Set Appliance State:

Mains Power Off
CH Demand
CH temperature control to maximum
DHW temperature control to minimum
Selector switch to heating and hot water position
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 at approximately 1 to 1.5 bar?