Replace timer with Hive, only 3 wires

[RandomGrinch]

Yes as I suspected that is a non-return valve for a filling loop, Do you have a short silver braided hose, somewhere ?

Possibly sitting on top of the boiler cabinet?

 

[tim888]

Yes as I suspected that is a non-return valve for a filling loop, Do you have a short silver braided hose, somewhere ?

Nope, no hose! That's to fill the system from the mains, right? Does that change the plan?

 

[ianmcd]

Nope, no hose! That's to fill the system from the mains, right? Does that change the plan?

THAT IS THE FILLING LOOP !