Magnetic filters: what they catch and what they cannot

What matters most with a magnetic system filter

• Most warranty terms on new boilers require a filter to be fitted
• Filters should be cleaned at every annual service, not left indefinitely
• They trap magnetite effectively but do nothing for dissolved limescale
• Fitting one after a flush protects the clean result going forward
• A full filter can restrict flow and mimic other circulation faults

Where a magnetic system filter usually comes from

Most manufacturers now specify a filter as a condition of the extended warranty on new boiler installs, because it protects the single most expensive component from the abrasive black sludge that steel radiators generate over time. It needs opening and cleaning periodically, usually at the annual service, where the trapped magnetite is rinsed out. A filter manages ongoing contamination; it is not a substitute for a flush on a system that is already heavily sludged.

Where we deal with this

• Cameley (BS39) · 5 mi
• Stanton Drew (BS39) · 8 mi
• Hinton Blewett (BS39) · 5 mi

Questions we get asked

Do I need a filter if my system is already flushed?

Yes — a flush removes existing sludge, but radiators keep corroding slowly, so a filter catches what forms afterwards.

Can I clean the filter myself?

Some are designed for homeowner cleaning between services, but check the manufacturer's instructions before opening one under pressure.