Takagi error code 311 indicates a flame loss during operation — distinct from initial ignition failure. The unit successfully started, ran briefly, then lost flame detection. This is the Takagi equivalent of Bosch EA, Rinnai 11, and other manufacturers' mid-operation flame failures.
What triggers Takagi 311
The flame sensor confirmed ignition successfully (initial flame detection signal received), but during operation lost the microamperage signal from the flame. Possible causes:
- Gas pressure drops under sustained load (gas line undersized)
- Flame sensor oxidation reduces sensitivity over time
- Combustion air intake restriction
- Exhaust vent partial blockage affecting combustion
- Burner orifice partial clog disrupting flame stability
Diagnostic steps for code 311
- Gas pressure under load: the most common cause. Connect manometer to gas valve test port, measure pressure with unit at high fire. Spec: natural gas 4.5" WC, LP 9.5" WC at the burner manifold. Significant drop from static indicates undersized gas line.
- Flame sensor inspection: remove and clean with fine emery cloth. Visible oxidation on the sensor rod tip is the most common cause of intermittent signal loss.
- Vent and intake inspection: verify both vent and intake (for sealed combustion units) are clear of obstructions. Insect nests, leaves, snow accumulation at terminations all cause code 311.
- Burner cleanliness: inspect for spider webs, dust, or other debris in the burner area. Clean as needed.
- Condensate trap (condensing models): blockage causes condensate backup that can flood the burner area.
Gas line sizing — the most common Takagi 311 cause
Takagi tankless units demand significant gas flow at high fire (typical T-H3 condensing 199,000 BTU/hr). A gas line sized for an old 40,000 BTU/hr tank water heater is meaningfully undersized for this demand. Even if static pressure is fine at 7" WC, dynamic pressure can drop to 4–5" WC under load — insufficient to maintain stable flame.
Gas line upgrade is often the resolution. Verify with manometer measurement before assuming the line is adequate.
Flame sensor cleaning procedure
- Shut off gas and power to the unit
- Remove the front cover to access the burner area
- Locate the flame sensor (typically a metal rod near the burner)
- Disconnect the sensor's electrical wire
- Remove the sensor from its mounting
- Polish with fine emery cloth (400 grit) — both sides of the rod
- Wipe clean
- Reinstall with sensor tip in the flame path (about 3/8" into flame zone)
- Reconnect, restore gas and power, test
Reset procedure
After addressing the cause, cycle power to reset code 311. Most Takagi models reset on power cycle. If code returns within minutes of operation, the underlying issue isn't resolved.
Multi-failure pattern
Sometimes code 311 appears in conjunction with codes 651, 652, 660 or others — indicating multiple developing issues. For Takagi units 8+ years old with multiple intermittent fault codes, consider full inspection:
- Gas pressure system end-to-end
- All sensors (flame, temperature, flow)
- Combustion air and exhaust paths
- Heat exchanger condition (scale buildup, soot)
- Control board functionality
Annual service prevents code 311
Most Takagi 311 events are preventable with annual service:
- Flame sensor cleaning (catches oxidation before signal failure)
- Burner inspection and cleaning
- Vent and intake clearance verification
- Gas pressure check under load
- Descaling in hard-water areas (scale changes combustion dynamics)
When to call Takagi authorized service
If code 311 recurs after standard maintenance, or if gas pressure measurement is outside your comfort zone, authorized Takagi service technicians have manometers and combustion analyzers to diagnose precisely. Authorized service is also required to maintain warranty validity for in-warranty units.