Tankless Water Heater Error Code 11 or 12, Ignition Failure, Frozen Condensate Drain Line, and Heat Exchanger Freeze-Burst¶

When a high-efficiency condensing tankless gas water heater (manufactured by Rinnai, Navien, Noritz, Takagi, Rheem, or Bosch) suddenly shuts down during a sub-zero winter cold snap and flashes Error Code 11 (11 / E003 / 111: Ignition Failure) or Error Code 12 (12 / E012 / 121: Flame Loss / Flame Failure), homeowners face two immediate hazards: losing all domestic hot water during extreme cold, and inadvertently making a single mistake, unplugging the unit's 120/230-volt electrical power cord, that destroys the 1,500-to-3,000-dollar copper or stainless-steel heat exchanger and floods the house within hours. Unlike a traditional 200-liter (50-gallon) storage tank water heater that holds a massive thermal reservoir and uses a standing pilot light, a condensing tankless water heater routes cold water through narrow 6-to-12-millimeter (0.25-to-0.5-inch) finned copper or stainless-steel tubing positioned directly inside an outdoor wall recess, unheated garage, or attic vented directly to freezing outside air.

During polar vortex freezes, the number-one hidden cause of Error Code 11 and Error Code 12 on a condensing tankless unit is not a bad gas valve, but a frozen exterior PVC condensate drain line, which backs acidic water up through the neutralizer trap straight into the secondary heat exchanger and combustion chamber, submerging the igniter and flame-rectification rod. Worse yet, if the household loses AC electrical power during a winter blackout, or if a frustrated homeowner unplugs a non-firing unit, the tankless heater's internal electric ceramic freeze-protection blocks (which draw 100 to 250 watts of AC electricity to protect down to minus 30 degrees Celsius / minus 22 degrees Fahrenheit) shut off immediately, allowing icy backdraft wind blowing down the exhaust vent to freeze and split the copper heat exchanger wide open. If you smell rotten-egg mercaptan gas odor or hear hissing gas, evacuate immediately and call your local emergency numbers (911 in the United States and Canada, 999 in the United Kingdom, 112 across the European Union, and 000 in Australia), and review our guides on tankless water heater freeze-burst, exhaust vent blockage, and error code lockout, frozen pipes, and winter power outages, prolonged blackouts, and grid freezes.

Do this now¶

If your tankless gas water heater flashes Error Code 11 or 12 in winter, stops flowing water, or loses electrical power during a freeze:

1. NEVER Unplug the Tankless Water Heater's Electrical Power Cord During Freezing Weather:
2. Even if your gas supply is out, your propane tank is empty, or the unit is flashing Error Code 11 or 12, leave the 120-volt (or 230-volt) electrical power cord plugged in and the breaker ON.
3. A tankless water heater's built-in ceramic freeze-protection heaters run on household AC electricity, NOT gas. As long as the unit has electrical power (even when turned "Off" at the keypad button), its internal thermostats automatically energize the electric heater blocks whenever internal pipe temperature drops below 5 degrees Celsius (41 degrees Fahrenheit). Unplugging the unit during a freeze guarantees a burst copper heat exchanger unless you manually drain every drop of water out of the unit first.
4. Error Code 11 or 12 in Winter: Check and Thaw the Condensate Drain Line First:
5. Condensing tankless heaters produce 2 to 4 liters (0.5 to 1 gallon) per hour of acidic liquid water (condensate, pH 3.2 to 4.5) that drains out the bottom of the unit through a 13-to-19-millimeter (0.5-to-0.75-inch) PVC pipe or clear vinyl tube.
6. Where that condensate pipe exits through an exterior wall or runs across an unheated garage/crawlspace, winter air freezes the slow trickle into a solid ice plug. The backed-up water fills the internal condensate trap, triggers the float switch or floods the burner box, and locks the unit out on Code 11 (No Ignition) or Code 12 (Flame Loss).
7. Action: Disconnect the condensate drain union nut or rubber coupling directly beneath the tankless unit over a 4-liter (1-gallon) bucket to drain the backed-up water, then thaw the frozen exterior condensate pipe with a hair dryer (set on low/medium) or warm wet towels (never an open blowtorch).
8. Check the Gas Supply, Outdoor Propane Regulator Vent, and Rooftop Exhaust/Intake Terminations:
9. Error Code 11 (No Ignition): Confirm the yellow manual gas shutoff ball valve beneath the unit is parallel to the pipe (open), verify other gas appliances (stove/furnace) light normally, and if on LP / Propane, brush snow and ice off the outdoor propane tank regulator vent screen (a frosted regulator vent locks off gas flow).
10. Error Code 12 (Flame Loss After Igniting): Step outside and visually inspect the 2-inch or 3-inch PVC concentric or twin-pipe intake/exhaust vent terminations on the side wall or roof for hoarfrost, snow drifts, or icicles blocking combustion air.
11. If Household AC Power Goes Out in Sub-Zero Weather (and You Have No Backup Power), Drain the Tankless Unit Within 30 Minutes:
12. During a winter power outage without a generator or UPS battery backup, cold outdoor air siphons backward down the exhaust pipe and freezes the thin copper water channels inside the heat exchanger in 30 to 90 minutes.
13. Execute the 5-Minute Blackout Manual Drain Protocol:
    1. Turn OFF the Blue Cold-Water Inlet Valve and Red Hot-Water Outlet Valve beneath the unit, and turn OFF the Yellow Gas Valve.
    2. Unscrew the brass caps on both the Cold Service Drain Port and Hot Service Drain Port, and open both service drain T-handles over a bucket.
    3. Unscrew the Cold-Water Inlet Filter Screen (strainer plug) at the bottom of the unit and open a hot-water faucet inside the house to break the vacuum so all 1 to 2 liters of water trapped inside the heat exchanger gravity-drains out completely.
14. If Hot-Water Faucets Produce Zero Flow (while Cold Faucets Flow Normally), Thaw Internal Pipes Gently Before Pressurizing:
15. If cold water runs at your sink but turning on the hot tap produces nothing at all, an ice plug has formed inside the tankless inlet/outlet pipes or heat exchanger.
16. Close the Blue Cold Inlet Valve immediately so municipal water pressure (400 to 550 kPa / 60 to 80 PSI) does not push behind a thawing ice split, open the bottom service drain ports over a wide tub, and warm the cabinet interior slowly with a hair dryer while watching for water dripping from the bottom of the sheet-metal casing (which indicates a split copper heat exchanger).
17. Never Repeatedly Press the Power Reset Button More Than 3 Times on Error 11 or 12:
18. If you have cleared the condensate line and intake vent and the unit still fails to light after 2 to 3 reset attempts, stop and call a licensed gas technician. Repeatedly cycling failed ignition attempts can accumulate unburned gas or damage the ceramic igniter and flame rod.

Decision tree: tankless water heater Error Code 11/12, condensate freeze, and blackout drain¶

flowchart TD
    Start["TANKLESS WATER HEATER WINTER EMERGENCY: Error Code 11 / 12, No Hot Water Flow, or Winter Power Outage"] --> Scenario{"What is the exact symptom?"}

    Scenario -->|"1. Error Code 11 (No Ignition) or 12 (Flame Loss)"| KeepPlugged["RULE 1: KEEP 120V/230V POWER CORD PLUGGED IN. Check condensate drain, gas supply, and outdoor vents in order"]

    KeepPlugged --> CondCheck{"Is the PVC/vinyl condensate drain line frozen outside or in garage?"}
    CondCheck -->|"YES: Backed-Up Condensate"| ThawCond["CONDENSATE DRAIN RECOVERY: 1. Place bucket under unit & unscrew condensate trap/drain cap 2. Drain backed-up acidic water from secondary heat exchanger 3. Thaw exterior PVC pipe with hair dryer; reroute or insulate pipe 4. Press Power OFF then ON to clear Code 11 / 12"]

    CondCheck -->|"NO: Condensate Drains Freely"| GasVentCheck["GAS & VENT DIAGNOSTIC: • Code 11: Verify yellow gas valve open; check other gas appliances; brush ice off outdoor propane regulator vent; clean inlet filter • Code 12: Clear snow/hoarfrost from outdoor intake & exhaust pipes; check for high-wind downdraft or fouled flame sensor rod"]

    Scenario -->|"2. Winter Power Outage (Blackout in Freezing Weather)"| BackupCheck{"Do you have a 120V Pure Sine Wave UPS or Generator?"}
    BackupCheck -->|"YES: Plug into UPS/Generator"| RunFreezeProt["Plug tankless 120V cord into battery UPS (draws only 100 to 250 W) to keep electric ceramic freeze heaters active"]
    BackupCheck -->|"NO: No Electricity Available"| ManualDrain["MANDATORY 5-MINUTE BLACKOUT DRAIN (Within 30 Min): 1. Close BLUE Cold Valve, RED Hot Valve, and YELLOW Gas Valve 2. Open BOTH bottom service valve drain ports over a bucket 3. Unscrew Cold Inlet Filter plug & open indoor hot tap to break vacuum 4. Leave all drain caps OFF until power and heat return"]

    Scenario -->|"3. Cold Taps Flow, But Hot Taps Have ZERO Water Flow"| FrozenUnit["FROZEN INTERNAL PIPES OR HEAT EXCHANGER: 1. IMMEDIATELY close BLUE Cold-Water Inlet Valve (prevent flood.) 2. Unscrew bottom service drain caps & cold inlet filter over bucket 3. Warm unit gently with hair dryer (NEVER fire burner or use torch) 4. Crack Blue Valve 10% open; if water leaks from bottom casing -> Heat exchanger is SPLIT (Keep Blue Valve OFF & call plumber)"]

Cross-brand error code translation table (Rinnai, Navien, Noritz, Takagi, Rheem)¶

While Error Code 11 and Error Code 12 are the industry-standard diagnostic codes originated by Rinnai, Noritz, and Takagi, other manufacturers use equivalent numeric codes for the exact same failure modes:

Why a frozen condensate drain line triggers Error Code 11 and Error Code 12¶

Non-condensing older tankless heaters (80 to 82 percent AFUE) vented scorching 150-degree-Celsius (300-degree-Fahrenheit) exhaust gases out a stainless-steel pipe. Modern high-efficiency condensing tankless water heaters (95 to 99 percent UEF) use a secondary stainless-steel heat exchanger to extract so much heat from the exhaust gas that the exhaust cools below its dew point (55 degrees Celsius / 131 degrees Fahrenheit), condensing water vapor mixed with dissolved carbon dioxide and nitrogen oxides into acidic liquid condensate (pH 3.2 to 4.5) at a rate of 2 to 4 liters (0.5 to 1 gallon) per hour.

HOW A FROZEN EXTERIOR CONDENSATE DRAIN TRIGGERS CODE 11 / 12 AND FLOODING

   +-----------------------------------+
   |   TANKLESS WATER HEATER CABINET   |
   |  +-----------------------------+  |
   |  |  Primary Burner & Igniter   |  | <-- 3. Backed-up water rises into combustion box,
   |  +-----------------------------+  |        drowning flame sensor rod -> ERROR 11 / 12.
   |  |  Secondary Heat Exchanger   |  | <-- 2. Acidic water fills internal collector pan
   |  +--------------+--------------+  |
   |                 ||                |
   |         [Internal P-Trap]         |
   +-----------------++----------------+
                     ||
           [Condensate DrainCap]       <-- EMERGENCY RELIEF: Unscrew cap here over bucket.
                     ||
                     ||====================[Exterior Wall]====> [SOLID ICE PLUG OUTSIDE]
                                                                (Slow trickle freezes at -5°C)

Step-by-Step: How to Clear a Frozen Condensate Line and Reset Code 11/12¶

1. Locate the Condensate Drain Pipe on the Bottom of the Heater:
2. Look underneath the tankless unit alongside the blue cold pipe and red hot pipe. You will find a 13-to-19-millimeter (0.5-to-0.75-inch) white PVC pipe, CPVC pipe, or clear braided vinyl tube attached to a plastic threaded stub (often routed through a cylindrical white limestone-chip acid neutralizer cartridge).
3. Relieve the Backed-Up Water Inside the Heater:
4. Place a 4-to-8-liter (1-to-2-gallon) plastic bucket directly under the unit.
5. Locate the condensate trap drain plug (on Navien units, a twist-lock cap with a pull-ring on the bottom right; on Rinnai/Noritz/Takagi units, the threaded union nut or spring-clamp hose fitting on the condensate stub).
6. Slowly unscrew the cap or disconnect the vinyl hose over the bucket. If 0.5 to 2 liters (1 to 4 pints) of water gushes out, you have confirmed that a frozen condensate line caused your Error Code 11 or 12.
7. Thaw the Exterior Condensate Pipe and Prevent Re-Freezing:
8. Follow the condensate drain pipe to where it terminates outside the house, in an unheated garage, or at a frozen floor drain.
9. Melt the ice plug using a hair dryer, heat gun on low setting, or hot water poured over rags wrapped around the PVC pipe.
10. Temporary Winter Bypass: While sub-zero weather persists outside, if the condensate line runs indoors before exiting the wall, you can temporarily let the disconnected interior condensate hose drip directly into a 20-liter (5-gallon) plastic bucket inside the heated garage or utility room (emptying the bucket daily into a laundry sink with baking soda to neutralize the mild acidity) until a plumber can reroute the drain to an interior laundry standpipe or floor drain or wrap it with self-regulating electric heat trace cable.
11. Reset the Unit: Reconnect the condensate trap cap, press the On/Off Power button on the front digital controller off and back on, and open a hot water tap to verify ignition.

How to manually drain a tankless water heater during a winter power outage¶

If a winter storm knocks out grid electricity, or if your gas supply fails and you cannot keep the unit powered, you must drain the 1.5 liters (0.4 gallons) of water out of the internal copper tubes before outdoor wind blowing down the exhaust flue freezes it solid.

BOTTOM PLUMBING MANIFOLD OF A TANKLESS WATER HEATER (BLACKOUT DRAIN SEQUENCE)

        +-------------------------------------------------------+
        |               BOTTOM OF TANKLESS UNIT                 |
        +-------+------------------+-----------------+----------+
                ||                 ||                ||
                ||            [YELLOW GAS]           ||
                ||             [SHUTOFF]             ||
        [Filter Screen Plug]  (Turn 90° OFF)         ||
        (Unscrew to Vent)                            ||
                ||                                   ||
          +-----+-----+                        +-----+-----+
          | BLUE COLD |                        |  RED HOT  |
          |  ISOLATION|                        | ISOLATION |
          |   VALVE   |                        |   VALVE   |
          | (Turn OFF)|                        | (Turn OFF)|
          +-----+-----+                        +-----+-----+
                |===[Service Drain Cap (OPEN)]       |===[Service Drain Cap (OPEN)]
                |           |||                      |           |||
          (Cold Supply)     vvv                (To House)        vvv
                      [Catch Bucket]                       [Catch Bucket]

The 6-Step Manual Winter Drain Checklist¶

Execute these six steps in exact sequence using a pair of channel-lock pliers, a flathead screwdriver, and a bucket:

• Step 1: Turn OFF the controller and unplug the 120V/230V power cord (ONLY because you are about to drain the unit completely dry; never leave a drained unit plugged in or the dry electric ceramic heaters can overheat).
• Step 2: Shut OFF the Yellow Gas Supply Valve by turning the handle 90 degrees so it sits perpendicular (crosswise) to the gas pipe.
• Step 3: Shut OFF the Blue Cold-Water Isolation Valve and the Red Hot-Water Isolation Valve by turning both main wing handles 90 degrees perpendicular to the vertical copper/PEX pipes.
• Step 4: Open the Hot and Cold Service Drain Ports: Place a bucket underneath the isolation valves, unscrew the round brass threaded dust caps on the side branches of both the Blue and Red service valves, and turn the small service port T-handles (or screwdriver slots) 90 degrees parallel to the drain spouts to open them.
• Step 5: Unscrew the Cold-Water Inlet Filter Screen (Critical Air-Lock Release): Locate the round plastic or brass knurled inlet water filter plug protruding directly from the bottom of the tankless cabinet just above the Blue cold valve. Unscrew it counterclockwise over your bucket. Removing this filter plug breaks the internal vacuum airlock so water trapped inside the upper copper coils of the primary heat exchanger can gravity-drain out.
• Step 6: Open the Condensate Trap Drain Cap and One Indoor Hot Faucet: On condensing models, pull the bottom condensate trap drain plug to empty the secondary collector, and open the highest hot-water faucet inside the house to vent the house hot-water line. Leave all drain caps and the inlet filter plug off in a plastic bag hung on the valve until power and heat are restored.

What to do if your copper heat exchanger is already frozen or split¶

If you wake up on a minus 15-degree-Celsius (5-degree-Fahrenheit) morning, turn on the hot water tap, and zero water comes out of the hot side while cold taps flow normally:

1. Do NOT Try to Fire the Burner: Even if you could force the burner on, heating a frozen block of ice inside a sealed copper tube creates a steam pressure rupture.
2. Close the Blue Cold-Water Inlet Isolation Valve Immediately:
3. The biggest danger of a frozen tankless water heater occurs 2 to 6 hours later when the morning sun or garage temperature rises above freezing.
4. Most copper heat exchangers split open at the moment they freeze, but the ice plug itself temporarily seals the crack so no water leaks while frozen. If you leave the Blue Cold-Water Inlet Valve open while the unit thaws, 400 to 550 kilopascals (60 to 80 PSI) of municipal water pressure will suddenly blast out of the split copper sheet-metal cabinet at 30 liters per minute (8 gallons per minute) while you are at work.
5. Thaw With a Hair Dryer and Perform the 10-Percent Crack Test:
6. With the Blue Cold Valve OFF, the power cord unplugged, and the bottom service drain ports open over a bucket, remove the front sheet-metal cover (4 screws) or blow warm air from a hair dryer up through the bottom pipe penetrations for 20 to 30 minutes until ice water finishes dripping out of the drain ports.
7. Once thawed, close the service drain ports, reinstall the inlet filter screen, and slowly crack the Blue Cold-Water Inlet Valve 10 percent open while looking up inside the bottom of the unit with a flashlight:
   • If water immediately rains down from inside the heat exchanger fins onto the burner manifold: The copper heat exchanger is freeze-split. Shut the Blue Cold Valve OFF immediately. You can still turn your main house water back on and have normal cold water throughout your entire house as long as the tankless unit's Blue and Red isolation valves remain closed.
   • If the unit holds pressure with zero internal dripping for 10 minutes: Open the Blue and Red valves fully, bleed air at a sink hot tap until a steady stream flows, dry all electrical connectors, and plug the unit back in.

Regional emergency numbers and gas appliance standards¶

Frequently asked questions¶

Why does unplugging a tankless water heater during a winter freeze cause the copper heat exchanger to burst?¶

Homeowners frequently assume that a gas tankless water heater uses its gas burner to keep itself from freezing, or that unplugging a malfunctioning unit in winter is a safe precaution. In reality, a tankless water heater never fires its powerful gas burner for freeze protection because doing so without water flowing would instantly boil the standing water into steam. Instead, every tankless unit relies on small 120-volt or 230-volt electric ceramic heating blocks clamped directly onto the internal copper pipes. As long as the unit remains plugged into a live electrical outlet, even if the gas is off or an error code is flashing, those electric blocks keep the heat exchanger above freezing. If you unplug the unit without draining the water out of the bottom service valves first, cold air blowing down the exhaust vent will freeze and split the copper heat exchanger within hours.

How does a frozen plastic condensate drain pipe outside the house cause Error Code 11 or Error Code 12?¶

High-efficiency condensing tankless water heaters extract so much heat from the exhaust gas that 2 to 4 liters (0.5 to 1 gallon) per hour of acidic liquid water condenses inside the secondary heat exchanger and drains out through a small 13-to-19-millimeter (0.5-to-0.75-inch) plastic pipe. Where that small pipe exits through an exterior wall or runs through an unheated garage, winter air freezes the slow trickle into a solid plug of ice. Once plugged, liquid condensate backs up inside the heater, fills the internal trap, and floods upward into the combustion chamber, submerging the flame sensor rod and igniter until the computer shuts down on Error Code 11 (No Ignition) or Error Code 12 (Flame Loss).

What is the difference between Error Code 11 and Error Code 12 on a Rinnai, Noritz, or Rheem tankless water heater?¶

Error Code 11 (equivalent to Navien E003 or Takagi 111) means "No Ignition": when you open a hot water tap, the flow turbine spins and the igniter sparks, but no flame is ever established, usually because the main gas valve is closed, the propane tank is empty or its outdoor regulator vent is iced over, air is trapped in the gas line, or backed-up condensate water has flooded the burner before startup. Error Code 12 (equivalent to Navien E012 or Takagi 121) means "Flame Loss": the burner successfully ignites, burns for a few seconds or minutes, and then suddenly goes out mid-cycle, most commonly caused by a partially frozen condensate line surging water into the burner, snow blocking the outdoor intake/exhaust PVC pipes, a dirty flame-rectification sensor rod, or insufficient gas line pressure when the home furnace kicks on simultaneously.

How can I keep cold water running in my house if my tankless water heater heat exchanger freezes and splits open?¶

If your tankless water heater freezes and splits its internal copper heat exchanger, you do not have to leave your entire house without running water while waiting days for a replacement unit. Simply walk up to the plumbing manifold directly underneath the tankless unit and turn both the Blue Cold-Water Inlet Isolation Valve and the Red Hot-Water Outlet Isolation Valve 90 degrees perpendicular to the pipes (the OFF position). Closing those two isolation valves completely disconnects the leaking tankless water heater from your home's plumbing system, allowing you to leave your main house water shutoff valve wide open so all toilets, sinks, and appliances have normal cold water.

Why did my cold faucets keep working while my hot faucets stopped flowing completely during a cold snap?¶

When cold water flows normally at every sink in your house but turning on any hot water faucet produces zero water flow (not even cold water coming out of the hot side), the blockage is located at the single bottleneck through which all hot water must pass: the tankless water heater itself or its exposed inlet/outlet pipes in the garage, attic, or exterior wall recess. Because the copper tubing pathways inside a tankless heat exchanger are only 6 to 12 millimeters (0.25 to 0.5 inches) wide and sit right below an open exterior exhaust flue, they freeze long before larger household wall pipes. Close the Blue Cold Inlet Valve immediately and thaw the unit gently with a hair dryer with the bottom drain ports open.

Can I power my gas tankless water heater with a computer UPS battery backup during a winter blackout?¶

Yes, because a gas tankless water heater uses natural gas or propane to generate 100 percent of its thermal heat and uses electricity only to run its circuit board, exhaust blower fan, and electric freeze-protection elements (drawing only 75 to 200 watts). However, you must use a "Pure Sine Wave" (True Sine Wave) UPS battery backup rated at 1,000 VA to 1,500 VA, never a budget "Simulated / Modified Sine Wave" computer UPS. A modified sine wave outputs a stepped square wave that causes the tankless heater's variable-speed combustion blower motor to overheat, stall, or throw fan-speed error codes.

This guide is for informational purposes and does not replace a licensed gas fitter or master plumber. Never open the sealed combustion chamber or adjust manifold gas pressures yourself, and consider taking a home emergency preparedness and first aid course through the Red Cross or St John Ambulance.

Related guides¶

• Tankless water heater freeze-burst, exhaust vent blockage, and error code lockout
• Water heater failure
• Frozen pipes
• Burst pipe
• Winter power outages, prolonged blackouts, and grid freezes
• Gas leak
• Emergency numbers

Sources and official guidance¶

• Rinnai Corporation, Navien Inc., Noritz America, & Takagi: Technical Service Manuals and Winter Freeze-Protection Bulletins (Diagnostic Codes 11/12 & E003/E012, Condensate Trap Drainage, and Manual Freeze-Drain Sequences).
• National Fire Protection Association (NFPA) & CSA Group: NFPA 54 / ANSI Z223.1 (National Fuel Gas Code) & CSA B149.1 (Natural Gas and Propane Installation Code - Direct-Vent Clearances Above Snow Grade and Condensate Disposal).
• Health and Safety Executive (HSE) & Heating and Hotwater Industry Council (HHIC, United Kingdom): Customer and Installer Guidance on Frozen Condensate Pipes (BS 6798 External 32-mm Insulated Pipe Standards).
• American Society of Plumbing Engineers (ASPE) & International Code Council (ICC): International Plumbing Code (IPC) & International Fuel Gas Code (IFGC) Standards for High-Efficiency Condensing Water Heaters.