Lochinvar F5 Error Code

What does Lochinvar F5 mean?

The F5 fault on Lochinvar Shield water heaters means that fuse F5 on the main control board has blown. Fuse F5 is a 5-amp slow-blow fuse that protects the pump circuit. When F5 opens, the control board cannot energize the circulation pump, which causes the unit to lock out to prevent operation without water flow.

Symptoms

• Circulation pump does not run during a call for heat.
• Unit displays a No Pump fault or similar pump operation failure message.
• High limit or temperature overshoot faults occurring due to no water circulation.
• Visible fuse failure confirmed with a multimeter at the F5 position on the control board.

Common causes

• Pump motor drawing locked-rotor current due to a seized pump, opening the fuse.
• Short circuit in the pump wiring harness or pump contactor.
• Failed pump relay on the control board sending excessive current through the fuse.
• Incorrect pump installed with higher amperage draw than the circuit is rated for.
• Fuse F5 weakened over time from repeated inrush current and failing on a normal start.

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Diagnostic steps

1. Disconnect power and locate fuse F5 on the control board

   Turn off power at the external disconnect. Remove the top access cover and control module cover. Identify fuse F5 — the 5-amp slow-blow fuse in the pump circuit position on the board.

2. Confirm fuse F5 is open with a multimeter

   Use a multimeter in continuity mode to verify that fuse F5 has blown. Do not assume the fuse is good based on visual inspection alone, as slow-blow fuses can fail internally without visible damage.

3. Inspect pump wiring for shorts before replacing the fuse

   Trace the pump wiring from the control board terminals to the pump motor connections. Look for damaged insulation, pinched wires, or corroded terminals that could cause a sustained short.

4. Manually test the pump motor

   With the fuse still removed, apply 120 VAC directly to the pump motor leads (per service manual procedure) to verify the pump spins freely. A seized motor will need replacement before the fuse is reinstalled.

5. Replace fuse F5 and restore power

   Install a new 5-amp slow-blow fuse of the exact specification. Reinstall covers, restore power, and verify that the pump energizes normally during the next call for heat.

6. Replace pump relay or control board if fuse blows again

   If the replacement fuse opens immediately or on the next heating cycle, the pump relay on the main control board has an internal fault. Replace the control board as directed on page 62 of the service manual.

When to call a professional

Pump replacement and control board diagnostics involve working with 120 VAC live circuits. If pump amperage draw testing, relay circuit measurement, or pump motor replacement is required, a licensed HVAC technician should perform this work. A seized or oversized pump can indicate a hydronic system design or commissioning problem that also needs professional evaluation.