Tankless Water Heater Not Working — No Ignition, Power, or Flow

Common Symptoms

• No hot water from any fixture — unit shows no signs of life
• Display is blank or dark — no power indicator at all
• Unit clicks repeatedly but never ignites
• Hot tap opens and nothing happens — no flow activation
• Error code on display — unit in lockout, won't fire
• Unit worked yesterday, completely unresponsive today
• Pilot light equivalent absent — gas not lighting on demand

Most Likely Causes

1. 1

   No Power Supply — Tripped Breaker or Blown Fuse

   Tankless water heaters require a dedicated electrical circuit even when gas-fired — the igniter, flow sensors, control board, and gas valve solenoids all run on 120V AC power. A tripped breaker or blown fuse cuts all power and makes the unit appear completely dead. Rinnai units use a 2-amp fuse on the PCB that can blow from a power surge. Navien units use a 5-amp fuse accessible from inside the front panel. Rheem RTGH and Noritz NRC series all require their own 15-amp dedicated circuit. Check the breaker panel first — before any other diagnosis.

2. 2

   Flow Sensor Failure — Unit Won't Activate on Hot Demand

   Tankless heaters use a turbine-style or paddle flow sensor to detect water movement and trigger ignition. When this sensor fails (from mineral scale buildup, debris, or bearing wear), the unit never sees the flow signal and never fires — even though water is flowing freely to the fixture. All major brands use this activation method: Rinnai (error code 65 when the flow control valve malfunctions), Navien (E047 air/gas ratio fault or E016 flow sensor fault), Noritz (error code 14 for flow sensor), Takagi (error code C4), and Rheem (error code C7). Flow sensor replacement is a straightforward DIY repair on most models.

3. 3

   Gas Supply Issue — Valve Closed, Low Pressure, or Empty LP Tank

   A tankless water heater that clicks repeatedly but never ignites almost always has a gas supply problem. Check: (1) The gas shut-off valve on the supply line to the unit — the handle must be parallel to the pipe (open). (2) Whether other gas appliances (furnace, range, dryer) are working — if multiple are out, the issue is upstream at the meter or utility. (3) For LP units, verify the propane tank level — LP tanks read 'empty' at roughly 20% capacity due to pressure behavior. (4) Supply pressure: natural gas should be 3.5–10 in WC at the appliance; LP should be 8–14 in WC. Tankless heaters draw significantly higher gas flow (up to 199,000 BTU/hr) than traditional appliances and expose undersized supply lines that were never a problem for tank-style heaters.

4. 4

   Venting Blockage — Blocked Exhaust or Intake Pipe

   All tankless water heaters use either a forced-draft vent system or a direct-vent (two-pipe) system. A blocked exhaust causes flue gas recirculation into the combustion chamber, triggering an immediate shutdown — Rinnai (error code 12), Navien (E030), Rheem (error code 76), Noritz (error code 11). A blocked combustion air intake starves the burner and prevents ignition. Common blockages: bird nests, wasp nests, mud dauber tubes, leaves, ice/snow in winter, or a disconnected PVC joint inside the home. Inspect both vent termination caps at the exterior wall every spring.

5. 5

   Igniter or Flame Sensor Failure

   The igniter generates the spark to light the burner, and the flame sensor (ionization rod) confirms ignition occurred. When the igniter wears out, the unit clicks without producing a spark. When the flame sensor is coated with carbon or oxidation, it cannot read the ionization current of a valid flame and shuts down after 2–3 seconds. Flame sensor cleaning (light polishing with fine emery cloth or 0000 steel wool) resolves a large percentage of no-ignition complaints across all brands. Rinnai, Noritz, and Takagi flame sensors are all accessible from the front after removing the cover.

6. 6

   Minimum Flow Rate Not Met — Unit Won't Activate at Low Draw

   All tankless heaters require a minimum flow rate to activate — typically 0.5 GPM (Rinnai, Navien, Noritz) to 0.75 GPM (some Rheem and Bosch models). At very low flow (dripping tap, partially closed valve, low-flow aerator in a single bathroom), the unit sees insufficient flow to trigger ignition and no hot water is produced. This is commonly misdiagnosed as a failure. Test by fully opening the hot tap at a tub or kitchen faucet — if the unit fires with full flow but not at the bathroom sink, the issue is flow rate, not a hardware failure. Rinnai and Navien offer reduced flow activation settings that can be adjusted in the controller menu.

Quick DIY Checks

1. 1Step 1 — Check electrical power first: go to your main breaker panel and locate the dedicated circuit for the tankless water heater (typically labeled 'Water Heater' or 'Tankless'). If the breaker is tripped (middle position or fully to the OFF side), reset it by pushing firmly to OFF then back to ON. Also check if the unit has a power switch or service switch mounted near it on the wall — these are sometimes accidentally turned off. For Rinnai and Navien units, check the internal fuse on the PCB: remove the front cover (power off first), locate the small glass or blade fuse near the board's power input terminals, and test with a multimeter for continuity. A blown fuse indicates a past surge — install a whole-home surge protector after replacing.
2. 2Step 2 — Verify gas supply is open and adequate: locate the gas shut-off valve on the line feeding the tankless unit. The valve handle should be parallel to the gas pipe (open position). Turn it back and forth once to verify it moves freely and is fully open. Then test another gas appliance — light a burner on your gas range, check the furnace, or verify the gas dryer fires. If those work, the gas supply to the house is fine and the issue is specific to the tankless heater's supply branch. For LP/propane units: check the tank gauge — propane tanks read low in cold weather due to pressure contraction, and most LP systems shut down when the tank falls below approximately 15–20%. If in doubt, call your LP supplier for a delivery.
3. 3Step 3 — Inspect vent termination for blockage: go outside and locate the vent termination caps on the exterior wall (usually PVC pipes terminating horizontally through the siding). Look inside both the exhaust pipe (center or single pipe on non-concentric systems) and the combustion air intake (outer ring on concentric systems or a separate pipe). Look for bird nests, wasp nests, mud dauber tubes, leaves, or ice. Clear any blockage with a flashlight and long-handled brush. For ice blockage in winter: the exhaust condensate can freeze at the termination during very cold weather — a drip elbow or ice shield (Rinnai and Navien sell these as accessories) prevents recurrence. After clearing, reset the unit and test.

1. 4Step 4 — Clean the inlet filter screen: turn off the cold water supply valve at the unit. Relieve pressure by opening a hot tap downstream. At the cold water inlet connection on the bottom or side of the unit, unscrew the inlet fitting or filter housing cap. Remove the small stainless or brass mesh screen. Rinse under warm running water and scrub lightly with a soft toothbrush to remove sediment and mineral particles. Reinstall, restore water supply, and test. A clogged inlet screen reduces flow below the minimum activation threshold — one of the most common causes of no-ignition on units 3+ years old that have never been serviced.
2. 5Step 5 — Clean the flame sensor: turn off the gas supply valve at the unit. Remove the front cover (typically 4–6 screws). Locate the flame sensor — a thin metal rod protruding into the burner area with a single wire connector. Disconnect the wire. Use fine emery cloth (220-grit) or 0000-grade steel wool to lightly polish the stainless sensing rod, removing any gray or black carbon buildup. Wipe clean with a dry cloth. Reconnect the wire, reinstall the cover, restore gas, and test. This single step resolves a significant portion of 'unit clicks but won't ignite' complaints across Rinnai, Noritz, Takagi, and Rheem units.
3. 6Step 6 — Verify minimum flow rate is met: fully open the hot tap on a bath tub or kitchen sink — these have the highest flow rates in the home. If the unit fires at full flow but not at a bathroom sink or kitchen faucet on half-turn, the problem is flow rate, not a hardware failure. Check for flow restrictors or aerators on the fixture. Tankless units need at least 0.5–0.75 GPM to activate. On Rinnai units (RU and RL series): navigate to the unit's 'Minimum Activation Flow' setting and confirm it is set to the lowest available (0.40 GPM on newer firmware). On Navien NPE units: access the remote controller menu → System Settings → Minimum Flow Rate and verify it is not set above your fixture's flow capacity.
4. 7Step 7 — Reset the unit after clearing any error lockout: many tankless heaters enter a lockout state after repeated failed ignition attempts and will not retry until manually reset. On Rinnai units: press the On/Off button to clear the displayed error code. On Navien NPE units: press the Reset button on the front of the unit or on the remote controller. On Noritz units: press and hold the reset button for 3 seconds. On Rheem RTGH units: press the reset button on the face of the unit, or turn the breaker off for 30 seconds and restore. On Takagi units: remove power for 30 seconds by flipping the breaker. On Bosch Therm units: hold the reset button for 5 seconds. After reset, wait 10 seconds, then test by opening a hot tap fully.

Repair vs Replace

Recommended Tools & Parts

• Tankless Water Heater Flow Sensor (Universal / Brand-Specific)

  Turbine flow sensor that detects water flow and triggers ignition. Available OEM by brand or universal-fit by connector type. Confirm your unit's model number and part number before ordering.

  $30–$80

  Buy on Amazon →

• Rinnai / Navien Inlet Filter Screen Kit

  Replacement stainless mesh inlet filter screens. Prevents debris from reaching the flow sensor and heat exchanger. Replace when torn or corroded.

  $5–$15

  Buy on Amazon →

• Tankless Water Heater Descaling Kit

  Submersible pump, hoses, and citric acid descaler for flushing scale from the heat exchanger. Annual descaling maintains performance and extends heat exchanger life.

  $40–$80

  Buy on Amazon →

• 2A / 5A PCB Replacement Fuse (Glass or Blade)

  Replacement fuses for tankless control board protection circuits. Rinnai units typically use 2A glass fuses; Navien uses 5A. Always identify the blown fuse before ordering a replacement.

  $3–$8

  Buy on Amazon →

Links are Amazon affiliate links (tag: fixitfastai-20). Prices are estimates.

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