UK: swapping a Honeywell t40 to a nest thermostat pro edition

[Hayon]

Hi

I currently have a honey well t40 thermostat in my hall way. I also have a invensys programmer. 

What is confusing me is the wiring. I currently have three wires in my thermostat and two wires (excluding the feed) in the 3 and 4 labelled HW and CH ON. 

With the three cables at the thermostat, do I loose one and connect them to the nest with the other ends in T1 and T2. This leaves the 3 and 4 wires in the invensys - where would they go? Or am I totally wrong?

[LarisaL]

Hi there, thanks for reaching out! You can check the wiring diagrams in the installation guide and see how you should wire the Heat Link depending on the system type you have. Basically you will need wires in the following terminals: L (live), N (neutral), for heating: 2 and 3 and for hot water in 5 and 6. You can find more information in the installation guide at page 19 and starting with page 20 are the wiring diagram.

[Hayon]

Thank you for this. I have an a S plan system. 
I do have a Grundfos pump wired in to my existing system. Where would this be wired to?

[AlexD]

I can't advise on different parts of the system as I don't know what the purpose is for them. It would be best to get in touch with a professional installer or Nest Pro to determine what to do with those elements. 

[AlexD]

Hi, I wanted to check in. Is there anything else I could perhaps try and assist with?