Rinnai Error Code 65 — Water Flow Rate Fault

Error Code 65 on Rinnai tankless water heaters (V series, RU series, and RL series) means the unit has detected insufficient water flow or a fault with the water flow sensor. Rinnai units require a minimum flow rate to activate the burner and prevent overheating — when flow falls below this threshold, Error 65 is triggered.

Jump to Fix

• Most Likely Cause
• Diagnosis Steps
• Parts

Minimum Flow Rate for Rinnai

• V75e / V94e / V65e: Minimum activation flow = 0.26 GPM
• RU Series (condensing): Minimum activation flow = 0.26–0.40 GPM depending on model
• Below this flow rate, the unit will not ignite and may display Error 65

Common Causes

Step-by-Step Diagnosis

Step 1 — Check inlet filter screen The most common fix for Error 65:

1. Turn off cold water supply to the unit
2. Disconnect the cold water inlet connection
3. Remove the filter screen (mesh disc inside the inlet)
4. Clean with water and a soft brush — replace if torn
5. Reinstall and retest

Step 2 — Check water pressure and flow

• Measure cold water pressure at the unit inlet (should be 15–150 psi)
• Partially open fixtures at multiple points to verify flow is reaching the unit
• Check for partially closed shutoff valves at the unit or upstream

Step 3 — Check the flow sensor

• The flow sensor is a hall-effect turbine inside the unit on the water inlet
• Remove and inspect for scale or debris lodged in the turbine wheel
• Clean with water — do not use metal tools on the turbine
• Spin the turbine wheel manually — it should rotate freely with no binding

Step 4 — Test the flow sensor signal

• With power on and water flowing, the flow sensor sends a pulsed signal to the control board
• If the turbine spins freely but Error 65 persists: flow sensor has failed — replace

Step 5 — Check for pipe scale (hot side)

• In high-scale areas, the hot water piping or fixtures can scale internally and restrict flow
• Disconnect the hot water outlet and measure actual flow from the unit
• If flow at the unit is adequate but fixtures are slow: scale is in the distribution piping

Replacement Parts

Reset Procedure

1. Fix the root cause (clean filter, restore flow)
2. Press the ON/OFF button to clear the error
3. Open a hot water fixture to verify the unit fires at normal flow
4. If Error 65 returns: flow sensor is failed

Pro tip: In hard water areas, descale the unit annually and clean the inlet filter every 6 months. A clogged filter is responsible for the majority of Rinnai Error 65 calls.

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See Also

• Rinnai RU130iN Tankless Water Heater Error Codes - Full Fault Guide
• Rinnai Error Code 52 — Outlet Water Temperature Sensor Fault
• Rinnai Error Code 29 — System Communication Fault Fix
• Rinnai Error Code 16 — Causes & Fix