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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.
Circulating pump flow rate
Age of existing system — used for an indicative sludge estimate
Maximum acceptable pressure drop through filter
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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: Choose a preset band (small ~60L, medium ~110L, large ~200L) or enter your own litre figure if you know it. As a rough guide, allow ~8–12L per large radiator plus the boiler and pipework.
- Filter size: Recommended no smaller than your primary pipe diameter (22mm / 28mm / 35mm), then scaled up for larger system volumes.
- Pressure drop: An indicative estimate only — modern filters use full-flow valves for minimal restriction. Always confirm against the chosen filter's data sheet; aim well within pump capability (often < 0.05 bar).
- Sludge estimate: An indicative rule-of-thumb of ~0.5g of magnetite per litre per year in an unprotected system (tapering as the system passivates), not a figure from any published standard.
BS 7593:2019+A1:2024 (the code of practice for preparing, commissioning and maintaining domestic heating systems) calls for the system to be cleaned, dosed with corrosion inhibitor and protected by a permanent in-line filter, with the inhibitor checked annually and re-dosed at least every five years. A magnetic filter is complementary to chemical inhibitor — not a replacement for it — and continuously captures magnetite between services.
Frequently Asked Questions
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.
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.
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.
Industry research (commonly cited by the Energy Saving Trust) suggests keeping a heating system free of sludge can improve efficiency by up to around 6%. Magnetite sludge coating the heat exchanger acts as an insulator and reduces heat transfer, so a magnetic filter that continuously removes these particles helps protect efficiency. Treat 6% as an upper-bound estimate rather than a guaranteed saving.
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. Use our Inhibitor Dosing Calculator for the correct dose rate.
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Last updated: March 2026
Verified against UK standards · estimates only, confirm with your supplier.