I have a Valliant 418 with VR430 controls. I have the Vallianthot water sensor in the tank. On the odd occasion the boiler while heating hot water has cut out and displays s28. Can anybody tell me the process for heating water, it appears to heat the water 15c above the HW target value which if set at 55c heats to 70c. But sometimes it heats to the max HW temp I have set of 80c.
