Vailant ecoTEC 630 Wiring Question

Hello. What is the purpose of wiring the heating / hot water controls across terminals 3 & 4 of this boiler? What additional protection / safety is gained by taking the supply to the controls from terminal 3 as opposed to supplying the controls direct from the spur supplying the boiler and then just sending the switch live to terminal 4?

I hope this query is clear?!

Thanks

The boiler PCB is protected from external faults by its own fuse.

On older Vaillants the OH stat could be bypassed by wiring this way.

To answer your question, the mains I/O switch on the facia does not isolate the boiler if you take an external phase conductor into 4. This obviously has safety implications for the unaware.