heat water up in boiler keep flowing into radiators?

potterton suprima.
put central heating on to heat up water in tank but in less than half an hour hot water starts to flow into radiators.Before I could heat water up in tank and it would not flow into radiators surley the boiler cannot heat up a full tank of water up in less than half an hour. I have checked all the controls and thet are still set the same as they have always been.