Most commonly, issues with valves lead to radiators working as usual despite the heating being switched off. Especially in warm summer climes and with a massive emphasis placed on keeping energy bills as low as possible, the prospect of home radiators turning on whilst the central heating is switched off is unappealing.
The problem will typically stem from troubles with diverter valves or check valves, with the latter often associated with older boiler or central heating system models. For a more detailed insight on why radiators seem to switch on by themselves, and how to combat the problem, take a look at our dedicated blog; Why are my radiators getting hot when the heating is off?