Set Appliance State:
Mains Power Off
Set Multimeter to DC Volts 
DHW temperature selector to 0 position
CH temperature selector to 0 position
Lower control box cover
CH temperature selector to 0 position
Lower control box cover
| WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure |
Measurement:
All heating controls calling.
Measure voltage across the CH 24 volt input control terminal and the main neutral on the PCB.
Expected voltage between 22 and 40 volts DC.
Note:
The input of the 24 volt connector is the upper of terminals on the connector.
Measure voltage across the CH 24 volt input control terminal and the main neutral on the PCB.
Expected voltage between 22 and 40 volts DC.
Note:
The input of the 24 volt connector is the upper of terminals on the connector.
Isolate as per TOPs P4
and refit as necessary