Radiator turns on full blast when off at thermostat

Hi, You have to ask a Gas Engineer to come and have a look at your boiler. Your Heating is continuously running in my opinion as you have mentioned the issue. It might be a thermostat which is faulty. If you use hot water and radiator gets hot then it can be a Diverter Valve as well. Kind regards

There could be several reason for this occurance. Firstly we need to know what type of boiler you have, is it a combi or central heating system boiler, combi type is where you get hot water straight from boiler to the taps and a central heating system boiler has a hot water cylinder. So if it is a combi boiler it is either the diverter valve or the plate to plate domestic hot water heat exchanger and if it is a central heating system boiler it could be a zone valve or mid-position valve letting by.