Bristol: Stop radiators heating up when central heating is off but hot water is on! (ended)

Stop radiators heating up when central heating is off but hot water is on!

The closest upstairs radiator to the hot water tank of a Baxi boiler system gets hot when the hot water is switched on but the central heating is switched off. This only presented itself when the heating was switched on for the first time a few weeks ago. I believe this is indicative of a failed valve. Unsure of exact age of the system as it was present when we moved in but would estimate that the boiler is around 10-15 years and tank is far newer.
Archive for Plumbers, Heating