Reasons why a motorized valve might fail
There are several reasons why a motorized valve might fail but usually the fault is in the actuator assembly. This is the motor section mounted on top of the valve. If the valve has seized up or is worn and leaking, then a new valve is required. A good indication of a seized valve is when it is difficult or impossible to move the manual override lever. If the motorized valve actuator has failed, you will still be able to use the manual override lever. Failed actuators usually default to the hot water open position. There is an override lever on the side of the casing which can be moved from auto to manual to open the heating circuit instead.
In replacing a faulty motorized valve actuators, the first thing to check is that there is power to the actuator. A mains tester can be used for this. Assuming the power supply is ok and that the actuator has indeed failed, you will need to buy a replacement. Buy an exact replacement. Nowadays, most actuators can be replaced without needing to drain down the system. With older types, this isn’t possible and you will need to drain before starting work. Switch off the boiler by removing the fuse from the plug or fused spur outlet. It is important to double check that there is no power to the terminal box for the motorized valve connection. Move the manual override lever to the manual position then remove the cover. Under this you will see the actuator which is held in place by two retaining screws. Undo these and cut the wires where they go into the terminal box. If you leave a small length of each wire still connected to the terminals it will make things easier when it comes to wiring up the new actuator assuming you have bought a similar replacement.
Now, remove one old wire at a time from the terminals and connect the new wires to their corresponding terminals. Position the manual override lever to manual on the new unit and gently fit it to the valve assembly. Don’t apply any force as this will damage it. Secure the new unit with the retaining screws and refit the cover. You can now put the override lever back to auto. Refill the system if it was drained down. Reconnect the power supply and switch the boiler back on. Check the operation of the system for hot water and heating.