Wiring for Vaillant Ecotec Plus combi and Honeywell twin channel zoning pack

[doitmyself]

The boiler will be controlled by the motorised valves via a relay across the 24v terminals in the boiler

Why do you need a relay? the auxiliary switches in the valves are volt free, so they can be wired directly to the 24V terminals on the boiler.

 

[mountainman]

Why do you need a relay? the auxiliary switches in the valves are volt free, so they can be wired directly to the 24V terminals on the boiler.

They can be volt free of course but that is not the usual way of wiring and the OP didn't seem confident in wiring the system so a relay is imo the safest option.

It has since been revealed that the OP is GSR & Part P, which makes the original question seem odd to me.