Magnetic heating filters: what they catch and who fits them

If you have had a new boiler fitted in recent years, there is a good chance a chunky cylinder sits on one of the pipes underneath it. That is usually a magnetic system filter. It catches the black iron oxide and debris that circulate in the heating water, so less of it reaches the boiler, the pump and the radiators.

This guide explains what a filter does and what to ask about. It is not a fitting guide. Work on the pipework at a boiler is a plumber's or heating engineer's job, and if it is on or next to a gas boiler, a Gas Safe registered engineer should be involved. See when to call a Gas Safe engineer.

What it catches

The water inside a central heating system slowly corrodes the steel in radiators and pipework. The main product is magnetite, a black iron oxide that forms sludge. Our heating inhibitor and sludge guide explains where it comes from and what inhibitor does to slow it.

A magnetic filter has a strong magnet inside. As the heating water flows through, magnetic particles stick to it and are held there instead of travelling on. Many filters are combined filters: as well as the magnet, they slow the water or pass it through a mesh so non-magnetic debris such as scale, sand or solder fragments settles out too.

The filter does not stop corrosion. It collects the results of it. That is why filters and inhibitor are usually used together.

Why so many systems have one

Modern boilers have compact heat exchangers with narrow waterways. Debris that an old cast iron boiler would have tolerated can block them or reduce efficiency. Pumps and valves are also sensitive to sludge.

Many boiler makers and installers recommend, or expect, a system filter to be fitted, alongside cleaning the system and adding inhibitor when a new boiler goes in. Whether it is a condition of a particular boiler warranty depends on the maker. Check the warranty terms you were given, or ask the installer.

Where it is fitted

A filter is usually fitted on the heating pipework near the boiler, on the pipe bringing water back from the radiators, so debris is caught before the water re-enters the boiler. Some go elsewhere if space under the boiler is tight. Valves either side let the filter be isolated for cleaning.

Fitting one is a plumber's job. It means draining down at least part of the system, cutting into pipework close to the boiler, making watertight joints, refilling, removing air and putting the inhibitor back to the right level. On a sealed system that also means repressurising. Any work on or next to the boiler should be done by someone competent, and anything involving the gas appliance by a Gas Safe registered engineer. We do not give a walkthrough.

Cleaning and servicing

A filter only helps if it is emptied. Most are designed to be cleaned at the boiler service: the engineer isolates the filter, removes the magnet or opens the body, and flushes out the collected sludge. If they find a lot of black debris, that tells them something about the system.

Things a householder can usefully do:

• Ask at each boiler service whether the filter was cleaned, and what was in it.
• Keep the filter's paperwork with the boiler documents.
• Look for drips around the filter body or its valves, and call your plumber if you see any.
• Do not open it yourself unless the maker's user instructions clearly say a householder can, and you can do it without touching the boiler. On a sealed system, opening the filter releases pressure and water, which then has to be restored; that is usually the engineer's job.

Signs the system needs more than a filter

A filter catches debris that is circulating. It will not clear a radiator that is already packed with sludge, or a heat exchanger that is already blocked. If radiators are cold at the bottom, or bleeding produces black water, a clean or flush may be needed. See radiator bleeding and balancing for how to tell air from sludge.

Products

We have not linked a filter here. The filters we found on the retailer pages we checked today were sold as boiler-maker accessories, and we do not link to boilers or gas appliance accessories. Ask your plumber or heating engineer which filter suits your boiler and pipework, and let them fit it.

Do I need a magnetic filter?

It is not a legal requirement for every home, but it is widely recommended, particularly with a modern boiler. If you are having a new boiler fitted, ask the installer whether one is included and how it will be maintained.

Can I fit a magnetic filter myself?

We do not recommend it. Fitting one means draining and refilling the heating system and cutting into pipework next to the boiler. Ask a plumber, and a Gas Safe registered engineer for anything involving a gas boiler.

How often should a magnetic filter be cleaned?

Usually at the annual boiler service, or as the filter maker recommends. A system with a lot of sludge may need it more often at first.