Direct answer: Navien code E110 usually means the unit sees a blocked exhaust path or abnormal combustion airflow. The first things to check are the outdoor vent openings, any visible vent pipe sag or separation, and whether condensate is backing up into the vent path.
Most likely: Most of the time, this turns out to be a blocked intake or exhaust termination, debris or ice at the vent, or condensate not draining correctly rather than a bad internal part.
Treat E110 as a venting safety problem until proven otherwise. If the code showed up after wind, freezing weather, heavy rain, nearby construction dust, or a recent vent change, that is a strong clue. Reality check: a lot of E110 calls end with clearing a blockage outside. Common wrong move: resetting the heater over and over without checking the vent ends.
Don’t start with: Do not start by ordering a control part or opening the sealed combustion area. On a gas tankless heater, airflow and venting problems come first.
You open a faucet, the unit tries to fire, then quickly throws E110 and stops heating.
Start here: Start outside at the intake and exhaust terminations. A blockage at the vent ends is the fastest check and the most common one.
The heater may have worked fine before the weather changed, then began locking out with E110.
Start here: Look for ice, snow, wind-driven debris, or condensate drainage trouble that could be letting water collect in the vent path.
You get warm water for a short stretch, then the burner drops out and E110 appears.
Start here: Check for a partial vent restriction, sagging vent pipe, or a condensate line problem rather than assuming a full blockage.
Power cycling clears it briefly, but the next shower or sink use brings the code back.
Start here: That usually means the underlying airflow problem is still there. Move on to a careful vent and drain inspection instead of more resets.
This is the most common field find. Leaves, lint, insect nests, snow, or a packed screen can choke airflow enough to trigger E110.
Quick check: Inspect both outside vent openings with the unit off. Remove loose debris by hand and make sure the openings are fully clear.
If condensate cannot leave the unit or vent properly, water can collect where it should not and upset pressure readings and combustion airflow.
Quick check: Look for water drips where they do not belong, a kinked condensate tube, a full neutralizer if one is installed, or signs of moisture in the venting area.
A low spot can hold water, and a loose joint can disturb draft and pressure enough to trip the code.
Quick check: Follow every visible section of vent pipe you can safely reach. Look for belly, disconnected joints, staining, or sections that shifted.
If the venting is clear and intact, the unit may be reading airflow wrong because of a pressure switch issue, blocked sensing tube, or another internal combustion-side fault.
Quick check: Only consider this after outside vent checks and visible condensate checks come up clean. This is usually a service call, not a first DIY parts purchase.
E110 is most often caused by something simple and visible at the intake or exhaust termination, and this is the safest place to start.
Next move: Restore power and run hot water for several minutes. If the code stays gone, the blockage was likely the cause. Move to the condensate and visible vent path checks.
What to conclude: A quick return to normal after clearing the vent ends strongly points to restricted combustion air or exhaust flow.
Tankless units make condensate, and when that drainage path plugs or kinks, water can interfere with venting and pressure sensing.
Next move: Restart the heater and test a hot water fixture. If E110 stays away, the drainage issue was likely affecting airflow or pressure sensing. Keep going and inspect the visible vent pipe run for slope and joint problems.
What to conclude: Water in the wrong place around a condensing tankless heater often means the vent or condensate path is not draining the way it should.
A partial restriction is not always at the outside end. A low spot, separated joint, or crushed section can create the same code.
Next move: If you corrected a simple support issue without opening joints and the code clears, monitor the heater closely during the next few hot water calls. If the vent looks intact but E110 remains, the problem is likely beyond a basic homeowner check.
A single careful restart helps confirm whether the problem is still immediate and repeatable after the easy outside checks.
Next move: If the heater runs normally through several minutes of hot water, keep using it but recheck the vent area over the next day or two for recurring blockage. If E110 returns with clear vent ends and no obvious condensate or vent defect, schedule qualified service.
Once the easy external causes are ruled out, the remaining causes are higher risk and often involve gas combustion, pressure sensing, or hidden vent problems.
A good result: A proper vent correction or confirmed internal repair should clear the code without repeated resets.
If not: If the code persists even after professional vent and combustion checks, the unit needs brand-specific diagnostic service rather than guesswork.
What to conclude: At this point, buying parts blindly is more likely to waste money than solve the problem.
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You can do one restart after checking the vent ends, but repeated resets are not the fix. If the code comes back, the heater is still seeing a venting or airflow problem.
Not usually. The common causes are blocked vent terminations, condensate trouble, or vent pipe issues. A blower or pressure-sensing fault is farther down the list and should be confirmed before any parts are ordered.
Yes. Snow, ice, wind-driven debris, and condensate freezing or backing up can all trigger this code. That is why an outside vent check is one of the first steps.
No. Because the code relates to combustion airflow or exhaust movement, treat it as a safety-related shutdown. Use the heater only after the cause is found and corrected.
Tell them when the code started, whether it followed wind or freezing weather, whether clearing the vent ends changed anything, and whether you saw condensate leakage, vent sag, or loose joints. That saves time on the call.