boiler firing when controller off

olbas_oil

I'm trying to diagnose an issue with my son's system on the house he has just bought. It is a vented system with a Potterton Suprima boiler and Drayton controller.

With both CH and HW set to off on the controller, the boiler is still firing. I thought that only when both controller was ON and Thermostat was ON that the boiler would run.

If the controller unit has failed (it looks quite old) would it default to the ON state?

molerat

Possibly a stuck diverter / zone valve. It is the valve that calls for heat, the controller just tells the valve to open so if stuck in open the boiler will fire as the valves are permanently powered. Is the radiator or DHW circuit heating up ? Is it a full burn or just a quick on / off which could indicate the daily pump protection cycle or frost stat fault.

olbas_oil

Many thanks for your very helpful explanation. I need to do a proper mapping of the pipework, but from memory I think it is the pipework up to the pump, and the coil in & out of the cylinder that was staying hot several hours after the controller had been switched off. It is a confusing situation because the original fault was a non-functioning thermostat on the immersion heater, which was being used for the first time in probably a long time. That caused continuous heating and venting back into the cold water tank. We've replaced the thermostat with a new dual safety thermostat and that it now fixed, but for a while all the pipework in that area was hot.
However I also noted the boiler firing on full, with the controller switched off, which I could not make any sense of until I read your explanation, that it is the valve that calls for heat from the boiler.