Before a tankless unit lights its burner, it first has to know water is actually moving through it, and that job belongs to the flow sensor.
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Quick answer
The flow sensor in a tankless water heater detects when water is moving through the unit, usually with a small turbine or paddle wheel that spins as water passes and sends a signal to the control board, which then triggers ignition and modulates burner output to match demand. Flow sensors fail mainly from mineral scale or debris fouling the paddle, mechanical wear on the turbine, or a worn magnetic pickup giving an inaccurate reading, which shows up as the unit failing to ignite on light flow, misjudging output, or not firing at all.
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A tankless water heater only needs to produce hot water while it is actually being drawn, which means the unit has to know, moment to moment, whether water is flowing through it and roughly how much. The flow sensor handles that job, typically using a small turbine or paddle wheel mounted in the cold water inlet path that spins as water passes through it. A magnetic pickup or similar sensor reads the speed of that spinning component and reports it to the control board as a flow rate.
The control board uses that reading for two things: deciding whether to ignite the burner at all, since most units require a minimum flow rate before they will fire, and modulating how much gas and burner output to apply so the outgoing water reaches the target temperature without wide swings as flow changes.
The turbine or paddle wheel is a small moving part sitting directly in the water stream, which makes it vulnerable to anything carried in that water. Mineral scale can build up on the paddle over time and either slow its rotation or cause it to stick, giving the control board an inaccurate reading even though water is actually flowing normally. Debris such as sediment from aging pipes or particles introduced during plumbing work can jam the paddle mechanically.
Beyond fouling, the magnetic pickup or sensor itself can wear or weaken over years of use, which produces an inaccurate or intermittent signal even with a clean, freely spinning paddle. Metro Vancouver's municipal reservoir water is relatively soft, which reduces scale buildup compared with hard-water regions, but sediment from older galvanized plumbing or from properties on well water in parts of the Fraser Valley can still be a factor.
| Symptom | Likely cause |
|---|---|
| Unit does not ignite even though a tap is fully open | Flow sensor is not registering enough flow to meet the minimum ignition threshold |
| Water temperature swings even though flow at the tap seems steady | Sensor reading is inconsistent, causing the burner to over- or under-modulate |
| Unit works fine at high flow but struggles at low flow, such as a single low-flow fixture | Paddle may be partially fouled, affecting sensitivity at low rotation speeds |
| Fault code related to flow appears on the unit's display | Control board is not receiving an expected or valid flow signal |
Because low flow rate situations, such as a single bathroom faucet, are inherently closer to the ignition threshold, a partially fouled flow sensor often shows up first as trouble with smaller fixtures before it becomes obvious on a full shower.
Diagnosis typically starts with checking the inlet water filter, since a clogged filter reduces actual flow and can mimic a flow sensor problem. The technician then inspects the sensor itself, checking whether the paddle spins freely and whether the reading it produces matches the actual flow through the unit. Descaling the sensor and surrounding fittings is a common first step if scale or sediment is visible.
If the sensor still gives an unreliable reading after cleaning, or the paddle or magnetic pickup shows physical wear, the sensor is replaced. Because the flow sensor usually sits close to the inlet filter and isolation valves, servicing it is often paired with a general inlet-side inspection.
Flow sensor issues are a routine, serviceable repair and are not a sign the unit itself needs replacing. Cleaning or descaling resolves many cases, with outright sensor replacement reserved for units where the sensor shows physical wear or the reading does not stabilize after cleaning.
Any work on the gas line, gas control valve, burner, or venting of a natural gas appliance must be done by a Technical Safety BC licensed gas fitter; CANRO holds gas licence LGA0043870 and boiler licence LBP0046450. If you ever smell gas, hear a hissing sound near the appliance, or notice soot, a burnt smell, or headaches and nausea that ease when you leave the house, leave the home immediately and call FortisBC at 1-800-663-9911 from outside before doing anything else.
Safety note
Any work on the gas line, gas control valve, burner, or venting of a natural gas appliance must be done by a Technical Safety BC licensed gas fitter; CANRO holds gas licence LGA0043870 and boiler licence LBP0046450. If you ever smell gas, hear a hissing sound near the appliance, or notice soot, a burnt smell, or headaches and nausea that ease when you leave the house, leave the home immediately and call FortisBC at 1-800-663-9911 from outside before doing anything else.
Most units require a minimum flow to ignite, and a partially fouled or worn flow sensor can push that threshold higher than it should be, so light demand fails to trigger ignition.
Often yes, since scale or debris fouling is a common cause, and cleaning the paddle and surrounding area frequently restores an accurate reading.
No, they are separate components that usually sit near each other; the inlet filter screens out debris, while the flow sensor measures how much water is actually passing through.
The municipal reservoir water is relatively soft, so scale buildup is less of a factor here than in hard-water regions, though sediment from aging plumbing can still be a contributor.
Yes, an inconsistent flow reading can cause the burner to over- or under-modulate, which shows up as temperature fluctuating even though the tap itself is steady.
A technician measures actual flow at the unit and compares it with what the sensor is reporting; a mismatch between the two points to the sensor rather than genuinely low water flow.
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