Electrical Issue With Central Heating

[CountryFan]

Are you sure it’s a neutral? Could be a switched live.

 

[Groover75]

I have disconnected all of the neutrals and the one from the master fused CH switch is the problem.

With the neutrals still disconnected I opened up the switch and I have 242V red to black going into the switch then when the switch is on 242V on the blue wire.

Note that the brown wire has a piece of insulation missing

The blue wire is in fact attached to the L terminal of the switch...

Good job I am super cautious!!

 

[CountryFan]

So it sounds like you have reversed polarity. Is that brown also connected to the earth terminal?

 

[Groover75]

With the switch turned on the brown wire is connected to earth, however my multimeter makes a warbling noise rather than a continuous beep, like it is connecting and disconnecting very quickly.

 

[CountryFan]

With the switch turned on the brown wire is connected to earth

Why is this?

however my multimeter makes a warbling noise rather than a continuous beep, like it is connecting and disconnecting very quickly.

Are you sure it’s a multimeter? Sounds like a 2 pole voltage indicator. I would rewire the switch with just LNE and connected to the correct terminals

 

[Daveydub]

I would rewire the switch with just LNE and connected to the correct terminals

If that brown is connected to Earth as it seems to be then changing it to Live would not be a good idea without altering other connections.
You appear to have L & N into that switch and just the blue wire leaving it. so no N to system.
This does not make sense as you say that the valve operates with the head removed.
If it is a Honeywell type valve with 2 fixing screws for head to body, sometimes over-tightening these screws can cause the motor to bind

 

[Groover75]

Are you sure it’s a multimeter? Sounds like a 2 pole voltage indicator.