My central heating is on even when heating switch is off and only water on. By searching web, i narrowed the problem to the actuator ( potterton msv322. Three port 22mm). The switch on top moves freely. With no change. So i was hoping just to change actuator for new one. I took it off and tried spinning valve by hand. This moves freely with water off, but with water on it won't move. Does this mean the valve is broken as well, or is the valve only meant to move when water stops flowing through it?