Display shows E005 and unit shuts down; may occur during long hot water draws.

Safety: Water in the system may be extremely hot. Allow the heater to cool and close the cold water supply before removing any sensors or piping.

Steps:

• 1. Check temperature setpoint: Verify the setpoint on the Lochinvar control is within safe limits (typically 120–140°F for domestic hot water). Reduce if set excessively high.
• 2. Inspect outlet temperature sensor: Locate the outlet NTC sensor on the hot water outlet. With power off, disconnect and measure resistance; compare to Lochinvar's temperature chart. Replace the sensor if readings are out of range.
• 3. Check for scale buildup: On condensing models, heavy scale on the heat exchanger can cause localized overheating. If the unit has not been descaled, schedule a professional to perform a heat exchanger flush using an approved descaling pump and solution.
• 4. Verify circulation: For systems with recirculation or primary/secondary loops, ensure the system pump is running and valves are open. A failed pump can cause hot water to stagnate and overheat at the outlet sensor.
• 5. Inspect mixing valve: If a thermostatic mixing valve is installed, verify it is not stuck or misadjusted, which can cause high outlet temperatures and nuisance trips.