🧪 How it forms
Steel radiators, pipework and fittings develop electrochemical corrosion when in contact with water containing oxygen. The result: black magnetic particles that continuously break loose.
Magnetic Filters & Deaerators: Why every heat pump needs them
Inside every closed heating or cooling circuit, an invisible "cocktail" of enemies circulates alongside the water: black sludge (magnetite), rust, microscopic air bubbles, and suspended particles. These enemies block, corrode and shorten the life of every component.
Magnetic filters and deaerators are the two "weapons" that protect your equipment - and in many cases they are a mandatory warranty requirement for heat pumps.
1. Black Sludge (Magnetite): The invisible enemy inside your radiators
Black sludge is not "dirt" from the mains supply. It is produced inside the circuit itself by a reaction between water, iron, and dissolved oxygen. Its chemical name is magnetite (Fe₃O₄) and it is magnetic.
🚫 Where it settles
Sludge settles at low points (bottom of radiators, T-pipe junctions), blocks narrow pipes, and jams valves. In underfloor heating, it can block entire loops.
⚡ Equipment damage
Sludge wears out circulator mechanical seals, blocks plate heat exchangers, and reduces flow. In heat pumps, the damage can reach thousands of euros.
🌊 How fast?
In a new circuit without chemical treatment, significant sludge appears within 2-3 years. In older circuits (10+ years), the situation can be dramatic.
2. How a Magnetic Filter works
The magnetic filter (or dirt separator) is installed on the return pipe, before the boiler or heat pump. Inside it sits a powerful magnet bar (usually neodymium) that "attracts" the magnetic particles.
🧲 Magnet strength
Neodymium magnets produce a strong field ≥ 10,000 Gauss. As water flows through the filter, iron particles "stick" to the magnet while clean water continues circulation.
🔧 30-second cleaning
Modern magnetic filters feature a drain valve at the bottom. The technician removes the magnet, opens the valve, and the sludge flushes out in 30 seconds without dismantling anything.
📍 Correct placement
Always on the return pipe, immediately before the boiler/heat pump, so it filters sludge before it enters the precious heat exchanger. Placing it on the flow pipe is incorrect.
💶 Cost vs Benefit
A quality magnetic filter costs €80-200. A heat pump heat exchanger costs €800-2,000. The risk calculation is clear and leaves no room for doubt.
3. Deaerators: Eliminating micro-bubbles
Dissolved air in circuit water is invisible, but it silently destroys. It feeds corrosion, creates "gurgling" noises in radiators, and reduces heat transfer efficiency.
🌊 How it works
The deaerator creates a low-velocity zone in the circuit. There, micro-bubbles dissolved in the water gather on special metal "vanes", merge together, and rise to the top where they are automatically expelled.
🔇 No more gurgling
Trapped air in a radiator creates the familiar "gurgling" and "knocking" sounds. A properly installed deaerator eliminates these noises permanently.
⚡ Improved efficiency
Air in water reduces thermal conductivity. Deaerated water transfers heat far more efficiently. Result: faster heating and lower energy consumption.
📍 Installation
The deaerator is installed on the flow pipe (hot water), at the highest point after the boiler or heat pump. There, temperature is at its maximum, so air release is also maximised.
4. Why heat pump manufacturers require them: The warranty
Almost every major heat pump manufacturer (Daikin, Mitsubishi, Vaillant, Bosch, LG) explicitly states in their installation manuals that the warranty is void without a magnetic filter.
📋 What the manuals say
Manufacturers require: a magnetic filter, chemical cleaning (power flush) before connection, and a corrosion inhibitor in the water. Without these, the warranty is voided.
💰 Cost of non-compliance
A heat pump costs €5,000-15,000. If the heat exchanger blocks due to sludge and the manufacturer refuses the warranty, the repair cost falls entirely on the owner.
🧪 Power Flush
In older circuits, before connecting a new heat pump, a chemical clean (power flush) must be performed. Special chemicals circulate through the system dissolving sludge, which is then flushed out. Afterwards, a magnetic filter and inhibitor are installed.
✅ The correct recipe
1) Power flush before connection, 2) Magnetic filter on the return, 3) Deaerator on the flow, 4) Corrosion inhibitor in the water. These four steps ensure warranty validity and long equipment life.
💡 A magnetic filter + deaerator cost €150-300 together. A heat pump heat exchanger costs €800-2,000. Anyone who does the maths understands that "saving" by skipping the filter is an illusion.
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