A magnetic filter is installed on the heating return pipe that carries water back to the boiler, ideally on a vertical section after the last radiator. The work belongs to a Gas Safe registered engineer, and the system should be flushed and dosed with a suitable inhibitor to BS 7593 before the filter goes into service. A filter can be fitted with a new boiler or retrofitted to an existing system where there is enough room on the pipework. This page sets out the position on the pipe, the cleaning that comes first, the yearly clean manufacturers ask for, and what you may safely check yourself.
Where is a magnetic filter installed on a central heating system?
A magnetic filter is installed on the heating return pipe, the pipe that brings water back from the radiators to the boiler. Worcester Bosch's installation instructions for its system filter place the unit in the heating primary return, before the boiler and after the last radiator, on a vertical length of pipe. That position gives the magnet the best chance of catching debris before it reaches the appliance.
What the magnet collects is magnetite. Baxi describes it as a black substance in the pipework, formed when system water reacts with the metal in radiators and pipes. Vaillant describes the filter's job the same way: a strong magnet holds rust particles back so they do not reach the boiler and cause damage or blockages.
Filters are made in versions for 22 mm and 28 mm return pipes. Worcester Bosch notes the pump must have enough spare head to overcome the extra hydraulic resistance at higher flow rates. Pipe size and pump duty are therefore part of the sizing decision, not an afterthought.
Who is allowed to carry out a magnetic filter installation?
A magnetic filter installation is work for a Gas Safe registered engineer. Vaillant states that a filter can be added to an existing system by a Gas Safe registered engineer, provided there is enough space on the pipework. The job means draining part of a sealed heating system, cutting into the return pipe, fitting the body and valves, then refilling and recommissioning the boiler.
In England, the Gas Safety (Installation and Use) Regulations 1998 as amended require work on gas appliances and gas fittings in a home to be done by a Gas Safe registered engineer who is qualified for that work. HSE states plainly that it is illegal for anyone else to do it. Recommissioning a boiler after the system has been drained falls inside that duty.
A magnetic filter protects a clean system. It does not clean a dirty one. Flushing comes first, then the filter keeps the water clear afterwards.
Does the heating system need flushing before a magnetic filter is fitted?
Yes. Worcester Bosch's instructions for its system filter require the system to be flushed and a suitable inhibitor added to BS 7593, the code of practice for water treatment in domestic hot water central heating. Worcester Bosch also treats cleaning an existing system as good practice when a boiler is replaced, with cleaner circulated at operating temperature for at least an hour, and longer where deposits are heavy.
Where a system is already heavily sludged, Viessmann notes that a filter alone is not enough and a power flush is needed first. Baxi lists the signs of sludge: cold patches on radiators, with the top cooler than the bottom, unusual boiler noises such as gurgling or banging, and leaks. If those signs are present, the cleaning decision comes before the filter decision.
Which clean a system needs depends on how bad the deposits are. Our guide to power flush or chemical flush explains how the two differ, and power flushing covers the machine-assisted clean.
Magnetic filter fitted with a new boiler or retrofitted later: what differs
Most magnetic filters go in during a boiler installation, but an existing system can be retrofitted where there is room on the return pipe. The differences are about timing, space and how the system is cleaned.
| Point of comparison | Fitted with a new boiler | Retrofitted to an existing system |
|---|---|---|
| When the work happens | The filter goes in while the boiler is being installed and the system is already drained down. | A separate visit, with part of the sealed system drained specifically for the filter. |
| Cleaning the system | Worcester Bosch treats cleaning an existing system before the new boiler goes in as good practice. | The system is flushed and a suitable inhibitor to BS 7593 is added before the filter is commissioned. |
| Space on the pipework | Pipe runs can be set out around the filter position on the heating primary return. | Vaillant sets the condition as enough space on the return pipework near the boiler. |
| Heavily sludged system | Cleaning is handled during the changeover, before the new appliance runs. | Viessmann notes that a filter alone is not enough and a power flush is needed first. |
| Manufacturer warranty terms | Baxi requires the filter to be installed and maintained for the warranty period, or cover reverts to two years. | The same Baxi condition applies, alongside registration and annual servicing of the boiler. |
Whether a retrofit is practical is decided at the pipework, not on paper. An engineer looks at the return pipe near the boiler, the available vertical run and the pump duty before confirming it.
How often does a magnetic filter need cleaning after installation?
Manufacturers ask for the filter to be cleaned once a year by a qualified heating engineer. Vaillant states the filter should be cleaned annually so it keeps working. Baxi describes the annual boiler service as the point where the engineer checks that the system water is clean and clears the magnetic filter.
The clean is not automatic, so ask what a service visit covers before it is booked. Worcester Bosch also advises checking the inhibitor concentration every 12 months, or sooner if the system has lost water. A filter full of magnetite that is never emptied stops protecting the boiler.
Manufacturer warranty terms often depend on this upkeep. Baxi states that the magnetic filter must stay installed and maintained for the warranty period, otherwise cover falls back to two years. Ideal offers a parts warranty on its system filter when it is registered within 30 days, serviced annually and registered with a MAX boiler.
What can you check yourself, and what is engineer-only work on a filter?
You may safely check four things on your heating system yourself, and a magnetic filter is not one of them.
- Boiler pressure on the gauge, read when the system is cold, against the figure in your boiler manual
- Thermostat and programmer settings, including timings and the target room temperature
- The reset button on the boiler, where the manual says a reset is appropriate
- Bleeding a radiator with the heating off and the radiator cold, with a cloth underneath
Opening or emptying a magnetic filter is work for a Gas Safe registered engineer, as is anything involving gas pipework, the flue, appliance seals, the condensate inside the appliance or removing the boiler case. Vaillant notes that pressure dropping more often than roughly once a month points to a leak or a faulty pressure relief valve, which also needs an engineer.
If you are still weighing up the filter itself, our guide on whether a magnetic filter is worth it covers what it changes. Cold patches across several radiators or gurgling from the boiler are worth having looked at, and boiler service and repairs is the place to start.
Sources
- Vaillant – What is a magnetic boiler filter?
- Worcester Bosch – Greenstar System Filter installation and servicing instructions
- Worcester Bosch – Should I have my heating system treated or cleansed?
- Viessmann – Are magnetic filters worth it?
- Baxi – Do I have to have a magnetic boiler filter installed?
- Baxi – How do I know if I have got sludge in my central heating system?
- HSE – Domestic gas: frequently asked questions
- Gas Safety (Installation and Use) Regulations 1998
- Ideal Heating – Ideal System Filter