My water tank is now producing scalding hot water. I've checked the cylinder stat which seems to be OK - switches the boiler/pump on/off as I turn the stat up/down. Could it be the valve not closing, so water continues to be directed to the cylinder as well as the heating even when hot water is not called for? How do I check this and what can I do?
Two points. Firstly have you checked that the immersion heater is not switched on accidentally.
Secondly, after the heating has been off for a while and the pipes are cool, turn down the cylinder stat and then turn the heating on. Feel the pipes as they begin to warm on either side of the three port valve, if the one which goes to the cylinder gets hot, then in all probability the valve is faulty.
My last boiler had an overtemp. trip of 72 C, vented system, indirect cylinder, 210 litres hot water.
The diverter valve is your main suspect ; --- stuck valve spindle/blown motor, leaking gland can cause both.
OR, has someone switched on the immersion element.
Checking the thermostat on the side of the tank is the first task, easy for a competent DIY'er, with the whole boiler system isolated.