Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
CH Demand
Close the integral bypass by turning adjustment screw clockwise, counting the number of turns until it stops
Gas On
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Observation:

Do the radiators now start to heat correctly?
Note:
If radiators still do not heat reset the bypass to its original setting.