Thread

In a UK vented central heating system, if your hot water turns on when you select central heating only, it could be due to several reasons:

1. Combined Cylinder: Many vented systems use a combined hot water cylinder. If the central heating system is designed to heat both the radiators and the hot water cylinder simultaneously, activating the central heating will also heat the water.

2. System Configuration: The wiring or configuration of your system may be set up such that the central heating and hot water are controlled from the same thermostat or switch. In some systems, when the heating is activated, it may trigger the hot water as well, especially if there's no separation in the control mechanism.

3. Thermostatic Control Issues: Faulty or incorrectly calibrated thermostats can cause cross-activation of the heating systems. If the thermostat for the hot water cylinder is not functioning properly, it might activate the hot water heating when the central heating demand is sensed.

4. Diverter Valve Malfunction: In systems with a diverter valve (used in combi systems or systems with an indirect water cylinder), a malfunction could lead to the valve not closing properly, allowing hot water to flow to the taps or cylinder when it shouldn't.

5. Pump Control: If the pump is designed to circulate hot water through both the heating and hot water systems, turning on the central heating might lead to the pump circulating through the cylinder as well.

To resolve the issue, consider consulting a qualified heating engineer who can assess your specific system configuration and diagnose the root cause. Regular maintenance can help prevent such issues.