Gas Furnace — Anatomy

For homeowners

A gas furnace is a heat exchanger with a controlled fire underneath. Burners produce hot combustion gas inside a sealed metal compartment (the heat exchanger). Your home’s air flows across the outside of that compartment, picks up heat through the metal walls, and returns to the rooms warmer. The combustion gas — which is toxic and which you want kept entirely separate from your indoor air — exits through a flue vent to the outdoors.

Modern residential gas furnaces are one of two efficiency classes:

80% furnaces — single heat exchanger, atmospheric or induced draft venting, no condensate. Older technology, still installed widely as a budget option.

90%+ furnaces (“condensing” or “high-efficiency”) — two heat exchangers in series. The primary extracts heat from hot combustion gas; the secondary cools the flue gas further until water vapor condenses out of it, releasing the latent heat of vaporization into the airstream. This recovers roughly 10% additional efficiency at the cost of producing acidic condensate that must be drained.

The diagram shows a condensing upflow design — most common in residential basements and mechanical closets. The major components are the gas valve, burners, hot surface igniter, flame sensor, primary and secondary heat exchangers, high-limit and rollout switches, draft inducer and pressure switch, and blower. Each has its own topic in this reference.

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For technicians

Cabinet configurations. Residential gas furnaces ship in four orientations matched to installation requirements:

• Upflow — air enters at the bottom, leaves at the top. Used in basements and mechanical closets. Most common configuration.
• Downflow / counterflow — air enters at the top, leaves at the bottom. Used in slab homes with ducts in the floor or attic-installed downflow into ceiling supply. Common in the Florida market for attic installations.
• Horizontal — air enters one side, leaves the other. Used in crawl spaces and attics where vertical clearance is limited.
• Multi-position — designed to be field-configured for any of the above. Most modern furnaces.

Efficiency classes:

80% AFUE (atmospheric or induced draft, non-condensing): Single heat exchanger, hot combustion gases exit through a metal flue at 350–500°F. The remaining 20% of fuel energy is lost up the flue. Atmospheric draft furnaces rely on natural buoyancy of the hot flue gas to vent themselves; induced draft adds a small fan (the inducer) to pull combustion gases out, which improves combustion control and allows for tighter cabinet sealing.

90–98% AFUE (condensing): Two heat exchangers in series. After the primary, the flue gas is around 300°F and still carries significant water vapor (water is a combustion byproduct of burning hydrocarbon fuel). It passes through the secondary heat exchanger — typically a stainless steel coil — where it’s cooled to about 100–150°F. At that temperature the water vapor condenses to liquid water, releasing approximately 970 BTU per pound of water vapor as latent heat. That heat goes into the airstream. The cooled flue gas exits through a PVC vent (not metal — PVC can handle 150°F flue gas, but galvanized cannot survive prolonged condensation contact).

The trade-off for condensing operation is condensate. The water that comes out of the secondary HX is mildly acidic — pH 3–5, from dissolved CO₂ forming carbonic acid plus trace amounts of sulfuric and nitric acid from compounds in the fuel. Modern furnaces drain this condensate through a plastic trap and PVC line, sometimes routed through a limestone neutralizer cartridge to bring pH up before discharge into a household drain.

Burner types.

• In-shot burners — most common on modern residential furnaces. Each burner is a metal tube that shoots a stream of gas-air mixture into a heat exchanger cell. One burner per cell. Simple, reliable, easy to clean.
• Ribbon / matrix burners — used on some higher-efficiency models. A flat porous metal surface across which gas burns evenly. Lower NOx emissions, more even heat distribution.

Gas types:

• Natural gas — methane primarily, delivered through utility piping. Manifold pressure typically 3.5 in. w.c. on a residential furnace.
• Propane (LP) — liquid stored in a tank, vaporized for use. Higher heating value than natural gas. Manifold pressure typically 10–11 in. w.c. Requires different burner orifices than natural gas — never assume a furnace will work on the other fuel without proper conversion.

Control sequence overview. When the thermostat calls for heat:

1. 24V W signal reaches the control board
2. Board energizes the draft inducer
3. Inducer pulls air through the heat exchanger, generating negative pressure that closes the pressure switch
4. Closed pressure switch tells the board “vent is clear, safe to fire”
5. Board energizes the hot surface igniter
6. Igniter warms to 1800–2200°F over about 30–60 seconds
7. Board opens the gas valve
8. Gas mixes with combustion air at the burners, ignites from the hot surface
9. Flame sensor confirms ignition via flame rectification current
10. After a 30–60 second blower-on delay, the blower starts
11. System runs until thermostat satisfies
12. Gas valve closes, flame goes out, inducer continues briefly to purge the heat exchanger
13. Blower continues briefly to cool the heat exchanger, then stops

See Gas Furnace — Full Sequence of Operation for the detailed diagnostic breakdown.

Safety chain. A gas furnace has multiple safety switches all wired in series, any of which can stop operation:

• High-limit switch in the supply plenum, opens at 160–200°F if the heat exchanger overheats (usually because airflow has failed)
• Rollout switches mounted near the burner inlets, open at 250–350°F if flame escapes the heat exchanger (indicates HX crack or blocked vent)
• Pressure switch at the inducer, must close to prove the inducer is moving air before ignition can proceed
• Flame sensor must detect flame within a few seconds of opening the gas valve — if not, the valve closes immediately

Diagnostic LEDs and fault codes. Every modern gas furnace control board has a diagnostic LED that flashes fault codes — typically 1–12 different patterns indicating which safety opened, whether ignition was attempted and failed, lockout state, etc. The fault code chart is printed on the inside of the cabinet door. Reading it is the first step of any service call.

Florida considerations. Gas furnaces are less common in Florida than electric heat strips because heating loads are small and natural gas service isn’t available in many areas. Where gas furnaces are installed:

• Almost always downflow or horizontal (attic installations)
• Almost always 80% AFUE — the small heating load doesn’t justify the cost premium of condensing
• Almost always propane (no gas utility service in most areas)
• Frequently sized too large because the contractor sized for AC and the furnace came in the same package

The “oversized furnace” problem is real — a 100,000 BTU furnace satisfying a 25,000 BTU heating load short-cycles, never fully warms its heat exchanger, condenses moisture inside the heat exchanger on every startup, and rusts itself out from inside in 8–12 years instead of the 20–25 it should achieve.