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Navien · Combi boiler

E773 Improper gas type detected

Critical severity Navien NCB-HNavien NFC-H

What the control is telling you

The boiler detected a mismatch between the gas type set at install and the gas actually connected. The manual notes detection takes time and may also trigger from measuring tolerance with the correct gas. NFC-H manual lists the reset type as: Parameter reset.

Applies to: Navien fire-tube combi boilers, NFC-H series (NFC-250/200H, NFC-250/175H) and NCB-H combi boilers (NCB-250/150H, NCB-240/130H, NCB-240/110H, NCB-190/080H, NCB-190/060H)

Likely causes

  • Gas type DIP switch (SW2 #1) not matching the supply
  • Conversion kit / orifice not installed or wrong kit
  • Measurement tolerance (false trip)

Test and repair steps

  1. Warning: wrong gas causes abnormal combustion - stop and verify before running.
  2. Check the rating plate and DIP SW2 #1: OFF for natural gas, ON for propane.
  3. Confirm the correct orifice for gas and altitude is installed (see the orifice table).
  4. Verify CO2 at low and high fire against the model's CO2 table.
  5. Setting matches the gas but code won't clear? Phone Navien Technical Support, 1-800-519-8794.

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Source: Navien NCB-H Installation & Operation Manual, 11.7 Understanding Error Codes, p.129-130; Navien NFC-H Installation Manual, 11.7 Understanding Error Codes, p.127-128. 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.