Quick answer
Navien error code E110 means abnormal air pressure: the heater senses a restriction in its exhaust or intake air path. Do not keep running or resetting it; the lockout exists to stop flue gas spilling indoors. Check that the outside vent pipes are free of snow, ice or nests and clean the intake air filter. If the terminations are clear and E110 persists, call a gas technician.
Snow against the side of the house is the first thing to think of when a Navien shows E110. Navien error code E110 means abnormal air pressure: the unit has sensed a restriction somewhere between the air intake and the end of the exhaust pipe. With the air path choked, the fan cannot move combustion air in and flue gas out, so the control locks the burner out. The definition is from the NPE-2 manual; if the label inside your model's panel reads differently, follow the label.
Is it safe to run a Navien with E110?
No. E110 is a venting fault, and the lockout exists to stop flue gas spilling into the house instead of leaving through the pipe. Never bypass, jumper or disconnect the air pressure sensor to get hot water back; that removes the only thing standing between a blocked vent and a running burner.
Ontario's Hawkins-Gignac Act requires working carbon monoxide alarms in homes with a fuel-burning appliance. If yours sounds, get everyone outside and call 911.
Check the exhaust pipe and clean the intake air filter
Both steps are the manual's listed user actions for E110, and in winter the first one solves most cases.
- Find the vent terminations outside, usually two plastic pipes or one concentric fitting on a side wall.
- Clear snow drifts, ice build-up, leaves and anything nesting in the openings. Remove ice by hand or with warm water, never a hammer; cracked pipe is a bigger repair.
- Dig the snow back so there is open air around and below the pipes, not a tunnel.
- Indoors, turn the heater off, unplug it, and clean the intake air filter as your manual describes. Let it dry before it goes back in.
- Power up and try one hot tap. If the code clears, look at the terminations again after the next snowfall.
Wall terminations in the narrow side yards of 1970s–90s subdivisions and newer builds are the ones that drift over first. The exhaust from a condensing heater is also wet enough to build its own ice at the outlet in a cold snap. The furnace vent beside it deserves the same look, and what to do if a furnace vent is blocked by snow covers that one.
What restricts the air path, most likely first
- Snow, ice or a nest at the exhaust or intake termination. Birds, wasps and mice all use open pipe ends in the off-season.
- A dirty intake air filter that the fan can no longer pull enough air through.
- Condensate backed up in the unit. A condensing heater drains acidic water through a trap and a drain line; if that line is kinked, plugged or frozen, water rises into the exhaust path and blocks it.
- A sagging or separated vent run. Horizontal vent pipe is meant to slope so condensate drains, and a sag holds a pool of water that the fan has to push against.
What happens on the service call if the terminations are clear
When the pipes outside are open and E110 persists, the restriction is somewhere you cannot see. The technician checks the condensate trap and drain line, walks the vent run for sags, missing hangers and joints that have come apart, and inspects the fan and the air-pressure sensor's connections. The sensor itself is judged last, because a sensor reporting a real blockage is doing its job.
Vent repairs on a gas water heater are made with ULC S636-certified pipe and are work for a TSSA-licensed gas technician under CSA B149.1. A diagnostic visit for a venting lockout is $90–$150, credited to the repair. Most fixes fall in the $150–$800 range, and re-running a section of vent is quoted in writing because it depends on length and access.
A heater that froze during the same cold snap is a different problem, covered in what to do if a tankless water heater is frozen, and the rest of the codes are in the Navien error code guide. If E110 will not clear, leave the unit off and book tankless water heater repair with ZK Mechanical; we cover Burlington, Oakville, Milton, Hamilton and Georgetown. The line is (647) 801-1252, or send a message with a photo of the vent terminations.