Lochinvar F1 Error Code

What does Lochinvar F1 mean?

The F1 fault on Lochinvar Armor and Shield series water heaters indicates that fuse F1 on the main control board has blown. Fuse F1 protects the 24V DC supply or relay board section of the control circuit. When this fuse opens, affected control circuits lose power, preventing normal operation of connected relays and board functions.

Symptoms

• Control board relay section fails to energize connected loads.
• Unit does not respond to calls for heat despite the display appearing functional.
• Specific relay-driven outputs such as the pump or zone valve fail to activate.
• Multimeter confirms the fuse at F1 position on the control board is open.

Common causes

• Short circuit or overload in the relay board wiring connected to the F1-protected supply.
• Relay contact failure drawing excessive current through the fuse circuit.
• Damaged wiring harness causing an intermittent or sustained short on the F1 supply rail.
• Voltage transient from an external device connected to the relay board outputs.

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Diagnostic steps

1. Turn off power and access the control board

   Switch off power at the external disconnect. Remove the top access cover and control module cover to expose the main control board and relay board sections.

2. Locate and test fuse F1

   Identify fuse F1 on the control board. Use a multimeter in continuity mode to confirm it is open. Note that F1 on Armor/Shield series units is a slow-blow fuse — replace only with the exact specified type and rating.

3. Inspect wiring on the relay board supply circuit

   Before replacing the fuse, examine the wiring connected to the relay board supply for shorts, damaged insulation, or loose terminals. Repair any defects found.

4. Replace fuse F1 with the correct slow-blow fuse

   Install the manufacturer-specified slow-blow fuse in the F1 position. Do not jumper the fuse or exceed the rated amperage. Spare fuses are included in the bag attached to the unit cover.

5. Restore power and test relay board operation

   Reinstall covers, restore power, and verify that relay-driven outputs activate correctly during the next call for heat. Monitor for recurrence of the blown fuse condition.

When to call a professional

If fuse F1 blows repeatedly, a relay, wiring component, or the control board itself has a persistent fault requiring systematic electrical diagnosis. Tracing relay board shorts under live conditions is hazardous and requires a clamp meter and isolation procedures. A licensed technician should perform relay board diagnosis and control board replacement if required.