Why is my electrical cable getting hot inside the wall cavity when the heating turns on?

Finding that an electrical cable is getting hot inside your wall cavity when the central heating fires up is genuinely concerning, and you are right to take it seriously. Here is what is likely happening and what you should do. The most common reason is thermal insulation. When your home has been insulated, either during a retrofit or during original construction, cables inside wall cavities can become surrounded by or buried in insulating material such as mineral wool, rigid foam board, or blown-in beads. Electrical cables generate a small amount of heat during normal use. Under normal conditions this heat disperses harmlessly into the surrounding air. When insulation wraps around the cable, that heat cannot escape, and the cable temperature rises. Over time this can damage the cable's outer sheath and inner insulation, creating a real fire risk. The heating connection matters here because your boiler, pump, or zone valves draw current when the heating turns on, so cables supplying those circuits see their highest load at that moment. A cable that is borderline for its conditions may only overheat when it is under that full load. Another cause is a loose or corroded connection somewhere on the circuit. A poor connection creates electrical resistance, and resistance turns electrical energy into heat. This heat can travel along the cable and build up inside the cavity. What you can safely check yourself: 1. Feel any accessible part of the cable, for example where it enters a socket, switch, or fuse board. Warm is unusual. Hot is dangerous. 2. Check whether the circuit breaker or fuse for that circuit is correctly rated for the cable and appliances on it. 3. Think back to whether any insulation work was carried out recently, as this is often the trigger. What you must not do: 1. Do not open the wall yourself to inspect the cable unless you are a competent person. 2. Do not continue using the circuit if the cable or fittings feel hot. 3. Do not assume the problem will go away on its own. Call a registered electrician, ideally one registered with NICEIC, NAPIT, or ELECSA, and explain the symptoms. This situation can lead to a house fire if left unaddressed.