Rheem tankless water heater shows E1 and shuts down shortly after opening a hot water tap

Safety first: Turn OFF power and close the cold water inlet valve before servicing. Allow the unit to cool.

• Check inlet valves: Verify the cold water inlet and hot water outlet isolation valves are fully open.
• Clean inlet filter: Locate the cold water inlet filter screen on the bottom of the unit. Remove the cap, pull out the screen, and rinse debris under running water. Reinstall securely.
• Check household flow: Open a hot tap fully. If flow is weak at all fixtures, you may have a home plumbing restriction or partially closed main shutoff.
• Descale the unit: If you have hard water and have not descaled in the last 12 months, perform a vinegar or descaling pump flush through the service valves to remove scale from the heat exchanger.
• Inspect flow sensor: With the cover off, locate the flow sensor assembly (usually on the cold inlet manifold). Check wiring connections. If the impeller is stuck or wiring is damaged, replace the flow sensor.