Tankless Fires on Test But Not on Demand: Decision Tree

Why this matters

A tankless water heater that lights and runs in a forced test mode but refuses to fire when a fixture is opened is a textbook "ignition is fine, the trigger is not" fault. The test fire proves the gas, igniter, flame sensor, fan, and burner all work. What it does not prove is the part of the system that decides when to fire on real demand: the flow sensor, the minimum-activation-flow threshold, and the cold-inlet plumbing that has to deliver enough flow to wake the unit. The expensive mistake is condemning the gas valve or igniter when those already proved good on the test. The correct move is to recognize that a successful test fire moves the whole investigation onto the flow-detection and demand side, then walk that side in order.

Symptom presentation

The complaint is no hot water on demand: open a hot tap and the unit stays dark, no ignition attempt, or it lights then quits immediately. In service/test mode (the unit's forced firing diagnostic) it fires and holds a clean flame. The fault commonly appears worst at low-flow fixtures (a trickling bathroom faucet or a water-saver showerhead) and may work at a wide-open tub. Aerator debris, a partly closed isolation valve, a new low-flow fixture, or a fresh softener/filter install often precedes it.

Quick checks

• Open a high-flow fixture (tub spout, hose bib downstream) fully and watch for ignition. If it fires at high flow but not at a trickle, the issue is minimum-activation flow, not ignition.
• Check the cold inlet path: isolation valve fully open, inlet filter screen clean, and no debris in the flow sensor. Tankless units have a small inlet screen that clogs and starves flow.
• Verify the fixture and aerator are not clogging flow below the unit's minimum activation rate (commonly around 0.4 to 0.5 gpm to start).
• Confirm the unit has power and is not locked out on a fault code; a hard lockout blocks demand firing but a manual test may still run.

Isolation tree

The unit fires on test, so gas/ignition is good. Work the flow-and-demand branch.

• Branch A: Insufficient activation flow. The fixture/showerhead delivers less than the unit's minimum activation flow, so the flow sensor never signals demand. Worst on low-flow fixtures; works on a wide-open tap. Confirm gpm at the fixture. Clean aerators, open valves, or address the low-flow fixture.
• Branch B: Inlet filter / flow path restricted. A clogged inlet screen, a partly closed service valve, a fouled softener/filter just installed, or scale in the heat exchanger drops flow below threshold. Clean the inlet screen and confirm full inlet flow.
• Branch C: Flow sensor (turbine) stuck or failed. Debris jamming the impeller turbine, or a failed flow-sensor signal, means the unit never sees demand even with good flow. Inspect/clean the turbine; test the sensor's pulse output.
• Branch D: Crossover / mixing bleeding flow. A failed mixing valve, a crossed hot-cold connection, or a recirc check stuck open lets cold bypass and hot drain back, so opening a hot tap does not produce enough net flow through the unit to trigger it. Isolate and test for crossover.
• Branch E: Hot trickle / temperature-based start delay. Some units throttle or delay when inlet is already warm or flow is marginal; combined with a low-flow fixture this reads as no-fire. Confirm with a clean high-flow draw.
• Branch F: Control/lockout. A unit in a soft or hard lockout from a prior fault will not fire on demand though a forced test runs. Read and clear the fault code, then address its cause.

Confirming diagnosis

• Flow-threshold: measure gpm at the calling fixture with a flow bag/meter. A rate below the unit's published minimum activation flow with no ignition attempt confirms a flow-threshold fault, not an ignition fault.
• Inlet restriction: remove and inspect the inlet filter screen; a clogged screen plus restored firing after cleaning confirms it.
• Flow sensor: with adequate flow proven at the inlet, no demand signal confirms a stuck/failed turbine sensor.
• Crossover: shut the cold supply to the unit and open a hot tap; continued flow indicates a cross-connection bleeding the system and starving the unit's flow detection.
• Lockout: the unit's display code identifies a control lockout distinct from a no-demand condition.

Remediation

Restore adequate triggering flow: clean aerators and the cold-inlet filter screen, fully open both isolation valves on the service kit, and replace or adjust low-flow fixtures that fall below the unit's activation threshold. The inlet screen is the single most common field cause and the fastest to check and clean. Clean or replace a fouled flow-sensor turbine so the impeller spins freely. Descale the heat exchanger if scale is restricting flow, which is common after a softener swap exposes loosened scale or on hard water with no treatment. Correct any crossover at a failed mixing valve, a mis-plumbed hot-cold connection, or a stuck-open recirc check that bleeds flow and starves the sensor. Clear and resolve any control lockout, addressing its underlying cause rather than just resetting it. After repair, confirm the unit fires reliably on demand at the lowest-flow fixture in the home, holds a stable flame, and delivers hot water at that fixture, not just at a wide-open tub spout.

References

• NFPA 54 / ANSI Z223.1 National Fuel Gas Code (gas-fired instantaneous water heater installation, gas supply, and venting).
• ANSI Z21.10.3 / CSA standard for gas instantaneous (tankless) water heaters.
• Manufacturer installation and service manual for the specific tankless model (minimum activation flow rate, inlet filter, flow-sensor service, descaling, fault codes).
• AWWA water-quality guidance on hardness and scale effects on tankless heat exchangers and flow restriction.