Central heating motorised valve issue

Maybe no easy answer unless you can figure out what must be broken from the symptoms. Plenty of wiring diagrams on Google though. When you say the connectors are brittle, which parts do you mean exactly?

Perhaps time for a completely new box. If you need to extend wires, Wago is probably the easiest way to go (use the ones with levers for multi-strand wire). They should all be fitted inside a suitable box.

But don't get the wago mbox L32 (AIUI it's a bit tight on space for S-plan) instead get the Wago 207-4301

For mains electrical provision to the various devices eg pump, motorised valves, thermostats and the boiler the wiring uses the same Brown, Blue and green/yellow conventions as all other electrical devices only switched lives within the panel use different colours. So your suspect blue wire is probably a Neutral from something. You should find a number of linked terminals where neutral wires are connected and it is probably from there. I cannot see that trying that would cause any harm, an unused wire should be terminated off and not left floating inside the panel. I recently swapped a 3-wire thermostat for a 2-wire so simply put a Wago on the unused neutral.

Richard

As a starting point here is the typical wiring diagrams for S Plan:

The best way to find faults on these is with a volt meter, working your way though the sequence of things that need to happen. So check that when the programmer is calling for heat, it has a CH on output live. Then check that is present at the output of the room stat. Then at the input to the valve. Lastly check the orange wire out of the valve is also live. If it is then check at the input to the boiler. If the orang is not live, check there is also a permanent live into the valve (grey wire typically).

The most common failure IME, is the failure of the valve to move fully, and activate the miroswitch that makes the call for heat live to the boiler (usually a stiff spindle on the valve), or a failure of the synchron motor in the valve)

ASSUMING (note the emphasis) your system has been wired to the conventional colour scheme and from you description you need to force both CH and HW valves to move - neither will move itself- I?d suspect your stray wire has come off the Neutral pin on the controller back plate* or the point in your nest of wires that connects to it. ( Where the Neutrals all meet.)

The above explains why neither motorised valve moves itself, at least one must to fire up the boiler. The other neutrals would prevent only one valve or show different symptoms.

You could check with a meter on ohms (power off) and trace the circuit to check the motors have a neutral connection - if you can access it. Or, lift the motor brown and check between it and neutral and your stray wire.