over pressure on baxi 105e even though boiler is turned off?

i have a baxi 105e and even with the boiler turned off, it keeps overpressurising and lifting the safety valve. ive checked the expansion tank and the bladder pressure is 0.5 bar, which is correct according to the manual. it was suggested that it may be the hot water plate heat exchanger but i cant see how water gets into the heating circuit, could someone please enlighten me. much appreciated.