E352 High water pressure

What the control is telling you

System pressure is above the high limit. Manual action: check water pressure.

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

Likely causes

• Expansion tank waterlogged, undersized or isolated
• Fill/pressure reducing valve letting by
• System overfilled

Test and repair steps

1. Read Water Press in Status Info and compare with the system gauge.
2. Check the expansion tank charge and size and that it cannot be isolated from the boiler.
3. Check the pressure reducing / make-up valve is not over-pressurizing the system.
4. NHB-H system pressure 12-30 psi with a 30 psi relief valve. NFB-H: 12-80 psi; parameter 17 High Sys Pressure (40-80 psi, default 50 psi) should suit the relief valve rating.
5. Drain to the correct pressure and retest.

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

• E278 External system supply sensor not detected
• E279 External system return sensor not detected
• E291 Return temperature higher than supply
• E302 Low water pressure
• E353 Water pressure sensor fault
• E480 DHW tank sensor not detected
• E515 PCB fault
• E517 DIP switch setting fault