Any plumbers or heating engineers here that can help?

[Colbaker]

Hi, Im hopeing someone here will be able to help me with a heating problem i have, Basically i have recently moved into a new house which has a back boiler and uses a hot water tank, For some reason when all the radiators are turned off but only the hot water is turned on all the upstairs radiators also heat up

is this supposed to happen? if not what would cause it and is it easy to rectify?

Thanks