Noritz Water Heater Not Heating

Common Symptoms

• No hot water — unit powers on but does not fire
• Error 11, 12, 14, or 90 displayed on remote controller (RC-7651M) or unit panel
• Unit fires then shuts off within seconds — Error 12 flame loss
• Cold water sandwich — brief cold burst between two consecutive hot draws
• Gradually declining hot water output over weeks — Error 90 scale progression
• Unit won't activate at low-flow fixtures — below 0.5 GPM threshold
• Lukewarm water only — combustion throttled by scale (Error 90 precursor)

Most Likely Causes

1. 1

   Error 11 — No Ignition (Gas Supply, Igniter, Flame Sensor Cleaning)

   Error 11 on all Noritz models means the unit attempted ignition three times without establishing a stable flame and locked out for safety. Diagnostic hierarchy: (1) Gas supply — confirm the gas shutoff valve at the unit is fully open (handle parallel to pipe). Verify other gas appliances work normally. Measure inlet gas pressure at the unit's 1/8-inch NPT test port with a low-pressure manometer: natural gas must be 3.5–10.5 in. w.c.; LP must be 8–14 in. w.c. Test both static and dynamic (under firing load). (2) Igniter electrode — inspect the ceramic insulator for cracks. The spark gap should be approximately 3–4 mm; replace the assembly if cracked or worn. (3) Flame sensor rod — Noritz specifies cleaning the sensing electrode with 0000 (four-aught) super-fine steel wool to remove the carbon and oxidation layer that prevents the ionization current from being detected. This is the most common Error 11 cause after the first year of operation. Never use coarse abrasives or sandpaper on the sensing rod — they leave scratches that accelerate re-fouling.

2. 2

   Error 12 — Flame Loss During Operation (Gas Valve, Vent Obstruction)

   Error 12 means Noritz ignited successfully but the flame extinguished mid-cycle. Primary causes: (1) Dynamic gas pressure drop — measure manifold pressure under full firing load; if pressure dips below 3.5 in. w.c. (NG) or 8 in. w.c. (LP) while firing, the gas supply or regulator is undersized. (2) Gas valve fault — the Noritz modulating gas valve can develop intermittent sticking or solenoid failure, producing a flame that holds at low BTU but collapses when the valve is commanded to modulate up. Error 71 (gas valve) may appear alongside Error 12 on a failing valve. (3) Vent obstruction — partial blockage of the exhaust vent or combustion air intake starves the flame at higher firing rates. Inspect both terminals at the exterior wall for debris, bird nests, spider webs, or snow/ice accumulation. Noritz concentric venting is particularly vulnerable to blockage at the inner exhaust ring.

3. 3

   Error 90 — Combustion Abnormality (Scale Buildup, Descale Procedure)

   Error 90 is the signature Noritz fault for combustion instability caused by calcium carbonate scale inside the condensing heat exchanger on NRC and NRCP models. Scale acts as thermal insulation, causing localized hot spots that destabilize combustion — the control board detects abnormal exhaust temperatures or heat exchanger outlet temperature swings and codes Error 90. In areas with hard water (above 120 ppm TDS), Error 90 typically appears within 2–3 years without annual descaling. The descale procedure: (1) Shut off gas, close cold inlet and hot outlet isolation valves (NRC/NRCP units have built-in service port valves). (2) Connect a submersible pump to the cold service port and a return hose from the hot service port to a 5-gallon bucket. (3) Fill the bucket with 3 gallons of undiluted white vinegar or diluted CLR. (4) Circulate for 60–90 minutes — solution will turn cloudy as scale dissolves. Refresh with fresh vinegar if solution saturates before 60 minutes. (5) Flush with 5 gallons of clean water. Restore operation and verify Error 90 clears.

4. 4

   Error 14 — Thermal Fuse (Overheat from Unresolved Error 90)

   Error 14 means the thermal fuse inside the heat exchanger assembly has opened due to an overheat event. On NRC/NRCP condensing models, Error 14 almost always follows repeated unresolved Error 90 events — scale buildup creates localized hot spots that progressively stress the thermal fuse until it fails open. The thermal fuse is a one-time device that must be replaced. However, replacing the fuse without first descaling the heat exchanger (see Error 90 procedure) will result in the new fuse tripping again during the next firing cycle. Always complete the full descale procedure before replacing the thermal fuse on condensing models. On NR non-condensing models, Error 14 more typically indicates a blocked vent or recirculation of exhaust gases into the combustion air intake.

5. 5

   Minimum Flow Rate — 0.5 GPM Activation Threshold

   All Noritz tankless models require a minimum water flow rate of 0.5 GPM to activate the burner. If the flow rate at any fixture drops below this threshold — due to a clogged aerator, partially closed valve, low-flow shower head with a flow restrictor insert, or a degraded internal flow sensor — the Noritz will not fire and produces cold water that resembles a heating fault. Error 65 (flow sensor) may appear if the flow sensor paddle is fouled or sticking. Test: remove aerators from problem fixtures and retest. Install an inline flow meter on the cold supply line at the unit to verify threshold. If the unit fires with aerators removed, clean or replace clogged aerators. If the flow sensor paddle is the cause, the sensor body can be removed from the cold inlet manifold and cleaned without full disassembly.

6. 6

   Cold Water Sandwich Effect (Tankless Design Characteristic)

   The cold water sandwich is a brief burst of cold water between two consecutive hot draws — a design characteristic of all tankless water heaters including Noritz, not a fault. After hot water is shut off, residual hot water sits in the pipes. When the tap reopens, that residual hot water arrives first, followed by cold water from the heat exchanger body while the unit reignites, then newly heated water. Noritz NRC and NRCP condensing models support recirculation via the built-in recirculation pump port — installing an external pump and return line, then scheduling recirculation through the Noritz Connect app, eliminates the cold sandwich for most households.

7. 7

   Scale Buildup on Condensing Heat Exchanger (NRC/NRCP Models)

   Noritz NRC and NRCP condensing models use a secondary stainless steel condensing heat exchanger to extract additional heat from exhaust gases. This heat exchanger operates at lower temperatures than the primary, causing condensation and accelerated calcium carbonate precipitation from hard water. Annual descaling with white vinegar or CLR is required to maintain efficiency and prevent Error 90 and Error 14 progression. Noritz recommends annual descaling in areas with water hardness above 7 grains per gallon (120 ppm). In very hard water areas (above 15 grains per gallon), a water softener or upstream polyphosphate scale inhibitor cartridge significantly extends heat exchanger life.

Quick DIY Checks

1. 1Step 1 — Read the error code from the RC-7651M remote controller or Noritz Connect app: note the error code displayed. If the Noritz Connect Wi-Fi module is installed, open the Noritz Connect app (iOS/Android) to view fault history with timestamps and repeat counts — the history reveals whether a code is intermittent (appeared once) or recurring (daily at the same time). Recurring Error 90 at the same time each day typically indicates peak hot water demand stress on a scaled heat exchanger. If no code displays but the unit won't fire, verify the 120V circuit breaker and that the on/off button is lit.
2. 2Step 2 — Test gas supply and inlet pressure (Error 11/12): fully open the gas shutoff valve at the unit (handle parallel to pipe). Test other gas appliances to confirm supply is active. Connect a low-pressure manometer to the 1/8-inch NPT test port on the gas inlet connection — measure static pressure (natural gas: 3.5–10.5 in. w.c.; LP: 8–14 in. w.c.). Then trigger a hot water draw and measure dynamic pressure under full firing load. A pressure drop greater than 1 in. w.c. below static under load indicates an undersized supply line, failing regulator, or undersized meter — correct before proceeding with other Error 11 diagnostics.
3. 3Step 3 — Clean the flame sensor with 0000 steel wool (Error 11): shut off the gas 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 electrode positioned in the burner assembly with a single wire lead to the PCB. Remove the mounting screw, slide the rod out, and inspect the metal sensing tip for carbon deposits or oxidation. Clean the metal tip only with 0000 (four-aught) super-fine steel wool — the Noritz service specification calls for 0000 steel wool, not sandpaper, to preserve the surface finish. Do not clean the ceramic insulator body. Reinstall, restore gas and water supply, plug in, and test. A dirty flame sensor typically clears Error 11 immediately after cleaning.

1. 4Step 4 — Check minimum flow rate and clean aerators (no-fire, no code): remove aerators from all fixtures where no-heat complaints occur and retest. If the unit fires with aerators removed, the aerators are restricting flow below the 0.5 GPM activation threshold. Replace restrictive aerators (flow-restricted models below 0.5 GPM are incompatible with Noritz tankless operation). Install an inline flow meter at the cold supply to the unit for definitive measurement. If Error 65 (flow sensor) has appeared, shut off the cold supply, remove the flow sensor from the cold inlet manifold, clear any debris from the paddle turbine, and reinstall.
2. 5Step 5 — Descale the condensing heat exchanger (Error 90 or declining output on NRC/NRCP): confirm that isolation service port valves are installed at the hot and cold connections (standard on NRC/NRCP models). Fill a 5-gallon bucket with 3 gallons of undiluted white vinegar or CLR Calcium, Lime & Rust Remover (per label dilution). Connect a small submersible pump to the cold-side service port and run a return line from the hot-side service port back into the bucket. Open both service valves and circulate the solution for 60–90 minutes. If the solution turns cloudy within 30 minutes, replace it with fresh vinegar. After descaling, flush with 5 gallons of clean water. Restore operation and confirm Error 90 does not return on the next firing cycle.
3. 6Step 6 — Replace the thermal fuse after descaling (Error 14 on NRC/NRCP): after completing the full heat exchanger descale, the thermal fuse must still be replaced as it is a one-time device. Locate the thermal fuse on the heat exchanger assembly (position varies by model — consult the Noritz service manual for your model number). Discharge any residual water pressure, remove the fuse assembly, and install an OEM Noritz replacement. Using aftermarket thermal fuses with incorrect temperature ratings risks premature re-trip or, worse, insufficient protection. Restore operation and verify the unit fires without Error 14.
4. 7Step 7 — Set up Noritz Connect app for diagnostics and recirculation scheduling: download the Noritz Connect app (iOS/Android) and pair with the Noritz Connect Wi-Fi module (sold separately or included on some NRC/NRCP models). The app provides: real-time outlet temperature, fault history with timestamps, maintenance reminders, and recirculation scheduling. Schedule hot water recirculation during morning and evening peak demand windows to eliminate the cold water sandwich. The app's diagnostic mode can identify thermistor drift before a hard fault code appears — if outlet temperature readings fluctuate by more than 10°F while actual tap temperature feels stable, suspect a thermistor drift condition.

Repair vs Replace

Recommended Tools & Parts

• Noritz Flame Sensor Rod

  OEM sensing electrode for your Noritz model. Needed when 0000 steel wool cleaning no longer clears Error 11, indicating the sensor surface is too corroded.

  $15–$40

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• Noritz Thermal Fuse Assembly

  OEM one-time thermal fuse for NRC/NRCP or NR series. Always descale the heat exchanger before installing — using incorrect temperature ratings is dangerous.

  $25–$60

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• Noritz Flow Sensor

  Replacement flow sensor paddle assembly for Error 65 or activation failure at the 0.5 GPM threshold.

  $30–$70

  Buy on Amazon →

• Submersible Pump + Hose Kit for Descaling

  Small submersible pump (120 GPH or larger) and hose set for circulating vinegar or CLR through the heat exchanger service ports during descale procedure.

  $25–$50

  Buy on Amazon →

• Low-Pressure Manometer

  Required to test gas inlet pressure at the 1/8-inch NPT test port — confirms gas supply is within Noritz specifications (3.5–10.5 in. w.c. NG; 8–14 in. w.c. LP).

  $30–$60

  Buy on Amazon →

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

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