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Central Heating Magnetic Filter Cleaning

Central Heating Magnetic Filter Cleaning and Leak Fixing

Last updated on June 10th, 2024

A magnetic filter is fitted to a central heating system to collect iron oxide and sludge using a powerful magnet.

Sludge build-up inside parts on a boiler is the number one cause of a boiler breaking down.

Central heating magnetic filter cleaning is a great way to make your boiler last longer, help stop your boiler from leaking water, and keep your heating system running at its best.

I have been a full-time central heating engineer for over a decade and I have cleaned and replaced my fair share of magnetic boiler filters.

Here I’ll share my first-hand knowledge based on my on-the-job experiences over the years.

Magnetic Filter Location

The location of a magnetic filter should always be fitted on the return pipe if possible. It should also be after the last radiator on the heating system so right under the boiler is perfect.

This is the ideal scenario when having a magnetic filter installed to make it a more efficient heating system.

Fitting it here is best because when the sludge is building up in the radiators and pipes, it is caught before it enters the boiler and clogs up parts inside the boiler.

However, fitting it in this location on the heating system is not always possible.

There might not be enough room on the return pipe so you might have to fit it on the flow, or in a cupboard away from the boiler. This is fine.

If you can not fit it on the return pipe under the boiler then fitting it on the flow is fine.

Having a magnetic filter somewhere on the heating system is so much better than not having one at all.

Central Heating Magnetic Filter Cleaning

To clean a magnetic boiler filter you need to isolate it from the heating system and open it up.

Doing this (especially on old magnetic filters that haven’t been touched for years) can cause leaks on the valves or lid which might lead to you needing to replace the seal or even the filter itself.

Cleaning a Magnaclean filter is the most common I carry out as they are the most popular magnetic filters, but here is my process for cleaning magnetic filters on a boiler:

Cleaning Filter With Isolation Valves

  1. Turn the boiler off
  2. Place an old towel under the magnetic filter
  3. Isolate both valves
  4. Open the bleed valve by turning it anti-clockwise with a radiator bleed key to let the pressure out
  5. Close the bleed valve
  6. Remove the lid off with the plastic spanner
  7. Remove the sheath from the magnet
  8. Clean the sludge off the sheath with some kitchen roll, or run it under the tap if it’s not too thick of sludge
  9. Refit the sheath and magnetic in the filter and tighten up the lid
  10. Open valves and check for leaks

Cleaning Filter Without Valves

  1. Turn the boiler off
  2. Place an old towel and bucket or large tub under the filter to catch dirty water
  3. Unscrew the cap off the bottom of the filter if it has one
  4. Use some pliers or a screwdriver to open the valve at the bottom and let the water squirt into the bucket
  5. When the water is clear, close the valve and refit the cap

Central Heating Magnetic Filter Leaking

If your filter is leaking, this will usually be in one of three places:

  • The rubber O-ring seal inside the lid: The most common and easiest to repair
  • The isolation valves: Valves need to be replaced but probably better off replacing the full filter
  • The filter casing itself is split or damaged: The magnetic filter needs to be replaced

O-ring Leaking: How to Fix

By far the most common filter leak repairs I come across and carry out are when it is leaking from the O-ring.

The O-ring sits inside the lid and is the main seal that presses against the top of the unit when the lid is screwed on.

These can be hard to see and very tricky to get out but it needs to be removed. I usually remove it with my 3.5mm flathead screwdriver.

The first thing I do when the O-ring is removed is clean it up and clean all the dirt away from inside the lid where it sits.

Then I’ll cover it with thick silicone grease and also put some grease in the lid.

Then put everything back together and check for leaks. If it’s still leaking then the O-ring needs to be replaced.

You can find most (if not all) O-rings and kits in plumbing merchants and on Amazon.

Leaking Isolation Valves

The isolating valves on magnetic filters can leak when used for the first time in years.

If you don’t use the valves on magnetic filters (or a boiler) for many years then you close and open them for the first time they can leak.

This is a common reason for replacing central heating pump valves also.

When they leak they will need to be replaced.

You can buy new valves for some magnetic filters separately but if the filter is really old you might be better off replacing the full unit.

Old dirty leaking magnetic filter
Old dirty leaking magnetic filter


Fitting a magnetic filter somewhere on the heating system is much better than nowhere so if you don’t have one then get a Magnaclean installed now.

Central heating magnetic filter cleaning should be done every so often to make sure the sludge doesn’t build up too much and affect the flow of the water.

If you don't clean a magnetic filter out for many years then you risk them leaking when you eventually do clean them because the valves and seals haven't been disturbed for so long.

Feel free to ask me any questions in the comment section below and I’ll try my best to help.

Please share this post if you found it helpful.


Are magnetic filters worth it?

Yes, magnetic filters are definitely worth it. They can help your central heating system run more efficiently by collecting the sludge that builds up.

They can also stop your boiler from breaking down and help make it last longer.

Sludge inside boiler parts is the number one cause of boiler breakdowns on older boilers which is why boiler manufacturers will give an extended warranty for their new boilers being fitted with new magnetic filters.

How often should a magnetic filter be cleaned?

A magnetic filter should be cleaned as often as it’s needed. If you get an annual boiler service then you could have your magnetic filter cleaned then.

It can be left for years without being cleaned unless you think it might be full of sludge, then you should clean it.

How long does a magnetic filter last?

Good quality magnetic filters last many years. They can last 20-plus years without any problems but could also need to be replaced after 3 or 4 years.

It can depend on how well and often it is cleaned and maintained, but could also last for 20 years if it’s never touched.

When you don’t close the valves on a magnetic filter for many years and then close them for the first time, this can cause them to leak and need to be replaced.

It is recommended to replace your magnetic filter when you replace your boiler.


  • Steven Reid

    I am a full-time plumber and Gas Safe registered engineer. I incorporated Housewarm Ltd. in 2011 to provide heating and plumbing services to homes in Newcastle upon Tyne, England. I now blog about what I've learned over the years to help DIYers and plumbers.

    View all posts
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