The wire into 5 comes via the remote thermostat. Not how I would have wired it but it works.
Thanks. That makes more sense now.
So the way I see it the yellow and wire joined in the terminal block (to a black wire) should go to position 6...
I am attempting to replace a Horstmann Channel XL plus with a Nest thermostat.
I have not been able to find a wiring diagram for this...
Separate names with a comma.