Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Boiler DHW thermostat set to MAX
Attach U gauge to burner pressure test point
Hot water tap ON and fully open
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Allow the burner to ignite and the boiler to run in DHW mode and observe the U gauge.
When the burner pressure has risen to maximum reduce the
flow rate at the tap to 6 l/min and observe the U gauge.
The burner pressure should slowly start to reduce as the temperature
demand is met (modulation).
Does the burner pressure modulate as described?