New Tankless Water Heater Error on First Fire Decision Tree

Why this matters

A new tankless unit throwing an error code on its first fire is, in the large majority of cases, an installation issue the code is reporting accurately: undersized or unpurged gas, a venting fault, no minimum flow, condensate or air in the lines, or a dip-switch/setting left wrong for the gas type or altitude. The temptation is to call the code a defect and request an exchange. Tankless error codes are diagnostic by design; each family points at a specific commissioning miss. This tree groups the common first-fire error families (ignition, flame loss, gas/combustion, flow, venting, and overtemp) and gives the commissioning checks that resolve each without condemning the unit.

Symptom presentation

• Error at the very first hot demand, before the unit has ever run successfully.
• Unit attempts ignition and locks out (ignition-failure family).
• Lights then drops out repeatedly (flame-loss/combustion family).
• No error but no fire until flow is high, or errors at low flow (minimum-flow family).
• Error tied to venting length/termination or condensate (vent/condensate family).
• Overtemp/outlet-high error on first draw (scale-free new unit, so flow or cold-water-sandwich related).

Quick checks

1. Gas type and settings: confirm the unit is configured for the actual gas (NG vs LP) and that any altitude/region dip switches or menu settings match the install. A wrong gas-type setting causes immediate ignition or combustion errors.
2. Gas supply: verify line size and length support the unit's full BTU input, the line is purged of air, and dynamic gas pressure stays within spec under full fire (NG commonly around 3.5 in WC, LP around 10 to 11 in WC, dropping no lower than the manufacturer minimum under load).
3. Venting: confirm vent material, diameter, total length and elbows, and termination clearances match the manufacturer table; confirm intake is not obstructed.
4. Water side: purge air from the new piping, confirm minimum activation flow is met, and verify the cold/hot connections and any isolation valves are correct and open. Verify condensate drain (condensing units) is connected and not airlocked.

Isolation tree

Branch A: ignition-failure family.

• No spark/no flame establishment: air in the gas line, gas valve closed, wrong gas-type setting, or inadequate gas pressure.
• Bleed the line, confirm gas type/setting, and measure dynamic pressure under full fire.

Branch B: flame-loss / combustion family.

• Unit lights then drops: gas pressure sags under load (undersized line), poor combustion air, or a flame-sense/ground issue.
• Verify line sizing against the BTU input and check that dynamic pressure holds at full fire; confirm proper grounding.

Branch C: minimum-flow family.

• Burner will not stay on at low draw: the unit needs a minimum activation flow rate; a partially open valve, a clogged inlet filter screen (debris from new piping), or a flow sensor not seeing flow.
• Clean the inlet filter, confirm flow meets the minimum, and flush construction debris.

Branch D: venting / air-supply family.

• Vent-pressure or combustion errors: vent run exceeds the allowed equivalent length, wrong diameter, blocked termination, or intake obstruction.
• Re-measure the vent run against the manufacturer table and correct.

Branch E: condensate / overtemp family.

• Condensing units: a blocked or airlocked condensate path can trigger faults; confirm the trap is primed and draining.
• Overtemp/outlet-high on first fire usually traces to low flow or trapped air, not scale on a new unit.

Confirming diagnosis

Gas confirmation: a manometer on the inlet showing pressure within spec at idle but sagging below the minimum under full fire confirms an undersized or unpurged gas supply, which drives most ignition and flame-loss errors. Correct gas type/setting first.

Vent confirmation: measuring the actual equivalent vent length (straight run plus elbow equivalents) and comparing to the manufacturer maximum confirms a venting-family fault; correcting the run clears the error.

Flow confirmation: a flow that meets the published minimum activation rate and a clean inlet screen clears minimum-flow errors; a clogged screen from new-pipe debris is a very common first-fire cause.

Settings confirmation: verifying the unit's gas-type and altitude/region configuration matches the install rules out the single most common "defective unit" misdiagnosis.

Remediation

• Correct gas-type/altitude settings, purge and properly size the gas line, and verify dynamic pressure under full fire.
• Bring the vent run, diameter, material, and termination within the manufacturer table; clear intake obstructions.
• Clean the inlet water filter screen, purge air from water piping, and confirm minimum activation flow.
• Prime/clear the condensate trap on condensing units.
• Re-run the first-fire sequence and confirm a stable burn at both low and high draw, then verify outlet temperature.

References

• International Plumbing Code (IPC) 2021 Chapter 5 Water Heaters
• NFPA 54 / ANSI Z223.1 National Fuel Gas Code (gas sizing, pressure, venting, combustion air)
• ANSI Z21.10.3 / CSA 4.3 Gas Water Heaters Volume III (circulating and instantaneous, generic)
• International Fuel Gas Code (IFGC) 2021 Chapter 4 Gas Piping and Chapter 5 Venting
• ASSE 1017 Temperature Actuated Mixing Valves (outlet temperature control)