A magnetic filter is usually worth fitting when a new boiler goes in, because it collects the magnetite sludge that would otherwise reach the boiler. Magnetite is the black iron oxide debris produced as radiators and pipework corrode, and manufacturers describe it as a cause of cold radiators and damaged boiler parts. The filter goes on the return pipe and needs cleaning once a year by your engineer. It protects a clean system rather than rescuing a dirty one, and several boiler makers tie their longer guarantees to having one fitted and maintained.
Is a magnetic filter worth it on a central heating system?
A magnetic filter earns its place because it keeps magnetite out of the boiler. Magnetite is the black iron oxide sludge that forms as steel radiators and pipework corrode. Baxi describes sludge as a cause of cold spots on radiators, of the boiler working harder, and of boiler components becoming blocked or damaged. A filter on the circuit collects that debris before it reaches those parts.
Two things decide whether a filter is worth it in your home. The first is the state of the system now. A filter gathers debris as it circulates. It does not strip out a heavy build-up that is already sitting in the radiators.
The second is your boiler guarantee. Several manufacturers set longer guarantee periods when a system filter is fitted and looked after. That makes the filter part of the boiler decision rather than an optional extra bolted on later.
What does a magnetic filter catch, and what does it miss?
A magnetic filter catches magnetic debris, mainly rust particles travelling round the heating circuit. Vaillant describes the device as a strong magnet that pulls magnetic debris out of the system water. Baxi names the material magnetite, an iron oxide sludge produced by corrosion inside the pipework over time.
Non-magnetic dirt is a separate question. Some filters add a cyclonic separator, which spins heavier particles out of the flow. Worcester Bosch states that its Greenstar System Filter separates non-magnetic particles as well as capturing iron oxide. Ideal describes its own system filter as removing contaminants from the water passing through it, to protect domestic heating systems.
So what gets caught depends on the model fitted, not on magnetic filters as a category. Worth asking before the job, not after.
Where is a magnetic filter fitted on a heating system, and when?
A magnetic filter is fitted on the return pipe, the pipe carrying water back to the boiler after it has been round the system. Vaillant gives the return as the fitting position and names a new boiler installation as the usual time to fit one. At that point the pipework is open anyway and the system is being cleaned and refilled.
Adding one to an existing system is possible where there is enough room around the pipework. It is work for a Gas Safe registered engineer. Space is the usual sticking point. In a tight airing cupboard or a boxed-in kitchen run, the pipework has to be altered to take the filter body and still leave room to clean it.
We carry out a free home survey before a new installation, and you get a written fixed price before any work starts. You can see what that covers on our new boiler installation page.
How often does a magnetic filter need cleaning, and who should do it?
A magnetic filter should be cleaned once a year. Vaillant recommends annual cleaning to remove the collected debris and keep the filter doing its job, and says your heating engineer can do it at the annual service. Baxi says the filter should be cleaned at the annual service, with the system water checked for cleanliness at the same time. Ideal states that its system filter must be serviced annually.
Cleaning the filter is not a householder task. It means isolating and opening a component on a live heating circuit, and it belongs with the engineer who services the boiler.
What you can safely check yourself stays short: boiler pressure on the gauge, the thermostat and programmer settings, the reset button and bleeding radiators. Our guide on what to check before you call an engineer goes through those four.
Does a magnetic filter affect your boiler guarantee?
Manufacturers link guarantee length to a system filter, and those are their terms, not ours. Baxi states that on the Baxi 800 range a magnetic filter must be fitted and maintained for the length of the guarantee, or cover reverts to two years. Baxi also states that boiler damage caused by sludge is not covered.
Other makers set out similar conditions in their own words:
- Ideal: a 10-year heat exchanger guarantee on the Logic and Logic+ ranges when the boiler is installed with an Ideal System Filter.
- Worcester Bosch: extended guarantees tied to registration of a system filter, with the detail in its extended guarantee terms and conditions.
- Vaillant: a guarantee extension through its Advance Protection Kit, which includes a filter as part of the kit.
These are the manufacturers' own terms, and terms change and vary by range. Read the current conditions for your model before you decide, rather than treating the examples above as cover you already have. Worcester Bosch also states in its installation manual that failing to flush the system and add an inhibitor can invalidate the appliance guarantee.
A magnetic filter will not clear sludge that is already in the system
Fitting a filter to a system that is already heavily sludged does not solve the problem. Vaillant is clear that a filter will not remove an existing heavy build-up, and that severe cases need the system cleaning by a qualified heating engineer. The filter then keeps the water clean afterwards.
Baxi lists signs that point towards sludge in a central heating system:
- Radiators hot at the top and cold at the bottom.
- Needing to bleed radiators more often than before.
- The house taking a long time to warm up.
- Hot pipework leaving the boiler while the return pipe stays lukewarm.
- Gurgling noises, or kettling sounds from the boiler.
Those are symptoms, not a diagnosis. Ideal and Glow-worm both explain a radiator that is cold at the bottom as sludge blocking the lower channels inside it. An engineer confirms what is going on and whether a clean is needed first. Our guide on a power flush or a chemical flush explains how those two methods differ.
What the rules in England say about cleaning and protecting the system
Approved Document L, Volume 1: Dwellings applies in England and asks for domestic central heating systems to be prepared and commissioned in line with BS 7593. Before a new heat generator is installed, the heating and primary hot water circuits should be thoroughly cleaned and flushed. A suitable chemical inhibitor should then be added to the primary circuit to guard against corrosion and scale. In hard water areas, the guidance asks for suitable measures to treat the feed water and reduce scale build-up.
A filter sits in the water circuit, not the gas way. Work on gas fittings in a home in England falls under the Gas Safety (Installation and Use) Regulations 1998 as amended, which require anyone carrying out the work to be competent. In domestic premises that means a Gas Safe registered engineer. Opening the boiler casing, the flue, seals or the condensate system inside the appliance is that kind of work.
If your hot water comes from a stored cylinder, the same system water passes through its heating coil, so the filter protects that too. You can see how we handle Megaflo and unvented systems if a cylinder is part of your plans.
Sources
- Vaillant – What is a magnetic boiler filter?
- Baxi – What is a magnetic boiler filter
- Baxi – How do I know if I have got sludge in my central heating system?
- Baxi – Boiler warranty information
- Ideal Heating – 10 year heat exchanger warranty on Logic and Logic+
- Worcester Bosch – Introducing our new Greenstar System Filter
- Approved Document L, Volume 1: Dwellings (2021 edition incorporating 2023 amendments)
- The Gas Safety (Installation and Use) Regulations 1998