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Do I need a magnetic filter on my boiler?
If your heating doesn’t have one, almost certainly yes. A magnetic filter catches the black sludge that wears boilers out, it’s cleaned at every service, and Worcester Bosch’s longer guarantees depend on one being fitted with the boiler.
- 1 hrtypical time to fit one
- 1clean a year, at your service
- 12years: the longest Worcester guarantee, which needs one fitted with the boiler
1 hrto fit
Every central heating system slowly makes a fine black sludge. It comes from the inside of steel radiators corroding, and it travels round the system in the water.
Left alone, it settles in the boiler, the pump and the valves. That’s where a lot of expensive repairs start, and it’s why radiators go cold at the bottom.
A magnetic filter is a small fitting that catches it before it gets there. If your system doesn’t have one, here’s why it’s worth it.
What a magnetic filter does
A magnet on the pipe back to the boiler.
The filter sits on the pipe carrying water back from the radiators to the boiler. Inside, a strong magnet pulls the black sludge, called magnetite, out of the water as it passes. The chamber slows the water down, so heavier non-magnetic bits drop out too.
It’s fitted with two valves, so at your service the engineer can shut it off, empty it and put it back without draining anything. What comes off the magnet is sludge that would otherwise be in your boiler.
- Protects the heat exchanger and pump
- Helps the boiler run as efficiently as it was designed to
- Cleaned at every annual service
- Usually fitted in about an hour
Cleaned at every service
Do I need one?
The signs, and the guarantee side.
Signs your system needs one
- There’s no filter on the pipe under the boiler
- The water is dark or black when you bleed a radiator
- Radiators are cooler at the bottom than the top
- Your boiler is a few years old on an unprotected system
Your boiler guarantee
Worcester Bosch’s extended guarantees, up to 12 years on selected models, depend on a Greenstar system filter being fitted with the boiler by a Worcester Bosch Accredited Installer. A filter added later protects your system, but it doesn’t extend a guarantee. Baxi’s 10 year warranty on the Baxi 800 needs its filter fitted and looked after for the whole period.
The British Standard
BS 7593, the standard for heating system water, says a filter should be fitted to protect the system and cleaned every year. It also says a filter is never a replacement for cleaning a dirty system.
Checking the water
New boiler?
Every boiler we install includes a system flush and a magnetic filter in the package. You don’t need to ask for it.
What happens on the day
A tidy job, often done alongside your service.
Shut down and drain
The system is shut down and drained as little as the job allows.
Fit the filter
We cut it into the return pipe close to the boiler, with valves so it can be cleaned without draining anything.
Refill and dose
The system is refilled, vented and topped up with a fresh dose of corrosion inhibitor to slow new sludge forming.
Test and tidy
We run the heating up to temperature, check for leaks, show you where the filter is and leave the area as we found it.
Which one sounds like you?
- No filter on my systemGet one fitted
Often done at your service.
- Black water from the radiatorsClean the system first
A filter alone fills quickly.
- Already have oneKeep it cleaned
Emptied at every service.
- Getting a new boilerIt’s included
Flush and filter with every install.
Common questions
Is my boiler at risk without a magnetic filter?
Not straight away, but sludge builds up year on year and settles in the boiler’s heat exchanger and pump. A filter costs a small fraction of either, which is why we recommend one on any system that doesn’t have one.
Which filter do you fit?
On its own, we fit an ADEY MagnaClean Pro2 on the return pipe near the boiler. New boilers come with a filter as part of the installation.
Does a magnetic filter need looking after?
Only at your annual service. The engineer opens it, removes what it’s caught and puts it back. There’s nothing for you to do.
Will a filter fix radiators that are already cold at the bottom?
It stops the problem getting worse and catches what’s going round. If the radiators are already badly sludged, a power flush first will get them properly clean.
Does it make a mess?
No. We drain as little as we need to, use dust sheets, and take all the packaging away.
Do I need to be home?
Someone over 18 needs to let us in and be there at the handover. The work itself doesn’t need you on hand.
How much does it cost?
We quote one price for your home before we start, and nothing is added on the day. If we’re already there for your service, it can often be done in the same visit.
Related pages
Sources: Worcester Bosch, extended guarantee terms; Baxi, registering your warranty; PHPI, BS 7593:2019 explained. Checked 2 November 2026.
Not sure what you need? Give us a shout.
Book online in a couple of minutes or give the office a call, Mon to Fri, 9am to 5pm. We’ll tell you what we think, and if the answer is to leave things as they are, we’ll say that too.
Worcester Bosch Accredited Installer
Guarantees of up to 12 years on the boilers we fit. Gas Safe registered. Fife Trusted Trader. 400+ customer reviews.