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.

In my years of service calls, I usually see E773 pop up during peak usage when the unit is running at max capacity. It is the system's way of preventing the PVC exhaust pipes from literally melting. While other codes track air pressure, this one is strictly about dangerous heat levels. I often find it is caused by a bird nest or an installer who exceeded the maximum allowed vent length, causing heat to back up into the unit.

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?

Think of E773 as a fever for your Navien. This safety lockout triggers when the exhaust sensor detects temperatures that could damage the internal components or the plastic venting. I have found that nine times out of ten, the unit is either recycling its own hot air or the heat exchanger is so scaled up that it cannot transfer heat to the water properly.

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?

You cannot rush this reset because the thermal sensor needs time to physically cool down. Give the unit at least 45 minutes of downtime. Once the pipes feel cool to the touch, press the Info button three times to clear the code. If the error returns during the very next heating cycle, you likely have a physical vent restriction that a reset cannot fix.

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.

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

Frequently Asked Questions

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

Think of E110 as a breathing problem and E773 as a fever. E110 triggers when the pressure switch feels that air isn't moving at all, usually from a total blockage. E773 triggers when air is moving, but it is way too hot. In my experience, a partially blocked vent is the most common bridge between the two. The unit might pass the pressure test for E110 but fail the temperature test for E773 as heat builds up over several minutes of operation.

Why must I wait before resetting E773?

The sensor inside the exhaust path is a physical thermal switch. If you try to reset the computer while that switch is still physically expanded from the heat, the code will just pop right back up. It is like trying to reset a tripped breaker that is still glowing hot. Giving it 30 to 45 minutes allows the metal components to contract and the safety circuit to close, allowing a successful restart.

Can E773 damage my Navien if it happens repeatedly?

Absolutely. I have seen PVC vent pipes turn yellow and become brittle from repeated E773 events. Beyond the pipes, the constant overheating stresses the heat exchanger's metal, which can lead to pinhole leaks or cracks over time. If you keep resetting this code without fixing the airflow, you are essentially shortening the life of your water heater by several years and risking a dangerous exhaust leak in your home.

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

You need at least 12 inches of clearance from any wall or overhang. If the exhaust is tucked into a corner or under a deck, the hot air will bounce back and get sucked into the intake. This is called recirculation. In the field, I often see installers put the pipes too close together or too close to an inside corner, which is a guaranteed recipe for an E773 code during the winter months.

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

Closet installs are tricky. If the unit is pulling combustion air from the closet rather than from outside, it will quickly overheat the small space and the unit itself. These are high-efficiency condensing units that really should be direct-vented, meaning both pipes go to the outdoors. If your unit is missing the intake pipe and is just sucking in closet air, you need a technician to run a dedicated intake line to resolve the overheating 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

Written by

Raj Patel

HVAC & Water Systems Specialist · 15 years experience

Last verified for technical accuracy on March 14, 2026