Selecting hot water only on gravity fed GFCH instead heats upstairs radiators?

Sounds like the anti gravity or non return valve on the central heating circuit is letting by. This valve should only open when the pump runs for the c/htg. If its beginning to fail the upstairs rads will get hot. The valve is mechanical so no wires and it will look like a long compression fitting usually in the c/htg flow pipe above the boiler.