Set Appliance State:

Mains Power Off
Gas Off
Set Multimeter to AC Volts Set meter to measure AC Volts
CH Demand
External controls (i.e. room thermostat/programmer) calling
Pre-heat switch OFF
CH switch ON
Mains Power On

Measurement:

Measure voltage across terminal marked Rs1 (4th terminal from left on terminal strip on main PCB 23) and the terminal marked N (blue wire on main terminal block).
Note: If room thermostat is not fitted then a link should be present between the terminals Rs1 and Rs2 to enable heating demand.