Boiler not turning off

[JAllen]

Hi,

I currently have a fully pumped Y-plan open vented system with a Potterton Suprima 80 boiler which has started playing up recently. What's happening is that on some occasions the boiler refuses to turn off. We first notice this when it starts to get too warm in the house - the boiler refuses to turn off even though the room stat temperature has been reached. Also, the water in the hot water cylinder becomes much too hot implying that both the room and cylinder stats are being ignored. Bizarrely, even if the heating programmer (Drayton LPP241) is turned to the off setting the boiler still continues to fire up and run. The only way to get it to turn off is by power cycling the heating programmer at its mains switch off the fused spur.

I was going to replace the programmer but after doing a search for similar problems it seems that the mid-position valve might instead be the cause of problem. Does this seem plausible? Our mid-position valve was actually replaced some time last year but nothing else has been done to the system. So is it better to try replacing the mid-position valve first to see if this fixes the problem? The problem is intermittent right now but is there something I should be looking out for next time it happens to help pin point the exact cause?

 

[ianmcd]

what make of mid position valve do you have ?

 

[JAllen]

The mid-position valve is an unbranded one that was replaced by British Gas.

 

[TonyW2]

The motorised valve probably needs a new head unit, although one year is a short life!
The head unit has a power supply to the motor and this supply should only be "on" when the timer AND at least one of the c/h or h/w thermostats is calling for heat. That should operate the motor, opening the valve AND operating a microswitch.
The microswitch should have a permanent supply and when operated that supply should connect to the boiler (firing it and leaving it running until the thermostat(s)stop operating the motor). If the switch is stuck, then there will be a permanent feed to the boiler, regardless of the thermostats or timer.
Probably the microswitch or the operating cam is faulty, but swapping the whole head is easier and more reliable.

 

[D_Hailsham]

The microswitch should have a permanent supply and when operated that supply should connect to the boiler

That only applies to an S Plan; the OP has a Y plan.

You will find fault finding info in How a mid-position valve works

 

[TonyW2]

Very true