Component
Flow sensor
The part that detects water flow and tells the control to fire. A fouled sensor, or flow below the activation minimum, means no ignition even though everything else is healthy.
The trigger for everything
A tankless unit is dormant until water moves through it: the flow sensor (a turbine or similar device in the water path) reports the draw to the control, which then starts the fan, ignition and burner sequence. If the sensor cannot see the flow, the unit never fires, and if it misreads flow, the burner modulates wrongly.
Every tankless platform publishes a minimum activation flow: Rinnai's SENSEI units need about 0.4 GPM (1.5 L/min), and Navien lists 0.5 GPM on its A2 specifications, with a lower-draw option in recirculation modes. Very low-flow fixtures can sit below the threshold and never trigger the unit.
Fouling and failure
The sensor sits behind the cold-water inlet filter, which is why manufacturers clean that screen at service: debris that passes it can foul the sensor's turbine. A sensor that fails is tested by its output against the flow, and the inlet filter and sensor cleaning are part of the descaling visit.
Frequently asked questions
Why does my tankless unit not fire on a trickle of water?
Flow below the activation minimum (about 0.4 to 0.5 GPM on most units) does not trigger firing; the flow must reach the threshold. Low-flow fixtures and aerators can sit below it.
Can a dirty filter stop the unit firing?
Yes: if the inlet filter is clogged, the flow sensor sees less flow and the unit may not fire or may modulate wrongly. Filter cleaning is part of the service.
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Sources and verification
- CANRO Heating & Cooling — About · Canadian source · accessed September 7, 2026
- Rinnai RU series Installation and Operation Manual (activation flow) · 100000467 · accessed September 7, 2026
- Navien — NPE-240A2 specifications (minimum flow) · Canadian source · accessed September 7, 2026
Facts checked September 7, 2026. Last updated September 7, 2026. Technical reviewer: not yet assigned.