How a Gas Furnace Works (and Why It Stops)

When your thermostat calls for heat: the inducer fan starts and proves airflow through a pressure switch, the igniter glows or sparks, the gas valve opens, burners light, and a flame sensor confirms combustion within seconds. The burners heat a metal heat exchanger; once it's warm, the blower pushes household air across it and into your ducts. Exhaust gases vent outside the whole time.

Each step is a safety gate — if any check fails, the furnace shuts itself down rather than run unsafely. That's why "furnace clicks but never lights" is such a common winter call: one failed sensor stops the whole chain.

AFUE (Annual Fuel Utilization Efficiency) is the percentage of fuel that becomes usable heat. An older 80% furnace sends roughly 20 cents of every fuel dollar up the flue; a modern condensing furnace at 95–98% captures nearly all of it by wringing extra heat out of the exhaust with a second heat exchanger.

In a heating climate like central Iowa — January lows around 14°F, with genuine sub-zero stretches — that efficiency difference compounds over a long heating season. It's also why condensing furnaces produce water (condensate) that needs a working drain.

• Dirty flame sensor — the #1 "furnace starts then dies" cause. A ten-minute cleaning fix, but it strands homes on cold nights.
• Failed igniter — hot-surface igniters are consumable parts; they crack with age.
• Clogged filter — restricts airflow, overheats the exchanger, and trips the limit switch (furnace short cycles).
• Cracked heat exchanger — the serious one. Age and overheating cause cracks that can let combustion gases, including carbon monoxide, reach household air. This is why annual inspections include a CO check.
• Blocked exhaust/intake — snow and ice over sidewall vents shut down high-efficiency furnaces; worth checking after every storm.

Once a year, ideally in fall before the first hard freeze. A real inspection checks burner condition, the heat exchanger, gas pressure, safety controls, blower performance, and carbon monoxide levels — not just a filter swap. Regular maintenance is also a written condition of most manufacturer warranties, including Amana's.