Gas Furnace Service and Repair: First Service-Call Basics

What the work looks like

Gas furnace service is the bread and butter of residential HVAC in cold-climate regions (anywhere north of about Georgia). A service tech rolls to a no-heat call, diagnoses why the furnace is not firing, fixes it, and leaves. You will see 80 percent AFUE atmospheric (single-stage) furnaces, 80 percent induced-draft (single-stage and two-stage), and 90+ percent condensing furnaces (two-stage or modulating, PVC venting).

Sequence of operation on a modern 90+ furnace:

1. Thermostat calls for heat (R to W closes).
2. Inducer motor starts, pressure switch closes.
3. Hot surface igniter glows.
4. Gas valve opens, burners light.
5. Flame sensor proves flame (microamp signal to the board).
6. Blower delays about 30 seconds, then starts.
7. Thermostat satisfies, gas valve closes, inducer and blower run off-cycles.

If you can recite that out loud, you can troubleshoot 70 percent of no-heat calls.

Job titles: HVAC Service Tech, Residential HVAC Tech, Furnace Repair Tech. Pay $22 to $45 per hour depending on region, plus commission or spiff on parts and new-equipment sales.

Safety and tools

Gas work kills people. The risks are carbon monoxide (CO) poisoning from bad combustion, explosion from gas leaks, and burns from hot heat-exchanger components.

Every service call:

• Check for gas leaks with a combustible gas detector (Testo 316 or similar) or soap solution on joints. Never with a lighter.
• Test for CO with a personal CO monitor (required on every service truck). Ambient over 9 ppm near the furnace means investigate.
• Measure static pressure and temperature rise; if the rise is above or below the nameplate range, the furnace is not getting proper airflow and the heat exchanger is at risk.
• Inspect the heat exchanger with a camera or light. A cracked heat exchanger is a red-tag condemned-unit situation; you shut off the gas and leave a sticker.

Tools: Fieldpiece SMAN 480 or Testo 550 digital manifold (more for AC than furnace), combustion analyzer (Testo 310 or Bacharach Fyrite Insight), CO monitor, multimeter, micromanometer (static pressure), inspection camera, 1/4 inch nut driver, 5/16 inch nut driver, Klein 11-in-1, flashlight, voltage tester.

Your first exercise

Find your furnace (or one you can access). Locate the thermostat wires (R, W, G, Y, C) on the control board. Identify the inducer motor, the hot surface igniter (ceramic, pencil-shaped) or spark igniter, the gas valve, the flame sensor (single rod near the last burner), and the blower motor. Turn the thermostat up and watch the sequence: inducer, igniter glow, gas ignition, blower delay. If the sequence stalls at any step, that is where your call diagnosis begins.

Where to go next

Build on Gas Furnace Service with HVAC Fundamentals (Introduction to HVAC), Refrigeration Systems (Introduction to Refrigeration), HVAC Controls and Thermostats (Introduction to HVAC Controls), Ductwork Fabrication (Introduction to Ductwork Fabrication), and Heat Pumps (Introduction to Heat Pumps). Safety: Fire Safety & Prevention, Hazardous Materials Handling, Workplace Safety.