Set Appliance State:

Mains Power Off
CH Demand
CH temperature set to MAX
Attach temperature clamps to the flow and return pipes
underneath boiler, as close to the boiler as possible
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Allow the boiler to run for central Heating, until noise or kettling begins.
Measure the temperature differential across flow and return.
Expect a differential of approx 22°C or less.
Note.
1. Ensure all radiators are open and system is balanced correctly
to provide correct test results.
2.Initial temperature differential may be greater than 22°C when system is heated from cold, but should start to reduce as return temperature rises.