How does a gas furnace actually work?
A gas furnace burns fuel inside a sealed metal heat exchanger, then blows your home's air across the outside of that hot metal — the air you breathe never touches the flame or combustion gases, which vent outdoors through the flue. Everything else on the spec sheet (AFUE, stages, variable speed) describes how efficiently and smoothly the furnace delivers that heat across Puget Sound's long, mild heating season. Understanding that sequence is what makes repair quotes, efficiency tiers, and heat-pump comparisons readable instead of opaque.
The quick answer
- Air is heated by passing over a sealed heat exchanger — combustion gases vent outside
- AFUE is the share of fuel that becomes heat in your home: ~80% on older units, 90–99% on modern condensing furnaces
- Single-stage furnaces are all-or-nothing; two-stage and modulating units cruise at partial output
- Seattle's long, mild season is exactly where staging and variable speed earn their keep
Why understanding the sequence helps you
When you know the firing sequence, technician-speak turns into plain English. 'Flame sensor,' 'ignitor,' 'inducer,' and 'limit switch' are steps in a chain — and where the chain stops tells you what failed and roughly what fixing it costs.
It also helps you read a replacement quote. The price differences between models are mostly about the heat exchanger design (80% vs condensing) and the burner/blower staging — not mystery.
In Greater Seattle's October-through-May heating season, a staged furnace runs far more hours at partial output than a single-stage unit ever will. That's why two-stage and modulating equipment earns its premium here in comfort and wear — not in a climate where the furnace only fires on the coldest days.
The firing sequence, step by step
Your thermostat calls for heat. The inducer fan starts first, pulling fresh air through the burner box and proving — via a pressure switch — that exhaust can safely vent. The ignitor glows, the gas valve opens, burners light, and a flame sensor confirms fire within seconds or shuts the gas off.
The heat exchanger warms up, and only then does the main blower start, pushing house air across the hot metal and out through your supply ducts. When the thermostat is satisfied, gas stops first and the blower runs on to harvest the leftover heat.
What AFUE actually measures
AFUE (Annual Fuel Utilization Efficiency) is the percentage of the fuel's energy that ends up as heat in your home over a season. An older 80% furnace sends roughly a fifth of every gas dollar up the flue. Modern condensing furnaces (90–99% AFUE) add a second heat exchanger that wrings heat out of the exhaust until the water vapor in it condenses — that's why they drain water and vent through PVC instead of metal flue pipe.
Single-stage vs two-stage vs modulating
A single-stage furnace has one output: full blast. Two-stage units add a low fire (~65–70% output) they use most of the time, and modulating furnaces adjust in small steps. In a climate like ours — hundreds of mild 40–50°F days — a staged furnace runs longer, gentler cycles: steadier temperatures, quieter operation, better filtration, and less of the stress that wears parts out.
Where furnaces go wrong
Most no-heat calls trace to the cheap links in the chain: a dirty flame sensor, a failed hot-surface ignitor, a clogged filter tripping the high-limit switch. The expensive failure is the heat exchanger itself — cracks from years of overheating (usually airflow neglect) end the furnace, because replacement rarely pencils out on an old unit.
The safety systems are not optional equipment. A furnace that keeps shutting itself off is protecting you — bypassing switches to 'keep it running' is how carbon monoxide problems happen. Diagnose the cause instead.
Across Burien and West Seattle, the most common no-heat calls in the first cold snap trace to a dirty flame sensor or a filter so clogged the high-limit trips — both are tune-up items that cost far less than an emergency visit. That's the practical reason to understand the firing sequence before winter.
Questions Seattle homeowners ask
Does the air in my ducts touch the flame?
No. Combustion happens inside a sealed heat exchanger; your house air passes over the outside of that metal. Exhaust gases vent outdoors through the flue. That separation is exactly why a cracked heat exchanger is treated so seriously.
Is a 95% AFUE furnace worth it over an 80% one?
In Seattle's long heating season, usually yes — you keep 15+ more cents of every gas dollar, every hour it runs, and current Washington energy code strongly favors condensing equipment in most replacements. The honest caveat: venting changes (PVC out the side wall, condensate drain) are part of the install cost. PSE pays conversion rebates ($3,000–$4,000) when you go further and replace with a heat pump instead.
What's the clicking/whooshing sequence when my furnace starts?
That's the normal chain: inducer fan hum, ignitor click or glow, the whoosh of burners lighting, then the main blower ramping up about a minute later. What's not normal: repeated clicking without ignition (failed ignitor or gas valve issue), booms on startup (delayed ignition — call promptly), or the blower running cold air for long stretches without burners ever lighting.
Why does my furnace keep running after the heat shuts off?
The blower intentionally runs 60–90 seconds past the burners to move the remaining heat out of the exchanger — it protects the metal and saves the heat you paid for. Constant running beyond that usually means the fan switch is set to ON instead of AUTO, or a control issue worth a look.
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How we know this
Mechanical descriptions reflect standard forced-air furnace design and what Glendale technicians service daily across Greater Seattle. Efficiency definitions follow U.S. DOE AFUE standards.
Reviewed by the Glendale Heating & Air Conditioning team · Last updated 2026-07-15
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