Fernox TF1 Sigma Mini 22mm Slipsocket review
Combi or system boilers with a tight service cupboard, where a full-size in-line filter simply won't fit.
By Tom Calder
A well-made, compact neodymium magnetic filter that protects the boiler's heat exchanger from magnetite sludge, which is the main cause of banging noises, cold spots on radiators and gradually worsening efficiency. Fitting it means cutting into a sealed heating system, so treat this as a competent-DIY-or-plumber job, and get the inhibitor topped up afterwards in line with BS 7593:2019.
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- Genuinely compact — Fernox states as little as 105mm clearance needed, so it fits under boilers and in cupboards a standard filter can't
- High-powered neodymium magnet housed in a withdrawable dry pocket, so servicing via the drain point is quick and mess-free with no disassembly needed
- Established UK brand (Fernox invented the in-line magnetic filter category) with a 10-year warranty
- Noticeably pricier than the standard full-size TF1 Sigma for the same core filtration, so only worth the premium if space is genuinely tight
- Installation involves draining and cutting into the sealed system — not a first-time DIY job
Who shouldn't buy this
Skip it if you've got normal clearance under your boiler — the standard-size TF1 Sigma costs less, is just as easy to service, and gives the same magnetic protection.
Key specs
| What it is | Compact neodymium magnetic system filter |
| How it works | High-powered neodymium magnet |
| Fits | 22mm slipsocket connection |
| Install effort | Fitted into the sealed heating system |
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as of 20 Sep 2026, 12:29 BST · Details
Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on Amazon.co.uk at the time of purchase will apply to the purchase of this product.as of 20 Sep 2026, 12:29 BST · Details
Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on Amazon.co.uk at the time of purchase will apply to the purchase of this product.as of 20 Sep 2026, 12:29 BST · Details
Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on Amazon.co.uk at the time of purchase will apply to the purchase of this product.Want this cheaper?
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