When a tankless water heater can no longer accurately sense how much water is moving through it, ignition and temperature control both suffer.
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Quick answer
Flow sensor replacement on a tankless water heater is typically considered when the unit fails to ignite at low flow, shows temperature swings during otherwise steady use, or throws a flow-related fault code, and cleaning the sensor does not resolve the issue. A technician first checks the inlet filter and cleans the sensor before recommending replacement, since fouling rather than outright failure is the more common cause. Cost depends mainly on whether cleaning is enough versus a full sensor swap, unit accessibility, and whether related inlet-side components are serviced at the same time.
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| Sign | What it suggests |
|---|---|
| Unit will not ignite with a single low-flow fixture running | Sensor is not registering enough flow to reach the ignition threshold |
| Water temperature fluctuates during use even with steady flow at the tap | Inconsistent flow signal is causing the burner to over- or under-modulate |
| A flow-related fault code appears on the display | Control board is not receiving a valid or expected signal from the sensor |
| Unit has not had inlet-side service in several years | Scale or sediment has had time to accumulate on the sensor paddle |
These signs can also point to a clogged inlet filter or genuinely low water pressure, which is why diagnosis, not just symptom matching, determines the right fix.
A technician starts by checking actual flow and pressure at the unit's inlet, then inspects the inlet filter for clogging, since a dirty filter reduces real flow and can look identical to a flow sensor problem. The sensor itself is then inspected for scale, debris, and free rotation, and cleaned if fouling is found, with the unit retested before considering replacement.
If cleaning does not restore an accurate signal, or the sensor's paddle or magnetic pickup shows physical wear, replacement is the recommended next step.
Because the correct fix depends on what is found during inspection, cost is best treated as a ballpark, not a quote, confirmed after a technician has examined the inlet assembly rather than estimated beforehand.
A flow sensor repair is routine and does not suggest the unit needs replacing on its own. It becomes part of a larger conversation only if it is one of several age-related issues showing up on the same unit, in which case a technician can discuss whether continued repairs still make sense.
As a ballpark, not a quote, a new tankless installation in Metro Vancouver typically runs $3,500 to $7,500, which is relevant only if the existing unit is genuinely reaching the end of its service life rather than dealing with an isolated sensor issue.
Periodic inlet filter cleaning as part of routine tankless maintenance is one of the more effective ways to keep sediment and debris away from the flow sensor in the first place, reducing how often the sensor itself needs attention.
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.
No, it is a functional issue rather than a safety emergency, though it should be addressed since it affects the unit's ability to ignite and regulate temperature properly.
Yes, genuinely low pressure or flow can produce similar symptoms to a faulty flow sensor, which is why a technician measures actual flow before diagnosing the sensor as the cause.
If the flow sensor is confirmed as the cause, yes; if the real issue is elsewhere, such as the heat exchanger or gas valve, replacing the sensor alone will not resolve it.
There is no fixed interval, but including it in periodic tankless maintenance along with the inlet filter is a reasonable approach.
Typically the inlet needs to be isolated and depressurized to access the sensor, which a technician handles as part of the repair.
Warranty coverage is set by the manufacturer and varies by model and age, so it is worth checking your warranty certificate or having a technician confirm.
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