My Potterton Suprima 40 Boiler Won't Shut Down

[sporticus]

My Suprima 40L boiler won't shut itself down when the CH is on, even if it exceeds the thermostat required temp. We have to shut the system down by flicking the fuse switch to off. Even when we move the CH control panel to off and switch the fuse back to on the CH kicks back in and superheats the house using lots of gas!! A British gas engineer who serviced the system said it could be 2 things. 1 - some sort of frost setting?? or 2 - a problem with a switch by the hot water tank which is part of a high pressure system, I have looked and I have no idea what the switch might be. I was advised that when it happens again(like today) to leave the system running for them to see....however the house gets up to 28 degrees + and it wastes lots of gas, costing us lots of money.

[The Builder]

Between you and me, Sporty, BG doesn't have a great reputation with boiler servicing.

The roomstat could be faulty. Perhaps it doesn't switch off at the set temp. That would mean the pump is running continuously, trying to get the system to reach an unattainable setting.

Or, quite possibly a simple control problem. A control board fault. (Printed circuit board)

There are various heat sensors that could cause this.

I would suggest you contact Potterton's helpline. What you need from them is a few names of likely installers who know this product. BG are just far too general.

http://www.potterton.co.uk/28/

[Hymie]

If you follow the fault-finding section within the installation and service guide, this may help pin-point the fault.

But what you describe suggests more than one fault to me.

Within the fault-finding guide is the instruction to turn the boiler control temperature to 0 (once it has fired up); if the boiler flame does not go out, it states that 'The gas valve has remained open. Check gas valve and temperature control, temperature sensing thermistor may be open circuit.'

Therefore it would be worth checking the thermistor - but given the complete lack of control you have over the boiler, I would think that there is a fair chance that the control board is US.

There is no circuit diagram within the installation and service guide that would allow examination of a single component failure, leading to the boiler behaving as you describe.