Set Appliance State:
Mains Power Off
Set Multimeter to DC Volts 
Turn Hot Water Tap On
Boiler Operating Switch / Programmer set to WATER only
| WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure |
Measurement:
With the DHW tap fully open measure the DC voltage across connections 5 and 6 (Yellow wires) on connector X2 on the main PCB.
The voltage obtained should be approximately 20V DC.
Decrease the flow at the DHW tap to assist temperature rise.
Does the DC voltage measured decrease as the DHW demands a reduction in burner output?
The voltage obtained should be approximately 20V DC.
Decrease the flow at the DHW tap to assist temperature rise.
Does the DC voltage measured decrease as the DHW demands a reduction in burner output?
Isolate as per TOPs P4
and refit as necessary