Why does a furnace sometimes smell like dust burning when it first turns on for the season?
A light, brief dust smell on the first startup of the season is common — dust settles on the heat exchanger and burner over the off months and burns off quickly. A smell that's strong, lingers, or comes with visible smoke is different and worth turning the system off to check.
Why does a furnace need combustion air, and can that be blocked accidentally?
A furnace burns fuel and needs a fresh air supply to do it safely and completely — boxes, laundry, or clutter stacked against or too close to the unit can restrict that air supply, which affects both performance and, in worse cases, combustion safety.
How does a dirty filter affect furnace performance?
A clogged filter restricts airflow across the heat exchanger, which can cause overheating, short-cycling, and in worse cases trip the limit switch repeatedly. It's the single most preventable cause of a furnace service call.
Why does the furnace flame look yellow instead of blue?
A healthy gas flame burns steady blue. Yellow or flickering usually means incomplete combustion — often a dirty burner or airflow problem — which can produce excess carbon monoxide. Worth a same-day call, not a wait-and-see.
What causes a furnace to short-cycle?
A dirty flame sensor, a clogged filter restricting airflow, or a furnace tripping its high-limit switch from overheating are the most common causes. Short-cycling accelerates wear on the ignitor and blower motor.
Do all furnaces still use a pilot light?
Most furnaces built in the last couple of decades use an electric hot surface ignitor instead of a standing pilot flame — it glows hot enough to ignite gas on demand, saving the energy a pilot flame burns constantly. Both can fail and both are repairable.
Is a 95% AFUE furnace worth the difference over an 80% model?
AFUE (Annual Fuel Utilization Efficiency) is the percentage of fuel that actually becomes heat rather than escaping as exhaust. An 80% AFUE furnace vents more like a traditional flue; a 95%+ condensing furnace vents differently (usually PVC pipe) and typically costs less to run, though the venting change itself adds to install complexity on a replacement.
Can a cracked heat exchanger be seen just by looking at it?
A visual check with a flashlight and mirror catches some cracks, but many are only detectable with a combustion analyzer that checks for CO leaking into the air stream, or a specialized camera. That's part of why a proper inspection takes more than a quick look inside the cabinet.
What's a C-wire, and why do some furnaces need one?
The C-wire (common wire) provides continuous 24V power to a thermostat, which smart thermostats especially rely on. Without it, some thermostats run on batteries or borrowed power and can lose connection intermittently.
What does a cracked heat exchanger mean?
The heat exchanger separates the combustion gases from the air that circulates through the house. A crack can let carbon monoxide into that airstream, which is why a cracked exchanger is treated as a safety issue, not just a repair item.
What should you do if you smell gas near the furnace?
Leave the area, don't operate switches or the thermostat on your way out, call the gas utility's emergency line from outside, and then call a contractor. This is the one furnace symptom that isn't a same-day call — it's an immediate one.
What's the difference between single-stage, two-stage, and modulating furnaces?
Single-stage furnaces run at one output level only; two-stage units can run a lower output for milder days; modulating furnaces adjust output across a wider range continuously. More stages generally mean steadier temperatures and less on/off cycling, at a higher equipment cost.
What triggers a furnace's limit switch?
The limit switch trips when the furnace's internal temperature climbs too high, usually from restricted airflow — a clogged filter, closed vents, or a failing blower motor. It's a safety shutoff working correctly, not a random glitch.
Does furnace size affect how evenly a house heats?
An uneven-heating complaint is more often a duct-balancing or airflow issue than a furnace-sizing one — closed vents, long duct runs, or a return-air imbalance can all leave one room behind even when the furnace itself is correctly sized for the whole house.
Should a CO alarm near the furnace ever be ignored?
Never ignore it and never just reset it. Leave the home, get to fresh air, and call from outside — a sounding CO alarm is treated as an active emergency, not a furnace symptom to schedule around.
What causes a pilot light to keep going out?
Often a weak or misaligned thermocouple, which senses the flame and shuts off gas flow if it can't detect it — a safety feature, not a malfunction on its own, though the thermocouple itself may need replacing.
Is bigger always safer when it comes to furnace size?
Yes — an oversized furnace short-cycles the same way an oversized AC does, wearing components faster and producing less even heat than a correctly sized unit running longer, gentler cycles. Bigger isn't a safety margin here, it's a sizing mistake.
What causes a boom when the furnace kicks on?
Usually delayed ignition — gas builds up slightly before catching, then ignites all at once. It can also stress the heat exchanger over repeated occurrences, so it's worth diagnosing rather than living with.
How long does a furnace typically last before replacement makes more sense than repair?
Most furnaces run 15 to 20 years with regular maintenance. Past that range, especially with a heat exchanger issue, a contractor can walk through whether the specific repair still makes sense versus replacement.
Is it normal for a furnace to run for very long stretches in cold weather?
Long, steady runtime during the coldest stretch of winter is normal and often more efficient than frequent short cycles. A furnace that runs constantly and still can't reach the setpoint, though, points to a sizing, airflow, or mechanical issue worth having looked at.