My Radiators get hot through the return pipe??

Hi

2 of my radiators are getting hot through the return pipe when the water in the hot water cylinder is being heated, i have a conventional system with 2 tanks in the attic (1 cold water storage & 1 feed and expansion)and 2 motorised valves (1 for heating & 1 for hot water) in the airing cupboard, i have changed the valves but the problem persists as it is the return pipe thats heating the radiators . how can i prevent this?

Many thanks

Normally the cylinder should be the last T before the boiler. it sounds as though the returns from the radiators that you mention are after this causing them to heat up on hot water.
You could install non return valves in the return legs that get hot.
Lee