Glossary · Boilers and boiler types
What is a magnetic system filter?
A magnetic system filter is a small cylinder fitted to the heating pipework near the boiler. A strong magnet inside it catches the black iron oxide sludge that forms in radiators and pipes, and traps catch other debris. It keeps that dirt away from the boiler, pump and valves.
Key points
- The magnet captures black iron oxide sludge. Separate traps catch non-magnetic debris such as solder and flux.
- Worcester says to place it on the heating return before the boiler and after the last radiator.
- The filter needs cleaning every year. Worcester says to do this at the annual boiler service.
- A filter does not clean a dirty system. Worcester says the system should be flushed and an inhibitor added first.
- Vaillant and Ideal both point to a filter as a way to extend boiler life.
How a magnetic system filter works
Over time the metal inside a central heating system corrodes. Radiators and pipework shed a fine black sludge called magnetite, which is a form of iron oxide, and other debris such as solder and flux can be left from installation. If it keeps circulating, it settles in the places that matter most.
The filter sits in the pipework and gives that dirt somewhere to stay. Worcester’s manual for its Greenstar filter describes it as bi-directional, so water can flow either way through it. The water slows as it circulates round the filter, a powerful magnet captures the iron oxide sludge, and traps at the top and bottom hold the non-magnetic debris.[1]Worcester, Bosch Group, Greenstar System Filter: Installation and servicing instructions
The aim is to protect the heat exchanger, pump and valves. The manual says contaminated system water can cause pumps, heat exchangers and valves to block, and can leave radiator cold spots and even make a radiator corrode from the inside, causing leaks.[1]Worcester, Bosch Group, Greenstar System Filter: Installation and servicing instructions It also says a layer of sludge on the inside of radiators and the boiler heat exchanger acts as a barrier, so the system uses more fuel to reach the same room temperature.[1]Worcester, Bosch Group, Greenstar System Filter: Installation and servicing instructions
Where it goes
The same manual says the filter can be installed almost anywhere in a heating system, but not between the boiler and the feed and expansion pipes on an open vent system. For the best effect it should be on the primary heating return, before the boiler and after the last radiator, with enough access for servicing.[1]Worcester, Bosch Group, Greenstar System Filter: Installation and servicing instructions
| Point | What the manufacturer says |
|---|---|
| Position | Heating return, before the boiler, after the last radiator |
| Before fitting | Flush the system and add an inhibitor in line with BS 7593 |
| Existing sludge | Needs a chemical flush, a power flush or a MagnaCleanse flush before the filter goes in |
| Every year | Clean the filter at the annual boiler service |
These are the Worcester instructions for its own filter. Other makes differ in detail, so the manual for the filter on your system is the guide.
Cleaning, cost and who does it
Worcester says the system should be flushed and a suitable inhibitor added before the filter is installed, and that skipping this limits how well the filter works.[1]Worcester, Bosch Group, Greenstar System Filter: Installation and servicing instructions On a dirty older system the manual advises a chemical gravity flush at least, or for best practice a power flush or MagnaCleanse flush. BASI’s power flushing page covers that work, and our pages on power flush vs chemical flush and central heating inhibitor explain the options.
Vaillant says a Gas Safe registered engineer can install and clean a magnetic filter during the annual service, and that it prolongs the life of the boiler.[2]Vaillant, How long do boilers last? Ideal Heating also recommends fitting one, saying it removes impurities from the water and helps the boiler last longer.[3]Ideal Heating, How long does a boiler last? Where the filter sits on the boiler’s pipework at an annual service, the engineer needs to be Gas Safe registered anyway, because anyone employed to work on gas appliances in a home must be.[4]Health and Safety Executive, Domestic gas: frequently asked questions
One more point from the manual: the filter contains a strong magnet and carries a caution to that effect.[1]Worcester, Bosch Group, Greenstar System Filter: Installation and servicing instructions
Example
Gareth in Halifax is having a new combi fitted to an older radiator system. The engineer checks the system water, flushes it, adds an inhibitor and fits a magnetic filter on the return pipe near the boiler. At each annual service the engineer cleans the filter and checks the sludge it has collected.
What people get wrong
The usual belief is that a filter fixes a dirty system. It does not. It keeps clean water clean and catches what is still being shed, but it cannot remove sludge already coating the radiators. The system needs flushing first.
The second error is fitting a filter and never cleaning it. Worcester says it should be cleaned each year, and a full filter catches less.
The bottom line
A magnetic system filter is a cheap guard for the most expensive parts of a heating system. It catches sludge before it reaches the boiler, pump and valves. Have the system flushed and treated first, fit the filter on the return pipe and have it cleaned at every annual service by a Gas Safe registered engineer.
Common questions
What does a magnetic system filter do?
It catches iron oxide sludge with a strong magnet and traps other debris, so the dirt does not circulate through the boiler, pump and valves.
Where should a magnetic filter be fitted?
Worcester says it should go on the primary heating return before the boiler and after the last radiator, with room around it for servicing. Your installer will choose the exact spot for your system.
How often does a magnetic filter need cleaning?
Every year. Worcester says the filter should be cleaned each year during the annual boiler service, so the engineer can do it at the same visit.
Is a magnetic filter enough on its own?
No. Worcester says a flush and a chemical inhibitor are needed with it, and that an existing system showing sludge needs flushing before the filter is fitted.
Sources
We cite the original source for every figure: government, regulators, standards bodies and manufacturers' own documents. Each one shows the date we last checked it.
- Worcester, Bosch Group. Greenstar System Filter: Installation and servicing instructions. Checked 11 November 2026. ↩
- Vaillant. How long do boilers last?. Checked 11 November 2026. ↩
- Ideal Heating. How long does a boiler last?. Checked 11 November 2026. ↩
- Health and Safety Executive. Domestic gas: frequently asked questions. Checked 11 November 2026. ↩