Rheem tankless water heater error code 11: flame not detected

By Jason Dearmin · Last reviewed November 14, 2026 · How we check facts

What error 11 means

Rheem's troubleshooting manual (AP21822) describes 11 as Flame Not Detected by Flame Rod: "In the case of ignition failure, the water heater will attempt ignition 2 times. If no flame is detected, the water heater displays the 11 error code on the built-in controller or the remote controller."

Rheem diagnostic check items

1. Gas supply pressure
2. Gas filter
3. Venting
4. Dip switches
5. Igniter rod
6. Flame rod

Details from the manual

• Gas supply pressure: the manual gives correct gas pressure as 4.0 to 10.5 inches W.C. for natural gas and 8.0 to 13.0 inches W.C. for propane, measured at the gas connection. A static reading can look normal while the dynamic pressure at maximum combustion drops when another gas appliance runs, so the manual says to check dynamic pressure as well.
• Gas filter: check the filter at the gas connection of the water heater for a blockage and clean it.
• Venting: check vent connections and terminations. If the vent is disconnected or blocked, reconnect it or clear the termination (both the inlet and exhaust sides).
• Dip switches: confirm they are set for the venting and altitude.
• Igniter rod: if spark is not visible in the sight glass, the manual walks a technician through re-seating connectors, measuring 108 to 132 AC volts on the igniter circuit, cleaning the igniter rod, and replacing parts if still no spark.
• Flame rod: if the spark is visible but there is no flame, the manual walks through the flame rod and gas valve solenoid checks.

These steps involve gas and line voltage and are written for a technician. A homeowner can safely check that the gas valve is open, the propane tank has fuel, and the outdoor vent and intake terminations are free of snow, ice or debris.

Safe homeowner checks for Rheem tankless error 11

1. Check the gas valve and fuelMake sure the gas valve at the water heater is open and, if you use propane, that the tank is not empty.
2. Check outdoor vent endsFrom the ground, look for snow, ice or debris at the intake and exhaust terminations.
3. Reset onceTurn the unit off and on again one time. If 11 returns, call a gas technician or Rheem.

Frequently asked questions

How many times does a Rheem tankless try to ignite?

According to Rheem's manual, it attempts ignition 2 times and displays 11 if no flame is detected.

Is a Rheem 11 the same as a Rinnai 11?

The two are separate manufacturers with separate manuals. Rheem calls its code 11 "Flame not detected by flame rod"; Rinnai calls it "No ignition". The thinking behind both lists is similar, but use the manual for your brand.

Sources

• High and Super High Efficiency Tankless Water Heater Troubleshooting Manual (AP21822 Rev 02) (Rheem) Covers: Rheem high and super high efficiency tankless water heaters (condensing and non-condensing, indoor and outdoor) covered by this manual.
• Troubleshooting Tankless Water Heater Diagnostic Codes (Rinnai America) Covers: Rinnai tankless water heaters in the SENSEI, SENSEI RX/CX and non-SENSEI series. Not every code applies to every model.

Safety note: this page is general information drawn from the sources above. Gas, combustion and line-voltage repairs belong to a licensed professional. Your own equipment's label and manual take priority over anything here. Read the full disclaimer.