Zone valves in CH can I replace them myself???

My CH system is becoming v unpredictable. Sometimes the water doesn't heat up unless I run the radiators. Other times the boiler (oil) shows an amber lights but doesn't run, and recently I found the immersion heater trips the earth breaker on the consumer unit.
Now, I suspect the Zone valves. It's a 2-port system, Honeywell, fully controlled, i.e. two valves in the airing cupboard.
Is it a hard job to replace these - could it just be the motors or switches??
I phoned a plumber and apparently they are booked into the new year!
Jay
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Motors are available. Screwfix sell them.
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Jay wrote:

The complete head or just the motor or microswitch inside can often be exchanged. This means that you don't have to drain down the system. Turn off the elect to the CH system and take off the head of each valve. Check that you can operate the valve spindle manually with a spanner or pliers. If the valve itself is faulty then you will have to drain down the system to replace the whole valve. They are normally standard spacing between the inlet and outlet so you don't have to get the same make.
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to
If you do change the actuators, make a very careful note of how they're wired up before removing the existing ones. Each one should have two distinct sets of wires (although they may all be incorporated into a single multi-core cable). One set drives the motor to open the valve in response to a demand from your programmer and (room or cylinder) stat. The other set turns on the boiler and pump when either or both valves are open.
Roger
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