Navien Water Heater Error E351 – Causes & Fix
Navien combi boilers: the auto make-up water valve couldn't restore heating pressure within 5 minutes.
Safety first · Consult a professional
If water is spraying or pooling near electrical parts, switch the boiler off at its power switch or breaker before you get close, then shut the make-up water supply. Remember it's still a gas appliance: with a gas smell, leave the house and call 911 or your gas utility from outside; with a CO alarm, get outside and call 911.
- Gas appliance: no DIY repairs. If you smell gas, leave the building, do not use switches and call your gas utility’s emergency line from outside.
- Carbon monoxide is odorless. Keep a CO alarm near fuel-burning appliances; if it sounds, get everyone outside and call 911.
- Water and electricity: unplug first, keep towels ready and never stand in water while the appliance is connected.
At a glance
Severity: Moderate
- Meaning
- Auto Feeder Can't Refill the Heating Loop
- Typical repair cost
- $0–$600
- DIY difficulty
- Hard
E351 is a Navien NCB combi-boiler code – it doesn't appear on NPE tankless water heaters. These boilers have a built-in auto feeder (make-up water) valve: when the internal pressure sensor sees the heating loop running low, the boiler opens the feeder to top it up. According to the NCB-H installation manual, if the refill isn't completed within 5 minutes the boiler shows E351 and needs a manual reset. In practice that means water is leaving the heating loop faster than it can be replaced, or the make-up water never reaches the feeder – a closed supply valve, a leak, a relief valve dumping water or a failed feeder valve. Finding and fixing the underlying water loss is work for a licensed plumber or hydronic heating technician.
How it looks on the display
Model families & display variants
| Model family | Shows | Note |
|---|---|---|
| NCB-E condensing combi boilers | E351 | Manual: abnormal auto feeder valve (make-up water). Pressure is readable in the INFO menu (press Mode three times). |
| NCB-H condensing combi boilers | E351 | Manual adds that the code appears when automatic refilling isn't complete within 5 minutes and requires a manual reset. |
| NaviLink app (NCB with Wi-Fi module) | E351 | The app reports the lockout remotely; the reset and the valve check still happen at the boiler. Don't confuse it with E352 (high pressure) or E353 (pressure sensor fault). |
Symptoms
- Radiators, baseboards or radiant floors stay cold and E351 is displayed
- Pressure shown in the boiler's information menu is well below the normal cold-fill level
- Water dripping from the relief valve discharge pipe or a wet spot under the boiler
- Code appeared right after plumbing work, a water shut-off or a filter change on the supply line
- The boiler has needed topping up more and more often before this lockout
Possible causes
Ranked by how often each cause is behind this code (editorial estimate).
- Make-up water supply valve to the boiler closed or partly closed, or its line strainer clogged
- Leak somewhere in the heating loop – baseboard, radiant manifold, fitting or inside the boiler
- Relief valve discharging because a waterlogged expansion tank lets pressure spike when the loop heats
- Auto feeder valve stuck or failed
- Low incoming water pressure or a restricted backflow preventer on the fill line
- Water pressure sensor reading incorrectly (often accompanied by E353)
Fix it step by step
Deal with water and gas hazards first
If you see water spraying, cut power to the boiler and close the make-up water valve before investigating. Any gas smell or CO alarm means leaving the building as described in the safety box.
Read the heating pressure
On NCB-E units press the Mode button three times until INFO appears, then step through with + / – to the space-heating water pressure in psi. Navien specifies a cold fill of at least 12 psi; a reading far below that confirms the loop is losing water.
Check the make-up water valve
Follow the small cold-water line that feeds the boiler's auto feeder inlet, usually past a backflow preventer. Make sure the shut-off on that line is fully open – plumbing work elsewhere in the house often leaves it closed.
Tools: Flashlight
Hunt for the water loss
Look at the end of the relief valve discharge pipe for drips or mineral trails, check under the boiler, along baseboard covers and at radiant manifolds. Put a towel or bucket under anything that's dripping and note where it is.
Tools: Flashlight, Towels & shallow pan
Reset once and watch the pressure
Press the front-panel reset button and watch the pressure in the INFO menu. If it rises and then slowly falls again, or the relief valve spits water when the boiler heats up, stop and call a pro.
Leave the hydronic side to a professional
Don't cap or plug the relief valve and don't try to force water into the loop with hoses. A licensed plumber or hydronic tech can check the expansion tank pre-charge, repair the leak, test the feeder valve and purge air properly.
How to reset E351
- Make sure the make-up water valve to the boiler is open and there's no active leak.
- Press the Reset button on the front panel once.
- Watch the pressure in the information menu – it should climb as the feeder refills and then hold steady.
- If E351 returns or pressure keeps falling, leave the boiler off and call a licensed technician.
Where the parts are
- 1Flue
- 2Fan
- 3Heat exchanger
- 4Ignition electrode
- 5Gas valve
- 6Circulation pump
- 7Expansion tank
- 8Pressure gauge
- 9Temperature sensor
Repair options & costs
| Repair | DIY? | Estimated cost |
|---|---|---|
| Open make-up valve / clean fill-line strainer | No | $0–$150 |
| Find and repair a heating-loop leak | No | $150–$600 |
| Replace or recharge expansion tank and relief valve | No | $250–$550 |
| Replace auto feeder valve | No | $250–$500 |
| Replace water pressure sensor | No | $180–$400 |
Rough US estimates; technician call-out fees vary by region. Not a quote.
When to call a technician
Call a licensed plumber or hydronic heating technician if the make-up valve is open and E351 still returns, if you find any leak or a dripping relief valve, or if the boiler has needed frequent refilling. Constant fresh make-up water brings oxygen and minerals into the loop and shortens the heat exchanger's life, so don't just keep resetting.
Frequently asked questions
What is the auto feeder on a Navien combi boiler?
It's a built-in valve that lets fresh water into the heating loop whenever the pressure sensor reports low pressure. It replaces the manual fill valve older boilers used, which is why low pressure shows up as a feeder error instead of a simple low-pressure warning.
What pressure should my NCB boiler show?
Navien calls for a cold fill of at least 12 psi. It rises a few psi as the water heats; residential relief valves are commonly rated 30 psi, so a reading that climbs toward that point suggests an expansion tank problem.
Why does the relief valve drip when the heating runs?
Water expands when heated. If the expansion tank has lost its air charge, pressure spikes and the relief valve opens, then the auto feeder refills and the cycle repeats until it can't keep up. A technician checks and recharges or replaces the tank.
Is it safe to keep resetting E351?
It's not dangerous in itself, but each reset pours in more fresh water that feeds the leak and brings in oxygen and minerals. Find the cause first; one reset to confirm the valve fix is fine.
Sources
- Official Navien user documentation and support pages (facts only; wording is our own)
- Methodology: How We Build Code Pages · Data Sources We Use · Editorial Policy