Set Appliance State:

Mains Power Off
CH Demand
Selector switch to heating and hot water position
CH temperature selector to MAX
DHW temperature selector to MIN
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Observe the system pressure gauge, while allowing the burner to fire and the CH system to heat.
Does the system pressure remain constant between 1 and 1.5 bar?