E016 Heat exchanger overheat (check flow / system filters)

What the control is telling you

Heat exchanger temperature too high. Manual action: check flow and clean system filters.

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

Likely causes

• Low water flow
• Dirty system filter or strainer
• Air in system
• Scaled heat exchanger

Test and repair steps

1. Warning: If overheating occurs, close the manual gas valve but keep the circulator powered.
2. Confirm the boiler pump runs (Test Mode > Boiler Pump) and that all isolation valves are fully open; the manual requires full-port ball valves.
3. Check the flow against the pump sizing table: e.g. at 20°F ΔT NHB-055H needs 5 GPM @ 7.2 ft, NHB-150H 13.5 GPM @ 19.8 ft; NFB-175H 16 GPM @ 4.2 ft, NFB-200H 18 GPM @ 5.3 ft (see Specs). With glycol use the corrected glycol values.
4. NFB-H only flow through the boiler must stay between the minimum 7.0/8.0 GPM and maximum 23.1/26.4 GPM (175H/200H).
5. Purge air from the boiler and system (Air Purge Mode, air vent cap fully removed during fill) and check the air separator.
6. Clean the strainer/filter in the system return; debris restricts flow.
7. Primary/secondary piping closely spaced tees no more than 4 pipe diameters or 12 in apart.
8. Compare supply and return temperatures in Status Info; a large rise points to low flow. Check Service Status Info for the HEX overheat count.
9. If scale is suspected, check water quality (hardness up to 200 mg/l, pH 6.5-8.5) and flush the heat exchanger per the User's Information Manual.

Settings for these units

• Installation clearances (indoor) · Navien NHB-H
• Installation clearances (indoor) · Navien NFB-H