Furnace Ignition System Check

Modern furnaces use one of two ignition systems. Hot surface ignition (HSI) uses a silicon carbide or nitride element that glows red hot (2,500°F) to ignite the gas. HSI systems are common in 80% and 90%+ AFUE furnaces built after 1990. The ignitor glows for 15–30 seconds before the gas valve opens. Intermittent spark ignition (ISI) uses a spark electrode to create a spark at the burner when gas flows. Older furnaces (pre-1990) may use a standing pilot—a continuously burning flame that lights the main burners when needed. Standing pilots waste gas (thermocouple and pilot flame) and are less efficient. Some mid-efficiency furnaces use direct spark ignition where the spark and gas flow simultaneously. Knowing your ignition type helps diagnose no-heat situations.

When your furnace will not heat, follow these diagnostic steps: 1. Check the thermostat—set to HEAT, temperature 5°F above room temp. 2. Check the furnace power switch (looks like a light switch on the unit)—ensure it is ON. 3. Check the breaker—a tripped breaker shuts off the furnace. 4. Open the burner access panel and watch the ignition sequence through the viewport or inspection window. 5. Listen: you should hear the inducer motor (a whirring sound), then the ignitor heating (or spark clicking), then the gas valve opening (a soft click), and finally the burners lighting. 6. If the sequence stops before ignition, note where it stopped. Watch the control board LED for error codes. 7. A furnace that tries to light but fails 3–4 times enters lockout mode—reset by turning power off for 5 minutes.

A failed HSI ignitor is one of the most common furnace repairs. Signs: the furnace does not heat, and you can see the ignitor does not glow during the start sequence. Buy a replacement ignitor specifically for your furnace model (universal ignitors work for many brands). Turn off power and gas to the furnace. Remove the ignitor (usually mounted with one or two screws inside the burner compartment). Handle the new ignitor carefully—the ceramic is fragile and oil from your skin creates hot spots that cause premature failure. Wear gloves or use the plastic packaging to hold it. Insert the new ignitor, tighten the screw(s), reconnect the wire. Restore power and test. Cost: $20–$50 for the part, $150–$300 for professional installation.

The flame sensor is a thin metal rod that detects when the burner flame is present. If it does not detect flame, the gas valve closes after 2–4 seconds—this is a safety feature. A dirty flame sensor is the most common cause of a furnace that lights briefly then shuts off. The sensor gets coated with combustion byproducts over time, reducing its sensitivity. Turn off power to the furnace. Locate the flame sensor (a single wire connected to a rod near the burners). Remove it with a ¼-inch nut driver or screwdriver. Gently clean the metal rod with a fine abrasive pad (do not use sandpaper—it scratches the surface). Wipe clean and reinstall. This 10-minute fix resolves most short-cycling issues.

Working on a gas furnace involves natural gas, high voltage, and high temperatures. Always turn off the power at the breaker and the unit-mounted switch before opening the furnace. Turn off the gas supply at the valve on the gas line (a quarter turn to perpendicular). Wait for the furnace to cool completely before touching any components. Never bypass safety devices (flame sensor, limit switch, pressure switch). If you smell gas during work, evacuate immediately and call 911. If the furnace does not light after repeated attempts, call a professional—repeated ignition attempts can accumulate unburned gas in the combustion chamber, creating a dangerous condition. Carbon monoxide testing should be done after any furnace repair by a qualified technician.