Set Appliance State:

Mains Power Off
CH Demand
Mains Isolator Switch (ON/OFF Switch) in ON position
Selector Switch in CH/DHW Winter position
Heating Temperature Control set to Maximum
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Run CH and monitor system pressure. System pressure should remain constant.
Note:
System pressure should be approx 1.5 bar.