Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
CH Demand
Attach temperature clamps to the flow and return pipes of
the primary heat exchanger (as indicated by the red arrows)
Note:
Ensure a good connection is made with the flow & return pipes
or misdiagnosis will occur
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Measure the temperature differential at the
time the system noise occurs.
Expect a temperature differential of approximately 22°C or less.