Water heater will not start; display shows A01 and unit remains in standby despite heat demand.

Safety first: Switch off power to the Nibe water heater at the breaker and close the cold water supply valve before opening any covers.

This alarm indicates that the high-temperature safety thermostat (manual reset cut-out) has tripped.

• Step 1 – Allow cooling: Let the unit cool for at least 30–60 minutes. Do not attempt reset while the tank is very hot.
• Step 2 – Access the high-limit thermostat:
  • Remove the front service panel using a screwdriver.
  • Locate the high-limit safety thermostat on the immersion heater pocket or control panel (often a small round device with a red reset button).
• Step 3 – Reset:
  • Press the red reset button firmly until you feel a click.
  • If it immediately trips again when power is restored, switch power off and do not continue to reset repeatedly.
• Step 4 – Check circulation and controls:
  • Verify that the circulation pump (e.g. Nibe circulation pump on the primary circuit) is running when there is a heat demand.
  • Check that the control thermostat or electronic temperature sensor is not set excessively high (typically 50–60 °C for domestic hot water).
  • Inspect for heavy limescale on the immersion heater if accessible; scale can cause localized overheating.
• Step 5 – Restore power and test:
  • Open the cold water supply valve.
  • Restore power at the breaker and monitor the temperature rise and operation for at least one full heating cycle.

Call a professional if the high-limit trips again, the circulation pump is not running, or wiring/insulation appears damaged.