HVACopediaNoritz · Tankless water heater
90 Combustion abnormality
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
- Check the intake and exhaust terminations, screens, and any blockage (lint, nests, animals); fix wrong or restricted venting.
- Indoor air supply look for lint, dust or oil (laundry rooms) at the air supply; clean the intake filter.
- 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.
- Premix (EZ/NRCR/CDV) check the condensate drain is not clogged or frozen.
- Premix verify vent type/length and other settings with the PROCard app (DIP switches) or EZ Start Plus app / display (Pro models).
- 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.
- NRC/NRCP read MM49 fan speed with the front cover ON; normal is 100, above 100 means intake or exhaust restriction.
- 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').
Settings for these units
Indoor clearances (required and recommended) · Noritz NRCR
| Location | Required | Recommended (service) |
|---|---|---|
| Top | 12 in. (300 mm) | 3 in. (76 mm) above and below exhaust |
| Front | 4 in. (100 mm) | 24 in. (610 mm) |
| Sides | 3 in. (76 mm) | 8 in. (203 mm) |
Required clearances apply to both combustible and non-combustible materials.
Recommended space is for service and quick access to gas and water shut-offs.
Operation panel menu settings and defaults · Noritz EZ, Noritz NRCR
| Setting | Options | Default |
|---|---|---|
| Max Set Temperature (power off) | 100-140 F in 5 F steps; 37-48 C, 50-60 C | 120 F / 50 C |
| Recirc ON/OFF | ON / OFF | OFF |
| Recirc Mode | Always OFF, Always ON, Manual Timer, Auto Learning (NRCR) | Always OFF |
| Vacation Mode | Active / Inactive | Inactive |
| Flow Meter Alarm | 10-60 gal (5 gal steps), 70-100 gal (10 gal steps), OFF | Alarm OFF |
| Drain Water (power off) | Start / Cancel | Cancel |
| Scale Flushing (power off) | Start / Cancel | Cancel |
| Audible Alarm | ON / OFF | ON |
| Screen Lock | Active / Inactive (4-digit password) | Inactive |
| Units (first 10 min after power-up, power off) | [F/gal/ft] or [C/L/m] | [F/gal/ft] |
| Power Saving Dis. / Backlight | ON-OFF / Power Save ON-OFF | ON / Power Save ON |
| Brightness / Contrast / Touch Sound | Bright-Normal-Dim-Dark / -15 to +15 / ON-OFF | Normal / Standard / ON |
Setting 125 F (55 C) or higher needs Max Set Temperature raised first; the panel warns about scald risk.
Default Settings resets everything in Settings except Screen Lock (hold OK about 5 s).
- Water quality limits · Noritz EZ, Noritz NRCR
Models that show this code
12 models in Noritz EZ, Noritz NRCR, Noritz CDV, Noritz NRC/NRCP
Other codes on the same units
- 63 Recirculation abnormality (no recirculation flow)
- 65 Main water control servo abnormality
- 66 Bypass water control servo abnormality
- 73 Circuit board setting abnormality
- 91 Gas conversion not done correctly
- 901 Combustion abnormality (sub-code of 90)
- 902 Combustion abnormality (sub-code of 90)
- C1# Descaling reminder (C11-C19)
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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.