A furnace that never seems to shut off is either undersized for the job, losing heat somewhere, or reading the temperature wrong — three very different fixes.
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Quick answer
A furnace running almost continuously without the house feeling adequately warm usually comes down to one of three things: the furnace is undersized or underperforming for the space, heat is escaping faster than the furnace can replace it, or the thermostat isn't reading the room accurately. Distinguishing between these starts with noticing whether the house is actually getting warm eventually, or never quite catching up at all.
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This distinction matters more than it might seem. A furnace that runs long cycles but does eventually bring the house up to temperature, especially during a genuine cold snap, may just be working hard on a legitimately cold day — that's not necessarily a problem. A furnace that runs continuously and the house still feels several degrees below the thermostat setting is a different situation, and points toward the furnace not being able to keep pace with heat loss or demand at all.
A thermostat mounted somewhere unusual — a hallway with poor air circulation, a spot in a cold draft, or on an exterior wall — can read the temperature inaccurately and keep calling for heat even after the rest of the house is genuinely comfortable. This is one of the easier things to rule out: compare the thermostat's reading to a separate room thermometer placed a few feet away, away from any vent or draft.
Character homes with original single-pane windows, minimal wall insulation by today's standards, and a furnace that was never resized after later renovations are a common combination behind a furnace that runs constantly without ever quite winning. Adding a basement suite or a laneway house onto ductwork designed for the original house alone can have a similar effect, spreading the same furnace's output across more square footage than it was ever sized to heat efficiently. None of this means the furnace is broken — it often means the equipment and the current demand on the house have drifted apart from each other over the years.
Rather than assuming the furnace itself is faulty, the technician checks airflow and static pressure in the ductwork, confirms the blower is performing to spec, and compares the furnace's rated output against a rough sense of the home's actual heating load. If the equipment is genuinely undersized for the current house, that's a different conversation than a repair — sometimes it's about sealing and insulation improvements elsewhere in the home rather than anything mechanical with the furnace at all.
Safety note
If a constantly running furnace is ever accompanied by a burning smell, a gas odour, or unusual sounds, leave the house and call FortisBC's emergency line at 1-800-663-9911 from outside. Diagnosing airflow, blower or gas pressure issues is work for a Technical Safety BC licensed gas fitter.
Yes, to a point — longer cycles during a genuine cold snap are expected. What's worth investigating is a furnace that runs almost continuously and the house still doesn't feel like it's catching up to the thermostat setting.
Yes. A thermostat in a drafty spot, a poorly circulated hallway, or on an exterior wall can read colder than the rest of the house actually is, keeping the furnace running longer than it needs to.
It can, if the added space draws on ductwork that was originally sized for the house alone. That's worth mentioning to a technician, since it changes whether the fix is mechanical or about the overall system's capacity.
Sometimes, but not always — heat loss through old windows or insulation can outpace almost any furnace size, and oversizing brings its own problems like short cycling. A proper assessment looks at the whole picture, not just furnace output.
A furnace running far more than it should does mean more gas use, though the exact impact depends on how much more it's running and the underlying cause. Addressing the root issue, whatever it turns out to be, is the more useful long-term fix than accepting higher bills.
Yes — a blower running slower than it should due to a failing capacitor or motor issue can cause exactly this symptom, and that's a straightforward repair once it's diagnosed rather than a bigger heat-loss question.
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