What is a furnace?
Updated August 2026
A furnace is a forced-air heater that burns natural gas, propane, or oil (or runs electric resistance elements) and blows the warmed air through your ducts. A blower sits at one end of the cabinet, burners and a heat exchanger at the other, and a flue carries exhaust outside. Two specs describe almost every unit: AFUE, the share of fuel that becomes usable heat (80% on a standard model, 90–98% on a condensing one), and staging, meaning whether the burner runs at one fixed output or throttles between low and high fire.
Where it is
Look in the basement, a utility closet, the attic, or an attached garage for a steel cabinet about 3 feet tall with a large return duct on one side, a supply plenum on top, and both a gas line and a metal or PVC flue running to it. Upflow models discharge air out the top, downflow out the bottom, and horizontal units lie on their side in a crawlspace. Pop the front access panel and the data plate is on the inside wall. Its serial number encodes the build year, which is the fastest way to learn how old the equipment really is.
What it does
On a call for heat the control board runs a fixed sequence: inducer fan, igniter, gas valve, flame proved by the flame sensor, then the blower. House air crosses the outside of the hot heat exchanger and leaves the registers at roughly 120–140F while combustion gas exits the flue. Three problems generate most no-heat calls. A cracked heat exchanger is the safety one, because it can let carbon monoxide into the air stream. A weak igniter or a sooted flame sensor makes the furnace light and quit over and over. A dying blower motor screeches, hums, or trips on overheat, and runs $400–900 to replace.
Maintenance
Two habits carry a furnace to the end of its life: a fresh filter every 30–90 days, and one professional tune-up each fall. Book the pre-winter test run in September or October, before the first cold snap fills every schedule in town, and expect $100–200 for a combustion check, a safety inspection, and burner cleaning. Keep storage away from the cabinet and leave your supply registers open. A maintained gas furnace lasts 15–20 years; past that, start planning the replacement instead of waiting for a January failure.
What it costs to replace
A replacement furnace runs $4,000–8,000 installed for a mid-efficiency gas furnace, or $7,000–12,000 for a high-efficiency variable-speed unit.
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Frequently asked questions
What is the difference between an 80% and a 96% AFUE furnace?
The number is how much of the fuel becomes heat inside your house. An 80% furnace sends a fifth of it up a metal flue. A 96% condensing furnace pulls out so much heat that the exhaust is cool enough to vent through PVC and drip condensate into a drain. The condensing unit costs roughly $1,500–2,500 more installed and needs new venting, so it pays back fastest in cold climates with expensive gas and slowest across the mild South.
Why does my furnace turn on and off every few minutes?
Short cycling usually starts with airflow. A clogged filter or a pile of closed registers makes the furnace overheat and trip its high-limit switch, so it shuts down, cools, and restarts. Other suspects: a dirty flame sensor that loses the flame signal 5–10 seconds after ignition, an oversized furnace that satisfies the thermostat too quickly, or a thermostat sitting in afternoon sun. Change the filter, open the registers, and if the pattern survives a day, book a tech.
Is a two-stage furnace worth the extra money?
In a cold climate, usually yes. A two-stage burner spends most of the winter on low fire, which buys longer and gentler cycles, smaller temperature swings between rooms, and noticeably less blower noise. The premium runs about $500–1,000 over a single-stage unit of the same efficiency. In a mild climate where the furnace works 300 hours a year, the comfort gain is real but the payback never arrives, and single-stage is the sensible buy.
How can I tell how old my furnace is?
Read the serial number on the data plate inside the front panel. Most manufacturers bury the date in the first four characters, either as week-then-year or year-then-week, and a search on the brand plus the serial format decodes it. If the plate is unreadable, check the permit sticker on the side of the cabinet or the service tags techs leave behind. Anything showing a 1990s date has already outlived the normal 15–20 year window.