Rheem tankless water heater shows E5 and shuts down when trying to heat water

Safety first: Turn OFF power and gas. Allow the unit to cool completely before touching internal components.

• Check water flow: Ensure all isolation valves are fully open and inlet filter is clean. Low flow can cause overheating.
• Descale heat exchanger: Connect a pump and hoses to the hot and cold service valves. Circulate white vinegar or approved descaler for 45–60 minutes to remove scale from the heat exchanger.
• Inspect high-limit switch: Locate the manual reset high-limit (if equipped) on the heat exchanger. Press the reset button if present. If it trips repeatedly, the underlying cause (scale, flow, or sensor) must be corrected.
• Check thermistors: Verify inlet and outlet thermistors are properly attached and reading correctly. Replace if faulty.
• If E5 recurs: The high-limit switch or control board may be defective. Further diagnosis by a technician is recommended.