A boiler will not let its burner fire without confirmation that water is actually moving through the heat exchanger, and the flow switch is what provides that confirmation.
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Quick answer
A flow switch in a gas boiler confirms that water is actually circulating through the system before allowing the burner to fire, protecting the heat exchanger from overheating if it were to fire with stagnant or no water flow. It is typically a paddle-type or pressure-differential switch positioned in the water circuit. Flow switches fail from a paddle jammed by debris, a worn microswitch, or calcification, and the symptoms often overlap with circulator pump problems or trapped air, so proper diagnosis is needed before replacing parts.
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A gas boiler's heat exchanger is designed to transfer heat into moving water. If the burner were to fire while water in the system was not actually circulating, the heat exchanger could overheat locally, a genuinely damaging condition for the boiler. The flow switch exists to prevent that by confirming actual water movement before the control board allows ignition.
Depending on the boiler design, this is done either with a paddle-type switch, where a small vane physically deflects in the moving water and trips a microswitch, or a pressure-differential switch, which senses the pressure drop across the circulator pump that only exists when water is actually flowing. Either way, the switch's job is the same: no confirmed flow, no burner.
A paddle-type switch can fail if debris in the system water jams the paddle in a fixed position, either stuck open, giving a false confirmation of flow that is not really adequate, or stuck closed, preventing the boiler from firing even when water is genuinely circulating. The microswitch that the paddle triggers can also wear out mechanically after years of repeated cycling.
Calcification or scale on the paddle or switch housing can also interfere with free movement, particularly in systems where water chemistry has not been well maintained. On pressure-differential type switches, a failing circulator pump can itself cause the switch to read low flow even though the switch itself is functioning correctly, which is why a flow switch fault and a pump fault can look identical from the homeowner's perspective.
| Symptom | Possible cause |
|---|---|
| Boiler will not fire at all, showing a flow-related fault | Switch may be stuck closed, or genuinely low or no flow due to a pump or air lock issue |
| Boiler fires but shuts down shortly after, repeatedly | Intermittent flow confirmation, possibly a partially jammed paddle or failing microswitch |
| System was recently drained, refilled, or serviced | Trapped air can cause genuine low-flow conditions that mimic a switch failure |
| Circulator pump is noisy, weak, or not running | The actual root cause may be the pump rather than the flow switch itself |
Because a healthy flow switch reporting genuinely low flow looks the same, from a fault code perspective, as a faulty flow switch giving a false reading, distinguishing between the two requires actually checking whether water is moving, not just replacing the switch.
Diagnosis starts by confirming the circulator pump is running and that water is genuinely circulating through the system, since a pump or air lock issue is a common cause of what looks like a flow switch fault. If flow is confirmed adequate but the switch is still not reporting it correctly, the switch itself, its paddle, and its wiring are inspected for jamming, wear, or calcification.
This sequence matters because replacing a flow switch when the real problem is a failing pump or trapped air will not fix the underlying symptom, leaving the homeowner with an unresolved boiler and an unnecessary parts cost.
A flow switch itself is a relatively simple, low-cost component to replace once genuinely confirmed as the cause. The bigger diagnostic effort is in ruling out the circulator pump, trapped air, or a partially closed isolation valve elsewhere in the system, all of which can present with the exact same symptom.
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.
Not necessarily; a flow switch fault code simply means the boiler is not confirming adequate flow, which can be caused by the switch itself, the pump, or trapped air in the system.
By directly checking whether the pump is running and water is actually circulating before inspecting the switch itself, rather than assuming the switch is at fault from the fault code alone.
Yes, air pockets in the system after a drain-down, refill, or repair can cause genuinely reduced flow that trips the switch even though the switch itself is working correctly.
Yes, it protects the heat exchanger from firing without adequate water flow, which is a genuinely damaging condition, so a confirmed flow switch fault should not be bypassed.
Yes, debris circulating in the system water can jam a paddle-type flow switch, which is more likely on older systems that have not had their water chemistry maintained.
Depending on the switch's location, some drain-down may be needed to access and replace it, followed by refilling and purging air from the system afterward.
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