90
Noritz Tankless Water Heater
Severity: CriticalWhat it means
Noritz code 90 is a combustion abnormality — the unit watched how the burner was running and decided it wasn't burning cleanly or correctly, so it shut down.
The common causes are venting problems (a restriction, a vent run too long, exhaust being re-drawn into the intake), a burner or heat exchanger fouled with soot, or gas pressure that's wrong for the unit.
Because bad combustion is also a carbon-monoxide concern, code 90 is one to take seriously rather than just reset past.
Affected Models
- Noritz NRC66, NRC83, NRC98, NRC111
- Noritz NRCP series (with built-in pump)
- Noritz EZ98, EZ111
- Noritz NR66, NR83, NR98
- Noritz GQ-C series
Common Causes
- Vent or intake restricted, run too long, or terminals mounted so exhaust gets re-drawn
- Burner or heat exchanger fouled with soot or lint
- Gas supply pressure too high or too low
- Unit set up for the wrong gas (natural gas vs propane) or converted incorrectly
- Combustion fan underperforming, so the air-to-gas ratio is off
- Air pressure sensor or its hose faulty
How to Fix It
-
Check the vent and intake first.
Outside: clear the terminals of nests, lint, leaves, snow, ice, and make sure intake and exhaust caps are the spacing the manual requires so exhaust isn't being pulled back in.
Inside, breaker off: clean the unit's air inlet filter.
Bad venting is the most common driver of code 90. -
Reset and see if it returns.
Power off, ten seconds, power on, run a hot tap.
If you fixed a venting restriction, code 90 should be gone.
If it comes back, the combustion problem is deeper — burner, gas, or fan — and that's tech territory. -
Look for soot.
Breaker off, cover off.
Black soot on the burner, in the heat exchanger, or around the vent connection means combustion has been running dirty.
Soot both causes code 90 and is a CO red flag — at this point bring in a gas technician with a combustion analyzer rather than carrying on yourself. -
Confirm the gas type and pressure.
Make sure the unit is set for the gas you actually have — a natural-gas unit running on propane (or a botched conversion) burns badly and trips code 90.
Have a tech put a manometer on the inlet and check pressure under load; it has to sit within Noritz's spec. -
Have the fan and air sensor checked.
If venting, burner and gas all check out, the air side is next: a weak combustion fan or a faulty air-pressure sensor throws off the air-to-gas ratio enough to register as abnormal combustion.
That's a Noritz service-tech diagnosis and parts swap — don't keep resetting in the meantime.
Frequently Asked Questions
Is code 90 dangerous — should I keep using the water heater?
Treat it cautiously.
Code 90 means combustion isn't right, and incomplete combustion of gas can produce carbon monoxide.
The unit shutting itself down is the safety system working as intended — so don't defeat it by resetting repeatedly to limp along.
Clear an obvious vent blockage and retest once; if code 90 comes back, leave it off and get a qualified gas technician to look at it, and make sure you have a working CO alarm nearby.