Central Heating Filters: Magnaclean Pro 2 vs Fernox TF1 and Why Every System Needs One

Magnetite sludge is the primary cause of premature boiler failure in UK central heating systems. A single season without a magnetic filter on a large, older system can coat heat exchanger surfaces enough to reduce efficiency by 10–15% and trigger the kind of boiler faults that land you on a callback. Fitting a magnetic filter at every service or new installation is no longer optional — it's standard practice, and most boiler manufacturers now require one to maintain warranty.

This guide covers the products heating engineers actually use, how to choose between them, and how to fit them correctly first time.

What Magnetite Sludge Does to a Central Heating System

Magnetite (Fe₃O₄) forms when iron and oxygen react inside an unsealed central heating system. In a typical UK system — steel radiators, copper pipe, a cast iron or stainless steel heat exchanger — the corrosion is slow but continuous. Magnetite particles are carried through the pipework by circulating water and settle anywhere flow is slow: at the bottom of radiators, inside heat exchangers, and on pump impellers.

The consequences are progressive:

• Cold spots at the bottom of radiators — the classic sign
• Reduced flow rate as sludge restricts pipe bore
• Blocked heat exchanger — causes short cycling, F75 or E119 errors on Worcester, Vaillant, and Ideal boilers
• Pump wear from abrasive particles
• Premature boiler failure — most manufacturers cite sludge as the primary cause of in-warranty failures

A magnetic filter captures these particles before they reach the boiler. It doesn't prevent corrosion — inhibitor does that — but it removes the byproduct from circulation continuously.

Magnaclean Pro 2 vs Fernox TF1 Sigma: Which to Specify?

Adey Magnaclean Pro 2 — £111.92

The Magnaclean Pro 2 is the industry standard. Adey invented the magnetic filter category, and the Pro 2 is what you'll find specified on Worcester Bosch, Baxi, and Ideal warranty paperwork. It uses a powerful neodymium magnet and a stainless steel collection chamber that can be flushed without draining the system — a significant practical advantage on a service visit.

Key specs:

• 22mm compression connections (straight or angled installation)
• Flush without system drain — isolate, flush, re-open
• High-flow design: minimal pressure drop across the filter
• Compatible with all common inhibitors
• Boiler warranty compliance: accepted by Worcester Bosch, Baxi, Vaillant, Ideal, Glow-worm

At £111.92, it's the premium option. For a new installation or a system where the customer wants the best, it's the straightforward specification.

Fernox TF1 Sigma UB Filter 22mm — £58.99

The TF1 Sigma is Fernox's answer to the Magnaclean and a genuine competitor. The "UB" (universal bypass) design allows installation in tight spaces and gives you a full-flow bypass position for flushing — useful where there's no room to fit isolating valves above and below. At £58.99, it's almost half the price of the Magnaclean Pro 2.

The TF1 Sigma is also specified by major boiler manufacturers and complies with BS 7593:2019 (treatment of water in domestic hot water central heating systems). For cost-conscious retrofit work or where budget is a constraint, it's a solid choice without compromising system protection.

Calmag 22mm Central Heating Filter — £69.99

Calmag's standard filter is a reliable budget option, widely used by heating engineers who prefer to keep their own supply chain simple. 22mm compression, magnetic capture, suitable for both combi and system boilers. If you're fitting a large number of systems and cost per installation matters, this sits between the Fernox and the Magnaclean on price with no meaningful compromise on function.

Also available in 28mm for larger pipework (£89.99).

The Magnetic Filter Alone Isn't Enough: Inhibitor

A magnetic filter captures existing magnetite. It doesn't stop new corrosion forming. Inhibitor does that — it coats the interior surfaces of the system with a protective film that prevents the electrochemical reactions that produce magnetite in the first place.

At every boiler service and every new installation, you should:

1. Clean the filter (flush it)
2. Top up or fully dose inhibitor
3. Check inhibitor concentration with a test strip

BS 7593:2019 and most boiler warranty terms require inhibitor to be present at the correct concentration. If a boiler fails within warranty and there's no inhibitor in the system, the manufacturer will decline the warranty claim.

The Calmag Heating Pack C (£94.99) is an efficient way to cover a full installation in one purchase: it includes a 22mm filter, inhibitor, system cleanser, filling loop, and scale inhibitor. For new installations where you need everything in one go, this is the fast option.

22mm or 28mm: Which Size?

Most domestic central heating systems run 22mm flow and return pipes at the boiler, so 22mm is the standard filter size. Fit the filter on the return pipe, as close to the boiler as the installation allows — this is where the most magnetite-laden water arrives.

28mm applies to:

• Commercial or semi-commercial installations (larger boiler plant)
• Older gravity systems with 28mm main circuit pipes
• High-output boilers with 28mm flow/return connections

Where to Fit the Filter

The correct position is on the return pipe, between the last radiator and the boiler. This captures sludge before it enters the heat exchanger. Fitting on the flow works but is less efficient — you want the filter to catch particles on their way back, not on their way out.

Practical considerations:

• Fit with accessible service clearance — the filter body needs to come off for cleaning
• Install the arrow on the body in the direction of flow
• Fit isolating valves either side for service without draining — this is essential practice
• On a combi boiler with tight under-sink installation, the TF1 Sigma's bypass design often works better than the Magnaclean where space is limited

Products in Stock at APM

All in stock for same-day collection from our Acton trade counter or next-day delivery across the UK.

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