What it is

A single-stage gas furnace is the most common piece of heating equipment in American homes. "Single-stage" means it has one fire rate — the burners are either fully on or fully off, no in-between. One gas valve solenoid, one manifold pressure, one heat output. It's simple, cheap to build, cheap to repair, and if you understand this unit cold, every fancier furnace is just this with extra steps.

The cabinet bolts inline with the ductwork. Return air comes in one end, gets heated, and the blower pushes it out the supply end. Most residential furnaces run 40,000–120,000 BTU/h input.

How it's built

Walk the cabinet from the air-entering side to the air-leaving side and you hit every major part in order:

  • Blower compartment. The bottom (on an upflow) holds the blower assembly — a squirrel-cage wheel in a housing driven by a motor, usually a PSC or ECM. This is the air mover for both heating and cooling.
  • Control board. Mounted in the blower compartment, this integrated furnace control (IFC) is the brain. It sequences everything, makes the safety proofs, and switches line voltage to the inducer, igniter, and blower, plus 24V to the gas valve.
  • Burner box / gas train. Gas enters through the gas valve, runs down the manifold, through the orifices (spuds), and into the burners. The burners fire into the heat exchanger.
  • Heat exchanger. A set of steel tubes or a clamshell. Flue gas runs inside; house air runs across the outside. This is the wall that keeps combustion products out of the breathing air. Cracks here are a CO and replacement issue.
  • Inducer (draft motor). A small blower at the top of the exchanger that pulls combustion products through the tubes and pushes them out the vent. It also establishes the draft the pressure switch proves.
  • Igniter and flame sensor. A hot-surface igniter (most common) lights the gas; a flame rod proves the flame with a microamp signal.
  • Limit switches. A high-temperature limit on the heat exchanger and one or more rollout switches near the burners. These are the safeties.

How it's gassed and wired

Gas side: the gas valve regulates manifold pressure down to spec — typically around 3.5" w.c. for natural gas, 10–11" w.c. for LP, but always set it to the rating plate. The orifice size and manifold pressure together set the fire rate. You confirm the actual input by clocking the meter.

Line voltage: 120V feeds the furnace through its own circuit. The board switches 120V to the inducer, igniter, and blower. The blower has speed taps (PSC) or a control harness (ECM) so heating and cooling can run different airflow.

Low voltage: a 24V transformer (often 40 VA) powers the control circuit. The thermostat closes R to W to call for heat. R to G runs the fan, R to Y passes a cooling call to the condenser and energizes the blower on cool speed.

In the field — sequence of operation

On a call for heat: thermostat closes W → board energizes the inducer → pressure switch proves draft → igniter warms up (~15–45 sec) → gas valve opens and the burners light → flame sensor proves flame within the trial window → igniter shuts off → after a 30–45 sec delay the blower starts on heat speed → burners and blower run until satisfied → on shutdown the gas closes immediately, the inducer post-purges, and the blower runs its off-delay (60–180 sec) to pull remaining heat out. (The component-level detail of each proof lives in the gas-heat sequence article.)

Normal values & targets

  • Temperature rise: the single most important setup number. Subtract return-air temp from supply-air temp; it must land inside the range stamped on the rating plate, commonly 35–65°F (model specific). Measure supply far enough downstream that you're not seeing radiant heat off the exchanger.
  • Manifold pressure: set to the plate — ~3.5" w.c. NG, ~10–11" w.c. LP nominal.
  • Flame-sense current: roughly 1–6 µA DC healthy; below ~0.5–1 µA the board drops flame.
  • Inducer draft / pressure switch: switch closes within a second or two of the inducer reaching speed.
  • Blower amp draw: within the motor's rated FLA on the blower label.
  • Gas input by clocking the meter: should match the plate input within a few percent.

Common faults & what they mean

  • No heat, board flashes a pressure-switch code. Inducer runs but the switch won't prove. Cracked or plugged hose, condensate-blocked port, weak inducer, or a restricted vent.
  • Igniter glows, gas lights, flame drops in a few seconds, repeats. Classic flame-sensing fault — dirty rod or bad burner ground. Clean the rod, check the ground, read microamps.
  • Short cycles on the high limit. Burners light, then cut out on temp before or shortly after the blower starts. Almost always airflow: dirty filter, closed registers, undersized return, or a weak/failed blower. The limit is protecting the exchanger.
  • Temperature rise too high. Not enough airflow across the exchanger — dirty filter/coil, blower too slow, restrictive ducts. Runs the exchanger hot and shortens its life.
  • Temperature rise too low. Too much airflow or under-fired. Causes condensation in a standard-efficiency exchanger and cold-blowing complaints.
  • Cracked heat exchanger. Shows up as flame distortion when the blower starts, CO in the supply, or visible cracking. Condemn and shut down — this is a safety call, not a maybe.

Tech tips & gotchas

  • Always set and verify temperature rise. A furnace running outside its rise band is either cooking its exchanger (too low airflow) or sweating it (too high airflow). This one check catches a huge share of long-term failures.
  • Watch the sequence and note where it dies. The step that fails is the diagnosis. "Quits right after the inducer" points straight at the pressure-switch circuit.
  • A furnace that won't keep flame is rarely a bad gas valve. Clean and test the flame rod before you reach for the valve.
  • The blower off-delay is supposed to run after the burners quit. It's recovering heat, not stuck on. Tell the homeowner.
  • Don't jumper a limit or rollout to "make it run." Those exist to keep combustion products out of the house. Fix the cause.

Safety / code notes

  • Combustion-air openings are sized per the fuel-gas/mechanical code combustion-air sections (the general two-opening method works out near 1 in² per 1,000 BTU/h). Confirm the appliance has the air it needs.
  • Venting follows the appliance category and the fuel-gas code venting provisions — never improvise vent material or sizing.
  • Any suspected spillage, blocked flue, or cracked exchanger means verify draft and test for CO with an analyzer before the unit goes back in service.
  • Gas piping and sediment trap per the fuel-gas code; leak-check every joint you open.