A magnetic filter, also known as a magnetic separator or dirt separator, is a small device that can significantly extend the life of a boiler and circulation pump. It captures particles of magnetite and sludge from the heating water before they settle in the heat exchanger. However, it only does its job when installed in the right place and cleaned regularly.
A magnetic filter for a gas boiler is installed on the return pipe, before the water enters the boiler, and is cleaned for the first time 1 to 2 months after commissioning and then at least once a year. Below, we explain how to clean the filter, where to install it, and what the sediment collected on the magnet reveals about the system.
Table of contents
- Why is a magnetic filter important for a gas boiler?
- Cleaning a magnetic filter step by step
- How often should a magnetic filter be cleaned?
- Where and how to install a magnetic filter?
- What does the sediment mean, and when should you flush the system?
- Frequently asked questions
Why is a magnetic filter important for a gas boiler?
A modern gas boiler, especially a condensing model, has a heat exchanger with narrow water channels. Particles of magnetite and sludge that would pass through an old cast-iron boiler without any problem settle in this type of heat exchanger, restricting flow and reducing heat transfer. The boiler then starts to hum, switches off more often, and may eventually report flow or overheating errors.
In a combi boiler, sediment also gets into the plate heat exchanger for domestic hot water and the built-in pump. That is why the filter is installed on the return pipe, before the water enters the boiler: this is the last place where contaminants can be stopped before they reach the most expensive components. When installing a new boiler, it is also worth checking its manual and warranty terms to see whether the manufacturer recommends specific protection for the heating water.
Cleaning a magnetic filter step by step
Cleaning the filter is a simple maintenance task that a homeowner can also carry out with a little care. It should always be done with the boiler switched off and cooled down. Do not open the filter while the system is operating, as hot water under pressure can cause scalding.
- Turn off the boiler and wait for the system to cool down.
- Close the shut-off valves that isolate the filter from the system circuit.
- If possible, remove the magnet before draining the water so that the sediment settles at the bottom of the chamber. For other filters, skip this step.
- Place a container underneath and drain the dirty water through the drain valve.
- Unscrew the housing, remove the magnet and mesh, and then wash them with warm water. Do not use a wire brush or solvents, as they may damage the surface and seals.
- Check the condition of the seals, reassemble the filter, and tighten the housing.
- Bleed the filter, open the valves, and check the pressure on the boiler pressure gauge. If it has dropped, top up the water according to the instructions in the article about what to do when there is too little water in the central heating system.
It is always worth checking the exact sequence of steps for the specific model in its manual, as filter designs differ in how the magnet and drain are secured.
How often should a magnetic filter be cleaned?
The first cleaning should be performed 1 to 2 months after start-up, as the filter collects the most contaminants left over from installation during this period. Subsequent cleanings should be carried out at least once a year, preferably before the heating season, together with the boiler inspection.
In practice, the amount of sediment determines how often it needs to be cleaned. If the filter has been installed in an older system, it is worth opening it more frequently until the amount of sediment clearly decreases. If there is always a lot of sediment despite regular cleaning, the cause should be sought in the system itself.
A clogged filter restricts water flow. The radiators farther along the system then heat less effectively, while the boiler quickly reaches the set temperature and starts short-cycling. When you notice these symptoms, start troubleshooting with the filter.
Where and how to install a magnetic filter?
The filter is installed on the return line, after the last radiator and before the boiler. It is the return water that carries sediment collected from the radiators and heating loops, so the filter traps it before it reaches the heat exchanger and pump. We write more about this in the article on how the magnetic filter on the return line protects the pump, and discuss how the device works in the article about protecting heating systems with magnetic filters.
- Distance from the boiler: the instructions for IBO filters recommend installing them at least 1 m from the boiler.
- Shut-off valves: install one on each side. Without them, every cleaning requires draining the water from the system.
- Flow direction: the arrow on the body must point toward the boiler.
- Grounding: when installing on metal pipes, the instructions recommend grounding the filter.
- Other boiler types: for a solid-fuel boiler in an open system, do not install the filter between the boiler and the open expansion vessel, so as not to restrict the safety circuit.
Warning: strong magnetic field. The magnet in the filter may interfere with electronic devices and pacemakers. Instructions recommend keeping such devices and people with implanted pacemakers at least 35 cm away from the filter, especially during cleaning, when the magnet is held in the hand.
| Model | Body position |
|---|---|
| I-001 and I-002 | Vertical or horizontal |
| I-003 and I-004 | Vertical only |
| IBF-05 | Vertical, horizontal, or at an angle |
The filter should be installed near the boiler by a qualified installer, as this requires cutting the pipe and refilling the system. After filling, it is a good idea to bleed the central heating pump right away. You can find a suitable model that meets your system’s specifications in our collection of magnetic separators and filters for central heating systems.
What does the sediment mean, and when should you flush the system?
The filter’s contents reveal a lot about the condition of the system. Black, greasy sediment on the magnet is magnetite, a corrosion product of steel components. A small amount is natural, but if there is a lot every time you clean the filter, oxygen is continuously entering the system.
The most common sources of oxygen are leaks and frequent topping up of the water. The latter is often caused by an expansion vessel with incorrect pre-charge pressure; we describe how to check it in our guide how to inflate an expansion vessel. Rust-colored sludge on the mesh indicates fresh, oxygenated water, while sand and metal filings are usually remnants of installation.
The filter protects the system continuously, but it will not remove sludge that has accumulated in radiators over the years. If the amount of sediment does not decrease or the lower parts of the radiators remain cold, it is best to flush the central heating system first. After flushing, a magnetic filter will help keep it clean.
Frequently asked questions
Does a gas boiler need a magnetic filter?
Yes, if the system has steel components where magnetite forms. The filter protects the boiler’s heat exchanger, built-in pump, and domestic hot water heat exchanger from sediment.
Where should a magnetic filter be installed with a gas boiler?
On the return pipe, after the last radiator and before the boiler, with shut-off valves on both sides.
How often should a magnetic filter be cleaned?
For the first time 1 to 2 months after commissioning, then at least once a year, preferably before the heating season.
Should a magnetic filter be installed on the supply or return pipe?
On the return pipe, before the boiler. Only there does the filter trap sediment from the system before it reaches the boiler’s heat exchanger and pump.
Can a magnetic filter be cleaned without draining the system?
Yes, if it has shut-off valves on both sides. In that case, only the water in the filter itself needs to be drained.
What does black sediment in a magnetic filter mean?
It is magnetite, a corrosion product of steel. A large amount each time the filter is cleaned means that oxygen is continuously entering the system.
Can a clogged magnetic filter cause poor heating?
Yes. A clogged filter restricts flow, causing radiators farther along the system to heat less effectively, while the boiler starts short-cycling.
Does a magnetic filter replace system flushing?
No. The filter continuously traps sediment, but it will not remove sludge that has accumulated in radiators, so a dirty system should be flushed first.
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