E291 Return temperature higher than supply

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.

Settings for these units

Installation clearances (indoor) · Navien NHB-H

Bracket gives 5/8 in (16 mm) from the wall; studs at 16 in on centre.

Common vent installs: 36 in above the top and 24 in in front are recommended for service.

Installation clearances (indoor) · Navien NFB-H

Do not install on carpet; indoors only.

Remove the wood pallet and refit the 6 screws to keep the cabinet airtight.

Models that show this code

6 models in Navien NHB-H, Navien NFB-H

Other codes on the same units

• E205 Heating supply temperature sensor
• E218 Heating return temperature sensor
• E278 External system supply sensor not detected
• E279 External system return sensor not detected
• E302 Low water pressure
• E352 High water pressure
• E353 Water pressure sensor fault
• E480 DHW tank sensor not detected