My heating system has recently developed a fault. When only 'Hot Water' is selected on the controller, my radiators are heating up as well. I'm assuming the '3-way valve' is a key area to be looking at. However having read other queries, is there two types? How do I identify which mine is? . The valve positioner is a Danfoss type HSA 3. (T45). Can anyone advise a way of determining what the fault is or is it a case of replacing the head unit. C/H works fine. Thanks for any suggestions offered.