A boiler that ignites and then shuts down within seconds or minutes, over and over, is almost always a safety control doing exactly what it is designed to do.
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Quick answer
A gas boiler that fires and then shuts off shortly after, sometimes repeatedly, is typically being stopped by a safety control that has detected a real problem: a flame sensor failing to confirm rectification, a blocked or restricted flue triggering a pressure switch, an overheating limit tripping from restricted water flow, or a dirty flame sensor giving an inconsistent signal. This pattern is called short cycling, and a licensed technician diagnoses which specific control is intervening rather than just resetting the boiler repeatedly.
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Every gas boiler has multiple safety controls stacked between ignition and continuous run: a flame sensor confirms combustion is actually happening after the gas valve opens, a high-limit control shuts the burner down if water temperature rises too fast or too high, and a pressure switch confirms the venting system is drawing properly before allowing gas flow to continue. When a boiler fires and then shuts down within seconds to a couple of minutes, one of these controls has detected a condition outside its allowed range and interrupted the cycle, which is the system doing its job rather than malfunctioning randomly.
A flame sensor that is coated with a thin layer of combustion byproduct can still allow ignition but fail to reliably confirm the flame afterward, so the control board cuts gas flow as a precaution and the cycle repeats. This is one of the more routine causes and is often resolved with a simple cleaning. Restricted water flow through the heat exchanger, whether from a closed valve, a failed circulator, or sludge buildup in an older system, can cause the water to overheat locally even though the boiler just started, tripping the high-limit control almost immediately. A venting restriction, such as debris, ice, or a bird's nest at the termination, or a flue that is not sloped or sized correctly, can prevent the pressure switch from confirming safe exhaust flow, and the boiler shuts down before combustion is allowed to continue.
Gas supply issues, such as a partially closed shutoff valve or low incoming gas pressure, can also cause the flame to establish weakly and then fail rectification, producing the same fire-then-shutdown pattern from a completely different root cause.
| What you might notice | Likely control involved | Likely root cause |
|---|---|---|
| Fires, runs 5-15 seconds, shuts down | Flame sensor / flame rectification | Dirty or degraded flame sensor, weak flame |
| Fires, runs a minute or two, shuts down hot | High-limit control | Restricted water flow, closed valve, sludge, dead pump |
| Won't stay running, especially in windy weather | Vent pressure switch | Vent blockage, termination clearance, condensate backup |
| Repeated attempts before final lockout | Ignition control / lockout logic | Weak gas pressure, faulty igniter, wiring issue |
Note how many seconds or minutes pass between ignition and shutdown, since that timing alone helps narrow down which control is likely tripping. Check whether any circulator zone valves or radiator valves have been recently closed or adjusted. Look outside at the vent termination for obvious blockage such as snow, leaves, or nesting material, which is common in fall and after wind events in Metro Vancouver. Do not repeatedly cycle the power or reset button more than the manufacturer's instructions allow, since repeated attempts without addressing the cause can push some units into a full hard lockout requiring a technician reset regardless.
Most causes of this pattern, a sensor cleaning, a stuck valve, a blocked vent, are inexpensive, quick repairs. Where it becomes a bigger conversation is a boiler with a genuinely failing heat exchanger causing chronic flow-related overheating, or one old enough that control board and sensor issues keep recurring across multiple service calls. In that situation a technician can walk through the tradeoff between continued repairs and a boiler replacement, which ballparks $7,500 to $15,000 or more depending on system size and complexity, not a quote.
Safety note
A boiler that repeatedly attempts ignition and shuts down should not be reset over and over without a diagnosis, since one possible cause is a genuine combustion or venting fault. If you smell gas, see soot around the boiler, or notice a burning odor, leave the home immediately and call FortisBC at 1-800-663-9911.
Resetting once or twice per the manufacturer's instructions is usually fine, but repeated resets without addressing the underlying cause can mask a real safety fault and should be followed by a technician visit rather than continued resets.
Wind can affect how a boiler's vent draws or drains, and a marginal vent installation or partial blockage can be pushed past its limit specifically during gusty weather, which is a pattern worth mentioning to your technician.
Yes, low system water pressure or a closed valve restricting flow to the heat exchanger commonly trips the high-limit control almost immediately after ignition, which looks similar from the outside but has a different fix than a gas pressure issue.
There is no fixed universal interval since it depends on the specific boiler and fuel quality, but it is a routine item on an annual maintenance visit.
Not usually. Most causes are individual component issues that are repaired rather than requiring replacement, unless the pattern is caused by a heat exchanger that is failing on an older unit.
Cold weather can indirectly contribute if it causes condensate to freeze in the vent or drain, or if a marginal vent installation performs worse under certain wind and temperature conditions, but the underlying cause still needs to be identified rather than assumed.
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