baxi 80eco combi boiler?

I have a 2 year old baxi 80eco boiler the fault with it is that the heating stays on all the time,even if the selector switch is in the tap only position.initially when this problem started the heating would come on as soon as we opened the hot water tap and on closing the tap heating remained on,but now after two days the heating remains on all the time,as soon as i switch the boiler on.I have disconected the heating timer but the fault stays.The only way to switch the heating off is to switch the boiler off .Pls can any one help me to repair this fault. Thanks.