My Baxi Combi 80eco keeps switching the heating on when it should be off ?

My Baxi 80eco boiler has a problem in that the timer is set to heating only but when you turn the hot water tap on the heating comes on and it doesnt go off unless you turn off the power to the boiler then leave it a while before switching it back on at this point the heating is off but as soon as you turn the water on again we start off all over again