Navien E773 Error Code: Exhaust Overheat Thermal Limit

Quick Answer

Navien E773 is an exhaust overheat fault, distinct from E110 which is a pressure-based venting fault. While both involve the exhaust system, E773 is thermally triggered: the exhaust gases are running physically too hot. The most common reason is a vent restriction that forces hot combustion gases to recirculate or linger in the exhaust path rather than leaving quickly. Recirculation of exhaust gases, improper vent length, and a partially blocked heat exchanger are the three main causes. E773 requires the unit to cool down before a reset will hold.

NavienWaterheaterSeverity: highDifficulty: intermediate71% DIY Success

Time to Fix

30–90 min

Difficulty

intermediate

Parts Cost

$15 – $55

Tools Needed

Flashlight, Tape measure (for vent equivalent length calculation)

What Does the E773 Code Mean?

E773 means exhaust gas temperature exceeded the thermal safety limit. Unlike E110 (pressure-based), E773 is triggered by actual heat buildup in the exhaust path. Check for vent restriction, exhaust recirculation at the termination, and heat exchanger blockage. The unit must cool before a reset will hold.

Most Likely Causes

Based on aggregated repair data, here is the probability breakdown for this error code:

Vent restriction or blockage slowing exhaust gas exit and causing heat buildup40%

Exhaust gas recirculation at the termination cap (exhaust re-entering the air intake)24%

Scale-blocked heat exchanger unable to absorb heat properly22%

Improper vent length or too many elbows increasing back-pressure14%

Symptoms You May Notice

E773 on controller display after a heating cycle
Unit fires and operates for a period then shuts down with E773
Exhaust vent pipes feeling unusually hot to the touch
Unit may simultaneously show elevated exhaust temperature readings in diagnostics mode
E773 following a previous E110 venting event

Can you reset a Navien waterheater to clear the E773 code?

Do not attempt to reset E773 until the unit has cooled for at least 30-45 minutes. The thermal high-limit device requires actual temperature reduction before it will reset. After cooling, press the Info button 3 times for a soft reset. If E773 returns within the first few minutes of operation, the root vent restriction or heat exchanger issue has not been resolved. Do not continue resetting without addressing the underlying cause.

Tools Required for Diagnosis

FlashlightTape measure (for vent equivalent length calculation)Gloves and safety glassesScrewdriver setThermometer (to monitor exhaust temp if unit has a test port)

Diagnostic Checklist

Follow these steps in order. We start with the easiest external fixes before opening up the machine.

Allow the unit to cool completely before any inspection

Allow the unit to cool completely before any inspection. This typically means 30-45 minutes of no operation. After cooling, perform a soft reset (Info button x3). If E773 returns within 5 minutes of operation after a proper cooldown, the vent system has an active restriction.

Inspect the outdoor vent termination

Trace the full vent pipe run from the unit to the termination

Check the termination cap for any physical restriction: bird nest

If all vent conditions check out

Replacement Parts

If your diagnostic testing proves the component has failed, you will need a replacement. We recommend OEM parts over aftermarket for water-handling components.

Part Name	OEM Number	Estimated Price
Vent Termination Cap (concentric or dual-pipe)BH-2T or equivalent (check vent brand) · $20–$55	BH-2T or equivalent (check vent brand)	$20 – $55
Exhaust Thermal Fuse / High Limit30010618A (varies by model) · $15–$35	30010618A (varies by model)	$15 – $35
Condensate Trap Kit30010723A · $20–$35	30010723A	$20 – $35

Verify part fitment for your Navien model

Repair vs. Replace Calculator

Appliance Age (Years)7 Yrs

Labor Approach

Our Verdict

REPAIR

Estimated Cost: $35 (Parts) vs $1100 (New Machine)

Frequently Asked Questions

What is the difference between E110 and E773 on a Navien?

E110 is triggered by a pressure switch detecting inadequate airflow through the exhaust/intake path. E773 is triggered by a temperature sensor detecting that exhaust gases are physically too hot. A fully blocked vent will trigger E110 because pressure backs up. A partially restricted vent may allow enough flow to satisfy the pressure switch (no E110) but still slow the exhaust enough to cause heat buildup that eventually trips E773. E773 can also occur on a physically clear vent if exhaust gases recirculate back into the intake, continuously heating the combustion air and driving exhaust temperatures up.

Why must I wait before resetting E773?

The thermal high-limit device in the exhaust path is heat-activated. If the exhaust temperature is still above the reset threshold, pressing Info 3 times will appear to clear the error, but the lockout condition is still physically present and E773 will reappear within seconds of the next firing attempt. Waiting 30-45 minutes for the exhaust components to return to ambient temperature allows the thermal device to reset properly so the next firing cycle runs clean.

Can E773 damage my Navien if it happens repeatedly?

Yes, repeated overheat events stress the heat exchanger, solder joints, and plastic vent components. PVC vent pipe that repeatedly reaches very high temperatures can soften, warp, or eventually crack. Repeated thermal cycling also degrades condensate trap gaskets and can cause micro-cracking in the heat exchanger. E773 should not be treated as a nuisance code to reset and ignore; it requires resolution.

How far from a wall does the Navien exhaust need to be?

Navien requires a minimum 12-inch clearance between the exhaust outlet and any adjacent wall, window, door, or obstruction. The exhaust must also be a minimum of 12 inches from the air intake port on the same unit. Local codes may require larger clearances. Always refer to the installation manual for your specific model, as clearance requirements vary slightly between NPE-A and NPE-2 series.

My Navien was installed in a closet and keeps getting E773. Can I fix this?

An enclosed closet installation can cause E773 if the room is sealed and combustion air is depleted, or if exhaust gases leak back into the room and contaminate the combustion air supply. Navien condensing units require a direct vent (sealed combustion) installation where combustion air comes from outdoors through a dedicated air intake pipe, not from the room. If the unit is pulling room air and exhausting into the same space, the installation is incorrect and must be corrected by a licensed installer to resolve E773 permanently.

Related Navien Waterheater Error Codes

Code: HUBNavien Tankless Water Heater Error Codes: Complete NPE and NCB Series Guide Code: E110Navien E110 Error Code: Exhaust Blockage and Venting Fault Explained Code: RESETHow to Reset a Navien Tankless Water Heater and Clear Error Codes

Models Known to Experience E773 Errors

This repair applies to most Navien waterheaters with this error code. Common model numbers include:

NPE-240A, NPE-180A, NPE-210A, NCB-240E, NPN-199A

Last verified for technical accuracy on March 14, 2026