Set Appliance State:
Mains Power Off
Set Multimeter to DC Volts 
Turn Hot Water Tap On
Boiler On/Off Switch ON
| WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure |
Measurement:
With the DHW Tap fully open and the DHW Temperature Selector
(Potentiometer - see (1)) set to maximum, measure the DC Voltage across the Red wire and Black wire on the Stage 2 Gas Valve Solenoid (solenoid 3).
The voltage obtained should be approximately 24 V DC.
Decrease the DHW Temperature Selector (Potentiometer) and if required decrease the flow at the DHW Tap to assist temp rise.
Does the DC Voltage across the Solenoid modulate down as the DHW demands a reduction in Burner output?
(Potentiometer - see (1)) set to maximum, measure the DC Voltage across the Red wire and Black wire on the Stage 2 Gas Valve Solenoid (solenoid 3).
The voltage obtained should be approximately 24 V DC.
Decrease the DHW Temperature Selector (Potentiometer) and if required decrease the flow at the DHW Tap to assist temp rise.
Does the DC Voltage across the Solenoid modulate down as the DHW demands a reduction in Burner output?
Isolate as per TOPs P4
and refit as necessary