I am trying to replace a bathroom ventilation fan with a Vent-Axia Silent Fan. It has an overrun timer, which is supposed to make it start when the bathroom light is turned on and turn off about 15 minutes after the light is switched off. When I wired it up as per the instructions, the fan came on when the light was turned on, as expected, but it then never stopped. It even came back on after I switched the bathroom light off and turned the electricity supply on and off. Is the fan faulty or have I missed a setting somewhere?