Potterton Suprima 70 Not turning off

[stimpyian]

Hi, looking for some advice.

Potterton - suprima 70l
6years old, never serviced.
Doesn't shut down, and keeps firing up. The only way it seems to control it is by using the temperature control dial on the boiler (next to the reset button), if you turn this all the way down almost to where it clicks and is off, it does fire down (turn off) briefly, but then fires up again after a couple of minutes, and the only way to turn it off completely is to use this dial and turn it to the off position.
Using the reset button hasn't made any difference.
The dual control for timing CH & HW, makes no difference if everything is set to on or off.
The thermostat in the hallway is turned all the way down, and i have had to adjust each radioator to regulate the temperature, otherwise they become burning hot, and you can hear them sizzing with the heat.

Obviously something is wrong, advice appreciated.

 

[misterdubya]

Id get it serviced.

Mr. W.

 

[45yearsagasman]

It will be a fault with one of your motorised valves.At this stage I assume you have 2, one for ch and one for hw.The micro switch in one is staying closed and therefore firing boiler.Usually the powerheads are replaced when this happens although it is possible in some cases to repair the sticking parts.If the spindle in the valve body has siezed then new valve is the best option IMO.

 

[stimpyian]

Thanks for the advice, as i know nothing about boilers or Central heating systems, if i flick through the yellow pages to find a local plumber, what sort of ball part cost would this be including the call out ?