We have a heating system with a boiler, hot water cylinder and a controller to set the timers for the heating and hot water which it then switches between accordingly.
Lately, it intermittently (I haven't figured out the exact situation that causes it) sometimes continues heating the radiators although they've already reached the right temperature.
It seems that the controller is calling for hot water, but instead it continues pumping through the "radiators" side. It's as if the valve is set to the wrong side, but the valve itself isn't "stuck" etc.
Any ideas on which specific part may be causing this and is it something a DIYer could do themselves?