When I advance the hot water timer and the heating is not on, the radiators get hot. Also, when the heating is on and the timed water has been off, for about half an hour after the water has gone off the myson power extra valve in the airing cupboard makes a buzzing noise - please give some advice as to what might be the problem :)
Sounds like the actuator is on it's way out, if you're competent you could renew the actuator yourself, when you remove it check the spindle on the 3 port valve moves freely.
If the spindle doesn't move freely it's worth changing the valve as the heating may pack in again - and the actuator wont last long as it will struggle to turn the spindle and the aperture in the actuator will become chewed.
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