Nexus heat interface unit and lightwaverf boiler switch

[PhilMcG321]

Hi, I am in an apartment which has a nexus heat interface unit which provides hot water to taps and rads. Control/thermostat wired back to main pcb of the heat exchanger. This is cat5 and the thermostat has 3 batteries and connects using 2 wires of the cat 5 to two terminals on the hiu pcb. I wish to install a lightwave rf boiler switch and wireless thermostat to replace the current wired control thermostat. Heating is controlled by the existing wired control thermostat. Hot water is always available to taps. The lightwaverf boiler switch suggest installing to the main power supply. The hiu is always on unlike a boiler I guess so I am struggling as to how to wire the new lwrf boiler switch. Looking at the hiu I see an electrical motorised valve which connects back to the pcb with 3 core. Pretty sure this valve is controlling the flow of hot water to rads and taps. Should this be the source for the new switch?
Any help much appreciated