a mesh or chamber that catches non-magnetic debris such as scale, flux and bits of old pipework.
Sludge causes a long list of problems: radiators cold at the bottom, poor circulation, noisy boilers, blocked heat exchangers and failed pumps and diverter valves. A filter does not stop corrosion itself. That is the job of chemical inhibitor in the water. The filter catches what corrosion still produces so it cannot build up in the boiler.
Common makes include the Adey MagnaClean, Fernox TF1 and Sentinel Eliminator ranges, and boiler manufacturers sell their own, such as the Worcester Bosch Greenstar System Filter and Ideal's system filter.
Where it is fitted
A magnetic filter for a boiler is fitted on the heating return pipe, the pipe bringing cooler water back from the radiators, as close to the boiler as practical. That way everything returning to the boiler passes through it first. You will usually find it under the boiler as a round canister, often with the maker's name on the cap and two small valves either side so it can be isolated for cleaning.
Types of filter
Most homes have a compact filter sized for the 22 mm pipe under the boiler, with isolating valves built into its connections. Larger systems with 28 mm pipework use bigger in-line filters. Many designs add an air vent, a drain point and a dosing point for inhibitor on the cap, and Baxi notes that some newer filters can send app notifications when cleaning is due. The filter must be installed the right way round, as marked with a flow arrow on the body.
Do I need a magnetic filter on my boiler?
You do not strictly need one for the boiler to work, but it is strongly recommended, and on a new boiler it is expected. BS 7593:2019, the code of practice for treating water in domestic heating systems, says an in-line filter should be permanently installed to protect the system. Baxi describes fitting a filter as best practice when installing a new boiler, and most installers now include one as standard.
On an existing boiler without a filter, adding one is a sensible upgrade if the system has had sludge problems, the radiators are old, or the boiler has needed a new pump, diverter valve or heat exchanger. A filter is typically £150 to £300 supplied and fitted, depending on the make, pipe size and access. Prices are higher in London and the South East.
A filter catches sludge as it circulates, so it keeps a clean system clean but will not quickly clear a system that is already badly sludged. That needs a flush first. Our guides to radiators cold at the bottom and powerflush costs cover that step.
Does a magnetic filter affect the boiler warranty?
Yes, for many boilers it can. Several manufacturers require a filter to be fitted, and in some cases maintained, for their longer guarantees, and most exclude damage caused by sludge whatever the guarantee length. What the literature says for the main brands:
| Manufacturer | What its guarantee literature or manual says about filters |
|---|---|
| Worcester Bosch | Extended guarantee terms require a Greenstar system filter to be installed and registered with the boiler |
| Baxi | On Baxi 800 models, the filter supplied must be fitted and maintained for the 10-year warranty to stay valid; sludge damage is not covered |
| Ideal | The Logic Max installation guide says failure to install the system filter correctly will affect the boiler warranty |
| Vaillant, Glow-worm and others | Conditions vary by model and guarantee length; check the guarantee terms for your boiler |
Guarantee terms change, so the version that applied when your boiler was installed is the one that counts. Keep the installation paperwork and Benchmark checklist, and make sure the engineer records the filter clean on the service record each year.
If a new boiler quote does not mention a filter, ask whether one is included and which make. A manufacturer's own filter can be needed for the longest guarantee, while a third-party filter may be accepted for a shorter one.
How often should a magnetic filter be cleaned?
A magnetic filter should be cleaned at least once a year, at the annual boiler service. Baxi's guidance says the filter should be cleaned during the annual service, and Ideal's servicing instructions include cleaning the system filter as a service step. On an old or dirty system, it can fill quickly at first, so an installer may suggest checking it again soon after fitting or after a flush.
| What you notice | What it suggests |
|---|---|
| Thick black sludge on the magnet at the service | The system is corroding; inhibitor level needs checking |
| Filter needs cleaning much more often than yearly | Possible leak letting in fresh water and oxygen, or low inhibitor |
| Radiators cold at the bottom despite a filter | Existing sludge that needs a flush |
| Dark water from radiator bleed valves | Sludge in circulation; filter doing its job but system dirty |
If you are topping up the pressure regularly, fresh water is entering the system and speeding up corrosion. Our guide to a boiler losing pressure explains how to trace the leak.
How to clean a magnetic filter
Most people leave this to the engineer at the annual service. Some filter makers publish cleaning instructions for homeowners, and if yours does and you are comfortable doing it, the steps usually follow this pattern. Always follow the instructions for your exact filter, as caps, valves and drain points differ.
- Turn the heating off and let the system cool, so the water is not hot.
- Close the isolating valves on either side of the filter.
- Put a towel and a container underneath. Some water will come out.
- Release the pressure in the filter using the vent or bleed screw on the cap, if it has one.
- Unscrew the cap, usually with the tool supplied, and lift out the magnet in its sleeve.
- Clean the magnet. On most designs, sliding the magnet out of its sleeve lets the sludge drop off. Rinse the sleeve and wipe the inside of the canister.
- Check the sealing ring for damage and seat it correctly before refitting the cap hand tight or as the instructions say.
- Open the valves, bleed any air from the filter vent, and check for drips.
- Check the boiler pressure. Cleaning a filter usually lets some water out, so top it back up to 1 to 1.5 bar with the system cold using the filling loop.
Every drain and refill dilutes the inhibitor. Many filters have a dosing point on the cap for adding inhibitor, and an engineer can test the water and top it up at the service.
Only work on the filter with the system cold, and never loosen the cap while the valves are open or the system is under pressure, as hot, dirty water can spray out. Do not open the boiler case. If the filter keeps leaking after cleaning, close its valves and call a heating engineer.
What to do next
Look under your boiler to see if a filter is fitted and note the make. If there is one, ask for it to be cleaned and recorded at every annual boiler service, along with an inhibitor check. If there is not, ask your engineer whether one is worth adding, especially if you have had sludge problems or plan to keep the boiler for years. If you are replacing the boiler, check the filter and flush are included in the quote; our guide to new boiler costs explains what a typical installation covers.