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Navien · Boiler

E291 Return temperature higher than supply

Medium severity Navien NHB-HNavien NFB-H

What the control is telling you

Return water is hotter than supply water. Manual action: check for reversed piping.

Applies to: Navien NHB-H (NHB-055H/080H/110H/150H) and NFB-H fire-tube (NFB-175H/200H) condensing boilers

Likely causes

  • Supply and return connections piped backwards
  • Pump pumping in the wrong direction
  • Reverse flow from another circuit (missing check valve)

Test and repair steps

  1. Verify the supply and return pipes are connected to the correct boiler connections.
  2. Check pump flow direction and that check valves are installed at pump outlets.
  3. Compare Supply Temp and Return Temp in Status Info while firing.
  4. Check for reverse flow during pump off-cycles from other pumps in the system.

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Source: Navien NHB-H Installation and Operation Manual, 11.14 Understanding Error Codes, p.119; Navien NFB-H Installation Manual, 11.7 Understanding Error Codes, p.116; NHB-H User's Information Manual (V1.0 Apr 2024), 5.2, p.24. Summarized in our own words; check the original manual for your model.

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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.