Noritz Tankless Water Heater Not Working

Common Symptoms

• No hot water — unit shows an error code and locks out
• Error 11 or Error 12 — ignition failure or flame loss
• Error 16 — unit overheating (most common in hard water areas)
• Scale buildup reducing hot water flow rate over time
• Error 61 — combustion fan fault
• Error 65 — water flow sensor fault
• Error 59 — PCB control board fault
• Error 90, 91, or 92 — combustion air issues

Most Likely Causes

1. 1

   Error 16 — Overheating from Scale Buildup (Most Common Noritz Failure)

   Error 16 indicates the heat exchanger outlet temperature exceeded the safe limit. In the vast majority of cases, this is caused by calcium scale buildup on the heat exchanger tubes. Scale acts as an insulator — the burner must run hotter and longer to transfer the same BTUs through a scaled heat exchanger, eventually triggering the high-temperature limit. In areas with water hardness above 11 grains per gallon (approximately 180 mg/L), scale accumulates rapidly. Descaling every 1-2 years prevents Error 16. If the unit has never been descaled and is more than 2 years old in a hard water area, descaling should be the first intervention before replacing any component.

2. 2

   Error 11 — No Ignition on Startup

   Error 11 means the burner did not ignite within the startup sequence window. Common causes: gas supply interruption or low pressure, clogged inlet filter screen reducing flow below the minimum activation threshold, venting obstruction causing flue gas recirculation, or a dirty/failed igniter or flame sensor. Start with the basics: confirm gas is on (check other gas appliances), clean the inlet filter screen, check venting termination. Then inspect the igniter for carbon deposits and test spark with the unit cover open (observe for spark across the igniter gap). Clean the flame sensing rod with fine emery cloth if present.

3. 3

   Error 12 — Flame Failure During Operation

   Error 12 indicates flame was established and then lost during a heating cycle. Common causes: gas pressure fluctuation under high demand, venting blockage causing oxygen depletion during operation, scale buildup creating incomplete combustion, or a failing gas valve. On indoor units with PVC venting, inspect the exhaust and intake runs for sagging horizontal sections that pool condensate. On outdoor units, check that wind deflectors or hoods are properly installed and the termination is not facing into prevailing winds.

4. 4

   Error 14 — Thermal Fuse

   Error 14 indicates the thermal fuse has blown. The thermal fuse is a one-time safety device located on the heat exchanger that opens permanently if the exchanger reaches a critical overheat temperature. The thermal fuse itself is a $5-$15 part, but a blown thermal fuse is a symptom, not the root cause — in most cases it blew because of scale-related overheating (same cause as Error 16). Replace the thermal fuse and descale the heat exchanger. If the fuse blows again after descaling, the heat exchanger may be damaged and require replacement.

5. 5

   Error 20 — High-Limit Switch Tripped

   Error 20 indicates the manual reset high-limit safety switch has tripped. This is a resettable bimetal thermal switch that opens when the heat exchanger outlet exceeds approximately 212 degrees F. Causes are similar to Error 16: scale buildup, restricted water flow, or a failed thermostat allowing runaway heating. After the unit cools, the high-limit switch can be reset by pressing a small button typically accessible through an access hole in the front cover. Reset, then run a heat cycle — if Error 20 returns within the cycle, descale and check water flow rate.

6. 6

   Error 29 — Condensate Neutralizer Blocked (Condensing Models)

   Error 29 appears on Noritz condensing models (NRC98, NRC1111, EZTR75 and similar) and indicates the condensate neutralizer is blocked or the condensate cannot drain. Similar to Navien E439, the neutralizer pellets must be replaced annually. Remove the neutralizer cartridge (bottom of the unit), inspect and replace if the pellets are compacted. Clear the condensate drain line with a wet-dry vacuum and flush with dilute bleach.

7. 7

   Error 59 — PCB Control Board Fault

   Error 59 indicates a fault on the control PCB. As with other brands, attempt a 5-minute power cycle before condemning the board — transient faults sometimes clear with a full reset. Inspect all wiring harness connectors for corrosion, bent pins, or loose connections. If the error persists after power cycle and connector inspection, professional evaluation is warranted. PCB replacement ($150-$350 depending on model) versus unit replacement is a cost-of-ownership decision on units over 10 years old.

8. 8

   Error 61 — Combustion Fan Fault

   Error 61 means the combustion fan did not reach target RPM. Causes: debris in the fan assembly, a failing bearing (listen for rattling or grinding before full failure), or a failed motor. With gas and power off and the cover removed, spin the fan blade by hand — it should spin freely. Use a multimeter to check fan motor winding resistance. On Noritz units the combustion fan is an induction motor on most models — test for an open winding (OL reading) or short to ground (near-zero ohms to ground). A failed fan motor requires replacement.

9. 9

   Error 65 — Water Flow Sensor Fault

   Error 65 points to the water flow sensor (turbine-type sensor on the cold water inlet). This sensor triggers unit activation and measures flow rate for burner modulation. Scale, debris, or a stuck/corroded turbine paddle causes Error 65. Descaling the unit and cleaning the sensor housing often resolves this code. If the sensor paddle is broken or the magnetic pickup has failed, the sensor assembly requires replacement.

10. 10

    Error 90 / 91 / 92 — Combustion Air Issues

    Errors 90, 91, and 92 indicate problems with the air-fuel mixture during combustion — detected by the combustion analysis circuitry or exhaust sensors. Error 90 typically indicates an ignition trial failure related to air/gas ratio; Error 91 a partial blockage of combustion air intake; Error 92 an exhaust recirculation condition (flue gases entering the air intake). Check venting at both termination points and throughout the interior run. On outdoor installations, verify that adjacent landscaping or structures have not grown to create an exhaust recirculation path between the exhaust and intake terminations.

Quick DIY Checks

1. 1Step 1 — Descale the heat exchanger if Error 16 or performance has degraded: if the unit is more than 2 years old, especially in a hard water area (greater than 11 grains per gallon), descaling is the first step before any other diagnosis. Purchase a descaling pump kit. Locate the cold and hot water service ports on the plumbing connections — Noritz NRC and EZTR series have 3/4-inch male threaded service ports. Connect the descaling pump hoses to the service ports, with the pump drawing from a 5-gallon bucket of citric acid solution (2 pounds citric acid per gallon of water) and discharging back to the bucket. Run the pump for 45-60 minutes, then rinse with fresh water for 10 minutes. This single procedure resolves Error 16, improves hot water flow rate, and extends heat exchanger life.
2. 2Step 2 — Check venting clearances and inspect termination: locate the exterior venting termination. Noritz recommends a minimum clearance of 12 inches from the ground (for snow accumulation), 12 inches from an opening into the building, and the exhaust must not recirculate back to the intake. Measure the clearance between exhaust termination and intake termination — minimum 6 inches separation on concentric fittings, 12 inches on separate fittings. Check that no adjacent landscaping, fencing, or added structures have altered the clearances since installation. A recirculation path between exhaust and intake causes persistent Error 12 and Error 90-92.
3. 3Step 3 — Test the igniter spark and inspect the flame sensing rod: turn off gas at the shut-off valve. Remove the unit cover (typically 4-6 Phillips screws). Locate the igniter assembly — the spark electrode protrudes into the burner area. With the unit powered on (gas still off), press the On button and watch for a spark at the igniter tip while listening for the clicking sound of the high-voltage igniter transformer. A strong visible blue spark should occur. No spark indicates a failed igniter transformer or PCB driver. Also locate the flame sensing rod — a separate metal rod adjacent to the igniter. Clean the rod surface with fine emery cloth to remove oxidation, which is a common cause of Error 11.

1. 4Step 4 — Check the high-limit reset for Error 20: locate the front access panel — on most Noritz units there is a small knockout hole or labeled access point for the high-limit reset button. The high-limit switch requires manual reset after tripping — press and hold the button for 2-3 seconds until you feel a click or hear it reset. Restore power and run a heat cycle. If Error 20 returns, the root cause (scale buildup or restricted flow) has not been addressed. Proceed to descaling and flow rate verification before resetting again.
2. 5Step 5 — Clean the inlet filter screen and verify water flow: turn off the cold water supply valve. Open a hot tap to relieve pressure. Disconnect the cold water inlet fitting at the unit — there is a brass inline filter screen in the inlet. Remove, inspect, and clean under running water with a soft brush. Reinstall and restore water supply. Then verify the water pressure and flow rate: open the hot tap fully and check that flow is strong (minimum 0.5 GPM for ignition, typically 1-2 GPM for comfortable shower flow). Weak flow from all hot taps indicates undersized supply piping or a partially closed main valve. Weak flow from a single tap indicates a restricted fixture aerator or valve.
3. 6Step 6 — Verify gas supply pressure and check for Error 90-92 combustion issues: for Error 90, 91, or 92, confirm gas supply pressure is within Noritz specifications (natural gas: 4.0-10.5 inches WC; LP: 8.0-14.0 inches WC). Low supply pressure causes incomplete combustion and triggers combustion monitoring faults. If you have a manometer, check static pressure at the unit gas valve inlet test port. Also verify the combustion air intake is completely clear — pull any access panels and visually confirm the air intake path is free of debris, rodent nests, or insulation that may have fallen into the intake path.

Repair vs Replace

Recommended Tools & Parts

• Descaling Pump Kit with Citric Acid (Noritz Compatible)

  Submersible pump descaling kit for Noritz NRC and EZTR series tankless water heaters. Circulates citric acid solution through the heat exchanger via the service ports. Resolves and prevents Error 16 (overheating from scale). Includes pump, hoses, citric acid, and service port adapter fittings.

  $40-$80

  Buy on Amazon →

• Noritz Thermal Fuse Replacement

  Replacement thermal fuse for Noritz tankless water heaters. One-time safety device that blows if the heat exchanger reaches critical temperature. Always descale the heat exchanger after replacing the thermal fuse to address the root cause.

  $5-$20

  Buy on Amazon →

• Noritz Water Flow Sensor

  Replacement water flow sensor (turbine-type) for Noritz NRC and EZTR series. Resolves Error 65. Confirm model number compatibility before ordering.

  $30-$80

  Buy on Amazon →

• White Vinegar 1-Gallon (for light descaling)

  Food-grade white vinegar as an alternative to citric acid for light descaling. Less aggressive than citric acid — suitable for mild scale buildup or annual maintenance descaling. Use 1-2 gallons undiluted. For heavy scale, use citric acid concentrate.

  $5-$10

  Buy on Amazon →

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

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