Why do radiators warm up when the hot water is set to on?

When the hot water is set to on, the radiators also warm up. Two 2-port valve / cylinder/gas boiler system. Seemed possible likely that the valve to the heating was sticking open (partially?). This valve was replaced the problem remains. What could be other possible causes? When the heating is on, the hot water does not warm up, so I don't believe pipes have been crossed (we have had various building works done involving changes to the central heating and at the time believed this had caused some 'dirt' to get to the valve.