Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
CH Demand
DHW Selector in Minimum (-) position
CH Selector in Maximum (+) position
ON/OFF Switch in ON position
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Allow boiler to ignite and CH system to heat.
Monitor the system pressure gauge as the system heats up.
Does the system pressure remain constant as the central heating heats up?