E001 Heat exchanger overheat

What the control is telling you

The heat exchanger got too hot and the boiler stopped firing. The manual's first action is to check for proper system flow.

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

Likely causes

• Low or no water flow through the boiler (pump off, undersized or air-locked)
• Closed or partly closed valves, non-full-port valves
• Clogged system return filter/strainer
• Air in the system
• Scale or sludge in the heat exchanger

Test and repair steps

1. Warning: If the boiler overheats or the gas will not shut off, close the manual gas valve; do not cut power to the circulator.
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