Rheem E5 Error Code

What does Rheem E5 mean?

Error code E5 on Rheem condensing tankless water heaters signals a water flow fault — the unit detects that the incoming flow rate exceeds its heating capacity, preventing it from reaching the set temperature. The control board triggers a safety lockout to avoid delivering tepid water. The problem originates either in the flow sensor itself or in conditions that drive actual flow above the rated maximum GPM.

Symptoms

• E5 code displayed on the unit's control panel with no hot water output.
• Water temperature at fixtures is lukewarm or cold despite normal demand.
• Unit cycles on briefly then shuts down, repeating the lockout.
• Flow sensor LED or diagnostic indicator active on the control board.

Common causes

• Clogged inlet filter screen restricting flow and triggering erratic sensor readings.
• Mineral scale or debris fouling the flow sensor paddle or turbine.
• Simultaneous high-demand fixtures pushing total flow above the unit's rated GPM.
• Water supply pressure exceeding 80 PSI, driving flow rate above the unit's capacity.
• Faulty flow sensor providing inaccurate signals to the control board.
• Scale buildup inside the heat exchanger impeding heat transfer and causing apparent flow anomalies.

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Diagnostic steps

1. Power-cycle the unit

   Turn off the water heater at the circuit breaker or gas shutoff, wait 30 seconds, then restore power. If the fault was transient, the E5 code clears on restart.

2. Inspect and clean the inlet filter screen

   Shut off the cold water supply valve, remove the inlet filter from the water connection fitting, and flush sediment or debris under running water. Reinstall before reopening the supply valve.

3. Verify system water pressure

   Attach a pressure gauge to a nearby hose bib. Operating pressure must stay between 40 and 80 PSI. If pressure exceeds 80 PSI, adjust the pressure reducing valve (PRV) downstream of the main shutoff.

4. Clean the flow sensor

   Locate the flow sensor on the cold water inlet manifold. Remove it per the IOM procedure, rinse the turbine or paddle mechanism under clean water, and gently remove mineral deposits with a soft brush. Reinstall and test.

5. Reduce simultaneous fixture demand

   Confirm total active hot water draw is within the unit's rated GPM capacity. Close any unnecessary fixtures and retest; if the error clears under reduced load, the installation is undersized for peak demand.

6. Descale the heat exchanger if water is hard

   Connect a descaling pump with food-grade white vinegar or manufacturer-approved descaler through the service ports per the IOM flushing procedure. Run for 45–60 minutes, flush with clean water, and restart the unit.

When to call a professional

Call a licensed plumber or HVAC technician if the E5 code persists after cleaning the filter and flow sensor, adjusting pressure, and descaling. Replacing the flow sensor, control board, or heat exchanger requires isolating refrigerant-side components, working with pressurized gas lines, and verifying electrical connections — tasks that demand certified trade credentials and proper instrumentation.