Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Primary water circuit drained and discharged
Remove the flow and return pipes to the recuperator

Observation:

Inspect the recuperator's water ways for restrictions and/or blockages.
Note:
To confirm no restrictions and/or blockages are present it may be necessary to remove the recuperator from the boiler and pour water through the heat exchanger to confirm it is unrestricted and/or blocked.