A wet heating circuit can contain several metals and other materials. Corrosion products, scale and installation debris can obstruct waterways or interfere with valves and pumps. Magnetite is a magnetic iron oxide that can form part of black heating-system deposits.
The diagram shows one possible obstruction mechanism. A cold patch alone does not establish what is inside: investigate circulation and operating conditions as well as water quality. This lesson helps you understand an engineer’s findings and maintenance record; it does not authorise opening a filter or dosing an unknown circuit.