What size magnetic filter do I need?

For most domestic systems with 22mm pipework and up to 100 litres system volume, a 22mm compact magnetic filter is sufficient. Larger systems (100-200L) need a full-size 22mm filter, and systems over 200L or with 28mm pipework should use a 28mm filter. Always check the manufacturer’s maximum flow rate rating.

Where should a magnetic filter be installed?

Install the magnetic filter on the return pipe to the boiler, as close to the boiler as possible. This protects the boiler heat exchanger from circulating debris. The filter should be accessible for cleaning and have isolation valves either side.

How often should I clean a magnetic filter?

On new installations or recently flushed systems, check the filter after 2-4 weeks, then at the first annual service. On older systems, check every 6-12 months. If significant sludge is found, more frequent cleaning or a power flush may be needed.

Does a magnetic filter affect boiler efficiency?

Studies by the Energy Saving Trust and HHIC show that a clean heating system can improve boiler efficiency by up to 6%. Magnetite sludge coating heat exchanger surfaces acts as an insulator, reducing heat transfer. A magnetic filter continuously removes these particles.

Do I still need chemical inhibitor with a magnetic filter?

Yes. A magnetic filter captures magnetite (iron oxide) particles that have already formed. Chemical inhibitor prevents new corrosion from occurring. Both are needed for comprehensive system protection. The inhibitor concentration should be checked annually at each service.

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Magnetic Filter Calculator — System Protection

Size magnetic system filters based on system volume and flow rate. Calculate pressure drop compatibility and cleaning schedules.

System Size

Approximate heating system volume

Number of Radiators

Total radiators on system

Primary Pipe Size

Main flow/return pipe diameter

System Flow Rate

Circulating pump flow rate

System Age

Age of existing heating system

How We Calculate This

This calculator helps select the correct magnetic system filter based on your heating system's volume, pipe size, and flow rate. It also estimates sludge accumulation and cleaning schedules.

Sizing method

System volume: Estimated from number of radiators (× ~10L each) plus boiler and pipework
Filter size: Matched to pipe diameter and maximum flow rate
Pressure drop: Must be within pump capability (typically < 0.05 bar)
Sludge estimate: ~0.5g per litre per year of magnetite production in unprotected systems

Building Regulations Part L and BS 7593:2019 require heating systems to be cleaned and treated with corrosion inhibitor. A magnetic filter is a best-practice addition that continuously protects the system between services.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.