Rinnai Tankless Water Heater Not Working

Common Symptoms

• No hot water — unit displays an error code and will not fire
• Error 11 or Error 12 displayed — ignition failure or flame loss
• Cold-water sandwich effect — brief cold burst between hot draws
• Fluctuating water temperature during showers
• Error 25, 31, 32 — thermistor or bypass valve fault
• Unit clicks repeatedly but does not ignite
• Error 70, 71, 72, or 73 — PCB or gas valve circuit fault

Most Likely Causes

1. 1

   Error 11 — No Ignition on Startup

   Error 11 means the burner did not ignite within the startup window. Most common causes: insufficient gas pressure (check that other gas appliances are working), closed gas shut-off valve, clogged inlet filter screen restricting flow below the 0.5 GPM minimum ignition threshold, or a dirty/failed flame sensor that cannot confirm a flame. Check gas supply first, then clean the inlet filter screen. If gas pressure is good and the filter is clean, test the igniter and flame sensor. Flame sensor cleaning (light steel wool or fine emery cloth on the rod) resolves Error 11 in a significant portion of cases.

2. 2

   Error 12 — Flame Failure During Operation

   Error 12 indicates the flame was established but went out during operation. Causes include: gas pressure drop under high demand (check supply pressure during full flow), venting obstruction causing flue gas recirculation into the combustion air intake, condensation in the venting on direct-vent models, or a failing modulating gas valve. Inspect venting for birds, nests, or ice blockage at the termination cap. A partially blocked vent causes flue gas recirculation that starves the burner of oxygen mid-cycle.

3. 3

   Error 25 — Cold Water Sandwich / Bypass Valve

   Error 25 points to the cold-water sandwich bypass valve or an issue with the flow sensor during standby. The cold-water sandwich effect is water that sat in the pipe between the unit and the fixture cooling down — when you open the tap, you get hot (residual), then cold (cooled pipe water), then hot again. This is an inherent characteristic of tankless units but can be exacerbated by a faulty bypass valve. Check that the cold water bypass valve is fully open and moving freely. On units with recirculation, verify the pump and schedule settings.

4. 4

   Error 31 / Error 32 — Inlet or Outlet Thermistor Fault

   Error 31 is an inlet thermistor fault; Error 32 is an outlet thermistor fault. These NTC thermistors monitor incoming and outgoing water temperature so the control board can modulate the burner. Test with a multimeter in resistance mode — a functional 10kΩ NTC reads approximately 10kΩ at 77 degrees F. An OL (open) or near-zero reading indicates a failed sensor. Thermistor replacements are straightforward and cost $15-$40. After replacement, clear the error code by pressing the On/Off button and monitor the first few heat cycles.

5. 5

   Error 52 — Modulating Gas Valve Failure

   Error 52 indicates the modulating gas valve is not responding to control board commands. This valve regulates gas flow to match the burner output to demand. A failed valve usually requires replacement. Before condemning the valve, check that the gas supply pressure is stable and within spec (natural gas: 3.5-10 inches WC; LP: 8-14 inches WC). Low supply pressure can mimic a valve failure by preventing the valve from opening fully. If pressure is confirmed good, the modulating gas valve (Rinnai part: varies by model — reference the service manual) requires replacement by a qualified technician.

6. 6

   Error 61 — Combustion Fan Fault

   Error 61 means the combustion fan is not reaching the required RPM within the startup sequence. The fan draws combustion air through the unit and must spin up before the gas valve opens. Causes: debris in the fan blade, failed fan motor capacitor, or a failing fan motor. Access the fan by removing the unit cover (ensure gas is off). Spin the fan blade by hand — it should spin freely with no binding. Use a multimeter to test the fan motor resistance; an open winding indicates motor failure. Fan motor replacement is an intermediate-level repair.

7. 7

   Error 65 — Water Flow Control Valve

   Error 65 points to the water flow control valve (also called the flow sensor or flow control servo). This valve modulates water flow through the heat exchanger and is used by the control board to match burner output to demand. Scale buildup is the primary cause of Error 65 — hard water deposits restrict the valve's movement. Descaling the heat exchanger (citric acid flush) often resolves Error 65. If descaling does not clear the code, the flow control valve itself may need replacement.

8. 8

   Error 70 — PCB (Control Board) Failure

   Error 70 indicates a fault on the printed circuit board. This is generally not a DIY repair — the PCB is the brain of the unit and replaces at $150-$400 depending on model. Before condemning the board, verify that the error is not caused by a voltage spike or wiring fault: check all wiring harness connectors for corrosion or looseness. A power cycle (unplug or turn off breaker for 5 minutes) sometimes clears a transient Error 70. Persistent Error 70 warrants professional service or, on older units, a cost-vs-replacement analysis.

9. 9

   Error 71 / 72 / 73 — Gas Valve Circuit Faults

   Errors 71, 72, and 73 indicate faults in the gas valve drive circuit on the PCB — specifically the solenoid valve circuits (71 = first solenoid, 72 = second solenoid, 73 = third solenoid or valve modulating circuit). These errors point to either the gas valve solenoids themselves or the PCB driver circuits. Perform a resistance check on the gas valve solenoid coils (typically 10-30 ohms at room temperature). If the solenoids test good, the fault is on the PCB. These errors typically require professional service.

Quick DIY Checks

1. 1Step 1 — Check gas supply and pressure: verify other gas appliances in the home (furnace, range, gas dryer) are working normally. If multiple appliances are out, the issue is upstream (gas meter, utility supply). For a single appliance failure, locate the gas shut-off valve on the supply line to the tankless unit and confirm it is fully open (handle parallel to pipe = open). If you have a manometer, check supply pressure: natural gas should be 3.5-10 inches water column (WC) static, at least 3.5 inches WC during full flow; LP should be 8-14 inches WC. Low pressure under flow indicates an undersized supply line, a faulty regulator, or a low LP tank.
2. 2Step 2 — Inspect venting for blockages: go to the exterior termination cap of both the exhaust (concentric outer) and combustion air intake (inner pipe). Look for bird nests, wasp nests, leaves, ice, or snow blockage. On direct-vent units the intake and exhaust are typically concentric PVC pipes terminating on a side wall. A blocked exhaust causes flue gas recirculation that triggers Error 12 or causes sooting of the heat exchanger. A blocked intake starves the burner of combustion air. Clear any obstruction and reset the unit by pressing On/Off. Also inspect the interior vent run for disconnected joints or condensate pooling in low spots.
3. 3Step 3 — Clean the inlet filter screen: locate the cold water inlet connection at the bottom of the unit. There is a small brass inline filter screen where the supply pipe connects. Turn off the cold water supply valve, relieve pressure by opening a hot tap, then unscrew the filter housing or inlet fitting. Remove the screen and rinse under running water — a soft toothbrush removes sediment from the mesh. Reinstall, restore water supply, and test. A restricted inlet screen reduces flow below the 0.5 GPM minimum ignition threshold, causing Error 11. This is one of the most commonly overlooked causes of ignition failure.

1. 4Step 4 — Verify minimum flow rate and check error code on display: the Rinnai unit requires a minimum of 0.5 GPM to activate. At very low demand (single low-flow fixture, partially opened valve), the unit may not see sufficient flow to trigger ignition — resulting in an apparent no-hot-water complaint with no error code. Test by fully opening a hot tap. Also note the exact error code on the display panel before resetting — the code identifies the failed subsystem. Reset by pressing the On/Off button; the code clears but the unit logs the fault in memory. If the same error returns within two or three heat cycles, the underlying cause has not been resolved.
2. 5Step 5 — Perform flame sensor cleaning: turn off the gas supply at the shut-off valve. Remove the front cover of the unit (typically 4-6 screws). Locate the flame sensor rod — it protrudes into the burner area and has a single wire lead. Disconnect the wire. Use fine emery cloth or 0000-grade steel wool to clean the oxidation off the stainless sensing rod. Wipe with a dry cloth, reconnect the wire, restore gas, reinstall the cover, and test. A carbon-coated or oxidized flame sensor reads incorrectly low ionization current and causes the PCB to shut down the burner (Error 11 or 12) even when the flame is healthy.
3. 6Step 6 — Test inlet and outlet thermistors for Error 31 or 32: turn off power to the unit. Remove the cover. Locate the thermistor connectors — typically labeled TH-I (inlet) and TH-O (outlet). Disconnect and test each with a multimeter set to resistance (Ohms). At room temperature (approximately 77 degrees F), a good 10kΩ NTC thermistor reads 9.5-10.5kΩ. An OL reading = open/broken; near-zero = shorted. Replace any sensor outside this range. Rinnai thermistors are available through HVAC distributors and online suppliers by model number — confirm your model number on the rating plate inside the cover.
4. 7Step 7 — Descale the heat exchanger to resolve Error 65 or restore performance: if the unit is over 2 years old and has never been descaled, scale buildup is likely degrading performance and potentially causing Error 65. Purchase a descaling kit (includes a small submersible pump, hoses, and fittings for the service ports). The unit has two service port connections — cold water inlet service port and hot water outlet service port. Connect the pump and a 5-gallon bucket. Circulate white vinegar or a commercial citric acid descaler (Rinnai recommends the Rinnai Descaler Kit) through the heat exchanger for 45-60 minutes. Rinse with fresh water for 10 minutes. Annual descaling is required to maintain the Rinnai limited warranty in hard water areas.

Repair vs Replace

Recommended Tools & Parts

• Rinnai Inlet Filter Screen Replacement

  Brass or stainless mesh inlet filter screen for Rinnai tankless water heaters. Prevents debris from reaching the flow sensor and heat exchanger. Replace if screen is torn or corroded.

  $5-$15

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• Rinnai NTC Thermistor (Inlet or Outlet)

  OEM-compatible 10kΩ NTC thermistor for Rinnai tankless units. Resolves Error 31 (inlet) and Error 32 (outlet) fault codes. Confirm model number before ordering.

  $15-$40

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• Rinnai Descaler Kit (Citric Acid)

  Rinnai-approved descaling kit with citric acid descaler, pump, hoses, and service port adapters. Removes calcium scale from the heat exchanger. Annual use maintains warranty compliance.

  $30-$60

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• Universal Tankless Water Heater Descaling Pump Kit

  Submersible pump kit for descaling tankless water heaters — compatible with Rinnai, Navien, Noritz, and most brands. Includes hoses, bucket adapter, and citric acid packets.

  $40-$80

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

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