Rinnai Water Heater Not Heating

Common Symptoms

• No hot water — unit powers on but does not fire
• Code 11, 12, 14, 25, or 31 displayed on the Rinnai panel or Control-R app
• Cold water sandwich — brief cold burst between two consecutive hot draws
• Lukewarm water only — heat exchanger throttling due to Code 14 overheat
• Unit fires then shuts down within seconds — Code 12 flame loss
• Low hot water pressure — scale restricting RUR/RUS condensing heat exchanger
• Unit won't activate at low-flow fixtures (aerator or shower restrictor issue)

Most Likely Causes

1. 1

   Code 11 — No Ignition (3 Attempts: Gas Supply, Igniter, Flame Sensor)

   Code 11 on Rinnai V, RU, RUR, and RUS models means the unit attempted ignition three times without establishing a stable flame and locked out for safety. The diagnostic hierarchy: (1) Gas supply — confirm the quarter-turn shutoff valve at the unit is fully open (handle parallel to pipe). Check other gas appliances; if they also have weak flame the issue is upstream. Natural gas inlet pressure must be 3.5–10.5 in. w.c. measured at the unit's inlet test port using a low-pressure manometer — test both static and under firing load. LP gas requires 8–14 in. w.c. (2) Igniter electrode — inspect the ceramic insulator for cracks and verify the spark gap is correct (approximately 3–4 mm); replace the igniter assembly if ceramic is cracked. (3) Flame sensor rod — carbon and oxidation deposits on the sensing electrode prevent ionization current detection, causing a false 'no flame' signal even when the burner is lit. Remove the sensor rod and clean the metal tip with 400-grit emery cloth or fine steel wool. Code 11 that recurs after reset almost always means the underlying cause has not been resolved.

2. 2

   Code 12 — Flame Loss During Operation (Gas Valve, Inlet Pressure Drop)

   Code 12 means the Rinnai ignited successfully but the flame extinguished during the heating cycle — unlike Code 11 which is a failure to start. Primary causes: (1) Dynamic gas pressure drop under firing load — when the burner is at full BTU output, supply pressure can dip below the 3.5 in. w.c. minimum, especially if multiple gas appliances draw simultaneously or if the gas meter regulator is undersized. Measure manifold pressure with the unit firing, not just static. (2) Partially failed gas valve — the Rinnai gas valve modulates flow electronically; a sticking or leaking valve produces an inconsistent flame that holds at ignition but collapses at higher BTU demand. (3) Flue restriction — partial vent blockage from debris or a sagging vent section starves the flame of combustion air at full output. Inspect the vent termination cap and full vent run.

3. 3

   Code 14 — Thermal Fuse Trip (Overfire — Descale Heat Exchanger on RUR/RUS)

   Code 14 means the thermal fuse inside the Rinnai heat exchanger has opened due to an overheat condition. On V and RU series non-condensing models, common causes are a blocked flue restrictor, blocked vent, or recirculation of exhaust gases. On RUR and RUS condensing models, Code 14 is frequently caused by calcium carbonate scale buildup inside the condensing heat exchanger — scale acts as thermal insulation, causing localized hot spots that trip the thermal fuse at progressively lower BTU loads as scale accumulates. After Code 14 trips, the thermal fuse must be replaced (it is a one-time device). However, if scale is the root cause, the new fuse will trip again unless the heat exchanger is descaled first. On condensing models, perform a full white vinegar or CLR descale before replacing the thermal fuse to prevent recurrence.

4. 4

   Code 25 — Cold Water Inlet Temperature Sensor Fault

   Code 25 indicates the cold water inlet thermistor has failed or is reading an out-of-range temperature. The inlet thermistor tells the Rinnai control board the incoming water temperature, which it uses to calculate the required firing rate to reach the setpoint. A failed thermistor causes the unit to misfire — producing water that is too hot, too cold, or triggering a protective shutdown. The thermistor is a low-cost sensor ($15–$40) mounted on the cold inlet pipe inside the unit. Before replacing, check the wiring harness connector for corrosion or looseness — a poor connection produces intermittent Code 25 that clears temporarily when disturbed.

5. 5

   Code 31 — Heat Exchanger Outlet Temperature Sensor Fault

   Code 31 means the heat exchanger outlet thermistor has failed. This sensor measures water temperature leaving the heat exchanger and feeds that reading to the modulating control. A failed outlet thermistor causes the unit to overheat the water or shut down protectively. On RUR/RUS condensing models, Code 31 can also appear as a secondary fault after heavy scale buildup causes genuine outlet temperature anomalies — in this case, descaling first may clear the code without a part replacement. Test the thermistor with a multimeter: consult the Rinnai service manual for resistance vs. temperature specs (typically 10 kΩ at 77°F).

6. 6

   Minimum Flow Rate — 0.4–0.5 GPM Activation Threshold

   All Rinnai tankless models require a minimum water flow rate to activate the burner — 0.4 GPM on the V53DeP/V65IP and similar entry models; 0.5 GPM on RU and RUR/RUS series. If the flow rate at any fixture drops below this threshold (clogged aerator, partially closed valve, low-flow shower head with a flow restrictor, or a degraded internal flow sensor), the Rinnai will not fire and will produce cold water that resembles a heating fault. Test: remove aerators from problem fixtures and confirm improved flow; install an inline flow meter on the cold supply at the unit to verify activation threshold is met under normal use conditions.

7. 7

   Cold Water Sandwich Effect (Design Characteristic of Tankless Heaters)

   The cold water sandwich is a brief cold slug between two consecutive hot draws — a design characteristic of all tankless water heaters including Rinnai, not a fault. When hot water is shut off briefly then turned back on, residual hot water in the pipes arrives first, followed by cold water from the heat exchanger body before the unit fires again, then newly heated water. Solutions include: (1) Use the Rinnai recirculation-ready models (RUR, RUS series have a recirculation port; require an external pump and return line) to maintain pre-heated water at fixtures. (2) Connect the Rinnai Control-R Wi-Fi module to enable scheduling-based or on-demand recirculation control from the app. (3) Install a small buffer tank downstream of the unit to absorb the cold slug.

8. 8

   Scale Buildup in RUR/RUS Condensing Heat Exchanger

   Rinnai RUR and RUS condensing models are susceptible to calcium and magnesium scale accumulation in the secondary condensing heat exchanger, especially in hard water areas (above 120 ppm TDS or 7 grains per gallon). Scale reduces heat transfer efficiency, restricts water flow, increases outlet water temperature variability, and ultimately triggers Code 14 (thermal fuse) as overheat events become more frequent. Annual descaling with white vinegar (3 gallons undiluted, circulated for 60–90 minutes via the service port isolation valves and a submersible pump) prevents scale progression. Rinnai RUR/RUS units have built-in service port isolation valves that make the descale procedure accessible without specialized tools.

Quick DIY Checks

1. 1Step 1 — Read the fault code and check the Rinnai Control-R app: note the code displayed on the unit's LED panel. If the Rinnai Control-R Wi-Fi module (sold separately, compatible with RU, RUR, RUS, and most V series with external Wi-Fi adapter) is installed, open the Control-R app (iOS/Android) to view the fault history with timestamps and repeat counts — this history is more diagnostic than the panel display alone. Codes that repeat in the morning may indicate gas pressure drop during peak demand. Codes that correlate with very cold weather may indicate Code 03 freeze protection or thermistor issues. If no code displays but the unit won't fire, verify the 120V circuit breaker is on.
2. 2Step 2 — Test gas supply and inlet pressure (Code 11/12): confirm the gas shutoff valve at the unit is fully open (handle parallel to pipe). Verify other gas appliances are working normally. Connect a low-pressure manometer to the 1/8-inch NPT test port on the gas inlet valve — static natural gas pressure must be 3.5–10.5 in. w.c.; LP gas must be 8–14 in. w.c. More importantly, measure dynamic pressure under firing load (trigger a hot water draw to force the unit to fire). Natural gas must maintain ≥3.5 in. w.c. under full load; a pressure drop greater than 1 in. w.c. below static indicates an undersized supply or a failing gas meter regulator that will cause persistent Code 11 or 12.
3. 3Step 3 — Check minimum flow rate and clean aerators (no-fire, no code): remove aerators from all fixtures where no-heat complaints occur and retest. Connect an inline flow meter at the Rinnai cold supply — open the hot tap fully and confirm flow exceeds the unit's minimum (0.4 GPM for V53/V65 entry models, 0.5 GPM for RU/RUR/RUS series). Replace any shower head flow restrictors limiting flow below threshold. If the internal flow sensor paddle is suspected (unit worked fine with aerators removed but still won't fire), shut off cold supply, remove the flow sensor from the cold inlet manifold, clear debris from the paddle turbine, and reinstall.

1. 4Step 4 — Clean the flame sensor rod (Code 11): shut off the gas supply valve at the unit, close the cold water supply, and unplug from the wall outlet. Remove the front cover (typically 4–6 screws). Locate the flame sensor rod — a single metal sensing electrode in the burner assembly with one wire lead to the PCB. Remove the single mounting screw, slide the rod out, and inspect the metal sensing tip for black carbon deposits, brown oxidation, or white powdery scale. Clean the metal tip only with 400-grit emery cloth or fine steel wool — do not sand the ceramic insulator body. Reinstall, restore gas and water supply, plug in, and test. Code 11 from a dirty flame sensor clears immediately after cleaning.
2. 5Step 5 — Descale the condensing heat exchanger (Code 14 on RUR/RUS, reduced output): confirm isolation service valves are installed on the hot and cold connections at the unit (standard on RUR/RUS; if absent, a plumber must add them before the descale can be performed). Fill a 5-gallon bucket with 3 gallons of undiluted white vinegar or a CLR Calcium, Lime & Rust Remover solution (per label dilution). Connect a small submersible pump to the cold-side service port and run a return hose from the hot-side service port back to the bucket. Open both service valves and circulate for 60–90 minutes. Flush with 5 gallons of clean water after. On heavily scaled units the vinegar solution will turn cloudy within 30 minutes — refresh with fresh vinegar for the remainder of the cycle. After flushing, restore operation and verify Code 14 does not return.
3. 6Step 6 — Test the Code 25/31 thermistor: shut off the unit and locate the cold water inlet thermistor (Code 25 — mounted on the inlet manifold inside the unit) or the heat exchanger outlet thermistor (Code 31 — mounted on the outlet manifold). Disconnect the wiring harness connector and inspect the connector pins for corrosion or bent contacts — re-seat firmly. Set a multimeter to resistance (Ω) and measure across the thermistor terminals at room temperature. Consult the Rinnai service manual for your model's resistance-temperature table (typical value: ~10 kΩ at 77°F / 25°C). A thermistor measuring open (OL) or near zero is definitively failed. Replace with the OEM Rinnai thermistor for your model — aftermarket thermistors may have slightly different resistance curves that cause temperature offset and recurring faults.
4. 7Step 7 — Set up the Rinnai Control-R app for ongoing diagnostics: download the Rinnai Control-R app (iOS/Android) and pair it with the Control-R Wi-Fi module for your unit. Once connected, the app displays: current water temperature setpoint and outlet reading, fault history with timestamps, maintenance reminders, and recirculation scheduling. Use the app's diagnostic mode to monitor live outlet temperature during a hot water draw — if the temperature reading fluctuates wildly while hot water feels normal at the tap, a thermistor is failing. Use the fault history to catch intermittent codes that disappear before you can read them from the panel.

Repair vs Replace

Recommended Tools & Parts

• White Vinegar (Gallon Jugs) or CLR Calcium, Lime & Rust Remover

  Descaling solution for Rinnai RUR/RUS condensing heat exchanger flush. Use 3 gallons undiluted white vinegar or CLR (diluted per label). Circulate via service port isolation valves for 60–90 minutes to dissolve calcium carbonate scale. Prevents Code 14 thermal fuse trips and restores full BTU output.

  $10–$25

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• Submersible Pump for Descale Kit

  Small submersible pump rated for mild acids, used to circulate descaling solution through the Rinnai RUR/RUS service port isolation valves. Required for the descale procedure — gravity flow alone cannot flush both heat exchanger circuits adequately.

  $25–$50

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• Rinnai Flame Sensor / Igniter Assembly (model-specific)

  Replacement spark igniter and flame sensor rod for Rinnai V, RU, RUR, or RUS series. Fixes persistent Code 11 after cleaning. Model-specific part number — verify against the unit's model and serial number label before ordering.

  $40–$80

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• Rinnai Inlet or Outlet Thermistor (Code 25 / Code 31)

  OEM thermistor for Rinnai V, RU, RUR, or RUS series. Fixes Code 25 (cold inlet thermistor) or Code 31 (heat exchanger outlet thermistor). Use OEM only — aftermarket thermistors may have offset resistance curves causing recurring faults. Model-specific.

  $15–$40

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• Rinnai Control-R Wi-Fi Module

  Optional Wi-Fi module for the Rinnai Control-R app — enables remote temperature control, fault history, maintenance scheduling, and recirculation control via smartphone. Compatible with most RU, RUR, and RUS series; check Rinnai compatibility list for V series.

  $80–$120

  Buy on Amazon →

Links are Amazon affiliate links (tag: fixitfastai-20). Prices are estimates.

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