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Magnetite deposits on ferrous metal
7 min read

Magnetite: Black sludge – every heating system’s enemy

Just like an unhealthy lifestyle can lead to cholesterol clogging human arteries, magnetite deposits can lead to serious problems in a heating system’s pipes. They start with a risk of clogging and can, in the worst-case scenario, lead to the heating system experiencing a full-on “heart attack”. Find out here how magnetite is formed and how the damage to heating systems described above can be prevented right from the planning phase.

Just like an unhealthy lifestyle can lead to cholesterol clogging human arteries, magnetite deposits can lead to serious problems in a heating system’s pipes. They start with a risk of clogging and can, in the worst-case scenario, lead to the heating system experiencing a full-on “heart attack”. Find out here how magnetite is formed and how the damage to heating systems described above can be prevented right from the planning phase.

Iron-oxygen compound: What is magnetite?

Magnetite (also called magnetic iron, lodestone or Fe3O4) is a type of iron oxide, a chemical compound composed of iron and oxygen. It must not be confused with hematite, classic “rust”, which is also an iron-oxygen compound (iron oxide). Rust or hematite is formed when a large amount of oxygen is present during corrosion – for example, when the corroding surface is exposed to air. By contrast, it is more common for magnetite to form in closed environments with little oxygen. Its appearance is blackish rather than reddish.

In nature, magnetite occurs comparatively often. Due to its unusual properties it is mined in various countries around the globe. The mineral is as popular with the electronics industry as it is feared by heating system operators. It’s special properties are to blame: magnetite (as the name suggests) is one of the strongest magnetic minerals; it attaches to iron and ferrous alloys. This property can become problematic when magnetite deposits in pipelines, for example, where it can reduce the flow in the long term (see illustration below).

Black sludge fight: What damage can magnetite do to heating systems?

A heating system’s task is generally fairly simple: It circulates water, thereby transferring heat from a heat source to radiators or to the pipes of underfloor heating systems. Deposits or contamination are a hindrance. 

The problem is that magnetite is not contained in the water fill right from the start; it forms when the heating system is in operation: Over time, the smallest of crystals (up to 5 µm in size) generate so-called magnetite sludge. This is a black brew every heating specialist would have come across. It can have devastating effects on the entire heating system:

  • Malfunctions of thermostatic valves due to deposits on the valve seals.
  • An efficiency drop for radiators as magnetite attaches to the inside of the pipes and reduces heat transfer. In extreme cases heating circuits can be fully blocked and fail.
  • In heat generators (boilers, heat pumps, heat exchangers, etc.) magnetite hinders heat transfer, meaning that not the full thermal output is transferred to the system.
  • Magnetite clogs the filters installed, thus reducing the flow. The required thermal output is no longer reached. As filters need to be cleaned more frequently, operating costs increase.
  • Magnetite can deposit in different areas of the pump, increasing wear of rotating components and bearings. The required energy input rises. Pumps can even fail completely when the shaft is blocked (wet rotor) or the mechanical seal is defective (dry rotor).

This is why heating water should have certain properties that clearly make it harder for such deposits to form. (In Germany this is governed by the binding directive VDI 2035.)

Magnetite deposit on a mechanical seal

Clearly visible: black magnetite deposits on a mechanical seal


When the wrong elements come together: How is magnetite formed?

Magnetite requires iron, water and oxygen to form. Iron is contained in many pipelines and valves of large systems. But where does the oxygen come from? Several sources may be present:

  • Untreated, added water
  • Oxygen ingress due to incorrect pressure maintenance, leaking vent valves and air separators
  • Old stock of non-diffusion-tight rubber expansion joints, connection hoses or plastic pipes
  • Negative pressure in the system due to incorrect pressure maintenance, faulty safety valves and faulty air separators
  • System filled improperly and too fast. The standards and directives for installing and operating hot-water heating systems (such as DIN 18380 and VDI 2035) must be observed.

Decisive factors are the prevention of contaminants being introduced during installation, a correct pressure test and the flushing of the heating system prior to commissioning as well as the correct use of air separators and venting systems. BTGA regulation 3.002 applies. From the point of view of the BTGA (German Industry Association for Building Services Systems) this regulation was urgently needed to fill the void of comprehensive legal requirements and standards. BTGA’s specialist regulation is partly based on DIN EN 14336 introduced in January 2005, which is not quite as well-known in professional circles. This is the German version of EN 14336:2005. It specifies measures for installing and commissioning hot-water heating systems.


Sludge separator, magnetic filter: How can damage by magnetite formation be prevented?

The best option of preventing magnetite damage would certainly be not to let magnetite form in the first place. However, there is not much chance. Even when materials are selected that are corrosion-resistant and as identical to each other as possible and when the system is thoroughly and regularly serviced, magnetite deposits cannot be completely ruled out.

The only effective and practical solution to reduce magnetite in heating water is fitting a sludge separator with integrated magnets (magnetic or magnetite separator) or a magnetic filter. The working principle is quite simple: The heating water flows through a spiral-shaped cylinder. In the cylinder, the flow velocity is reduced, so non-magnetic particles can deposit successively, collecting at the very bottom of the separation chamber. In addition, the heating water flows around a magnetic rod integrated in the separator. The tiny magnetite particles suspended in the water attach to the rod. The liquid flow is not influenced by these measures; no major resistance is present. To dispose of the foreign matter a specialist simply pulls out the magnetic rod from the filter element, opens the separation chamber, and the sludge can flow out.

To prevent a “heart attack” in the heating system, this procedure should be carried out regularly as part of maintenance work; the exact intervals depend on the system design. 

Schematic drawing of a KSB magnetic filter / sludge separator

The magnetic filter is designed for filtering liquids, e.g. in circulation circuits, containing ferrous particles (magnetite, etc.) which have to be removed in order to protect the mechanical seal faces.


Preventing a “heart attack” in the heating system: Summary and conclusion

Magnetite is a chemical compound formed when low-oxygen water, air and ferrous pipelines come together. This is the case in almost every heating system, and it can have devastating effects: from efficiency loss and pump damage right through to a full-on “heart attack” in the heating system. If you would like to avoid this in the long term, make sure your system is fitted with functioning magnetic separators. This not only applies to new installations but also to older systems being renovated to improve their efficiency. Especially when replacing pumps, installing such a filter makes sense to fight magnetite before it becomes a problem.

*Source: IKZ 12/04/2008


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