A flow-related fault on a boiler can trace back to the flow switch itself, the circulator pump, or trapped air, which is why diagnosis matters before parts are ordered.
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Quick answer
Flow switch replacement is considered when a boiler will not fire, or fires and shuts down repeatedly, with a flow-related fault, and a technician has confirmed the circulator pump is running and water is genuinely circulating. Because low flow can be caused by the switch itself, the pump, or trapped air, a proper diagnostic check comes first. Cost depends mainly on the switch's location and accessibility within the boiler, whether the system needs draining and refilling to access it, and whether related components are addressed on the same visit.
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| Sign | What it suggests |
|---|---|
| Boiler will not fire, flow-related fault shown | Switch may be stuck closed, or genuine flow issue elsewhere needs ruling out |
| Boiler fires then shuts down shortly after, repeatedly | Intermittent or marginal flow confirmation |
| Fault appeared right after a drain-down, refill, or repair | Trapped air is a likely contributor and should be checked first |
| Circulator pump sounds weak, noisy, or is not running | Pump issue may be the actual root cause rather than the switch |
These signs are a starting point for a service call rather than a firm diagnosis, since several different issues can produce the same flow-related fault.
The technician first confirms the circulator pump is actually running and moving water through the system, and checks for trapped air, especially if the system was recently serviced or refilled. Only once genuine flow is confirmed and the switch is still not reporting it correctly does attention turn to the switch itself, its paddle, wiring, and microswitch.
This order matters because replacing the flow switch when the real cause is a weak pump or an air pocket will not resolve the fault, and the boiler will continue to show the same problem after the part is swapped.
Because the scope of the actual repair depends on the diagnosis, cost is best treated as a ballpark, not a quote, confirmed after a technician has assessed the system rather than estimated over the phone.
A flow switch replacement, once genuinely confirmed as the cause, is a routine boiler repair. It only becomes part of a larger replacement conversation if the boiler is showing several other age-related issues at the same time, in which case a technician can discuss whether continued repairs remain the right call.
As a ballpark, not a quote, full boiler replacement in Metro Vancouver typically runs $7,500 to $15,000-plus, which is a different scale of decision entirely from a targeted flow switch or pump repair.
Because a flow-related fault can come from the switch, the pump, or trapped air, paying for a diagnostic check before parts are ordered is what prevents replacing a perfectly good component while the actual cause remains unresolved.
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.
Yes, a weak or failing pump can produce the exact same flow-related fault as a bad switch, which is why the pump is checked first.
Yes, especially right after the system has been drained and refilled; purging air properly often resolves the fault without any parts needing replacement.
It depends on the switch's location and whether the system needs to be drained down, but it is generally a routine, same-visit repair once diagnosed.
The fault itself is the boiler's safety system correctly preventing it from firing without confirmed flow, so it should be diagnosed and fixed rather than bypassed or ignored.
Yes, debris in the system water can jam a paddle-type switch, which is more common on older systems without well-maintained water chemistry.
If the switch is confirmed as the cause, yes; if the actual cause is the pump or trapped air, those need to be addressed instead for the shutdowns to stop.
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