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Noritz · Tankless water heater

90 Combustion abnormality

High severity Noritz EZNoritz NRCRNoritz CDVNoritz NRC/NRCP

What the control is telling you

On EZ/NRCR/CDV (premix): the unit cannot reach set temperature, most often from restricted intake/exhaust, wrong DIP/vent settings, a clogged condensate drain, low gas pressure, or on older units a dirty fan/venturi. On NRC/NRCP (non-premix): two main causes, not reaching set temperature and/or the fan speed over-correcting (MM49 above 100) because of air restriction. Owner's guide shows 901 and 902 with the same meaning.

Applies to: All Noritz residential tankless (EZ, NRCR, NCC199CDV, NRC/NRCP)

Likely causes

  • Blocked or restricted intake or exhaust (lint, debris, animals, bad termination)
  • Wrong DIP switch / vent settings
  • Clogged or frozen condensate drain line (premix)
  • Insufficient inlet gas pressure
  • Dirty fan or venturi (older installs)
  • Bypass servo stuck (outlet temp low but HEX temp high)

Test and repair steps

  1. Check the intake and exhaust terminations, screens, and any blockage (lint, nests, animals); fix wrong or restricted venting.
  2. Indoor air supply look for lint, dust or oil (laundry rooms) at the air supply; clean the intake filter.
  3. Measure inlet gas pressure under full demand. Premix (EZ, NRCR, NCC199CDV) inlet gas: NG 3.5-10.5 in. WC, LP 8-14 in. WC. NRC/NRCP inlet gas: up to 199k Btu/h NG 4-10.5 in. WC, LP 8-14 in. WC; over 199k Btu/h NG 5-10.5 in. WC, LP 10.5-14 in. WC.
  4. Premix (EZ/NRCR/CDV) check the condensate drain is not clogged or frozen.
  5. Premix verify vent type/length and other settings with the PROCard app (DIP switches) or EZ Start Plus app / display (Pro models).
  6. Read MM31 (outlet) and MM32 (heat exchanger): MM32 high but MM31 low = bypass servo problem; both low = burner not heating enough, usually not enough gas.
  7. NRC/NRCP read MM49 fan speed with the front cover ON; normal is 100, above 100 means intake or exhaust restriction.
  8. Older installs clean the fan (air compressor, vacuum or plastic brush) and venturi; vacuum the burner, never blow it out (see guide 'Cleaning the fan and burner - Noritz EZ/NRCR/CDV').

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Source: Noritz EZ/NRCR/CDV Level 2 Maintenance & Troubleshooting (Premix R2 V2), 90 error code and MM31/MM32, p.16-17, 20-21; Noritz Residential Level 2 Maintenance & Troubleshooting (NRC/NRCP and older, R2 V1), 90 error code and MM49, p.10-11, 15; Noritz NRCR Pro training (NRCR92/NRCR111, Printable V3), Gas line conditions, p.20; Noritz EZ/NRCR Owner's Guide (Rev. 10-25), Checking for error conditions, p.31. Summarized in our own words; check the original manual for your model.

Homeowner looking at this code? See what it means for you and when to call a technician.

For licensed technicians. Gas, electrical and refrigerant work must be done by a qualified, licensed person. Shut off power and gas before servicing, never bypass a safety control, and follow the manufacturer’s manual for the exact model; it and local codes always take priority. HVACopedia is not affiliated with any manufacturer; brand and model names only identify the equipment.