Furnace Ignition Systems: Hot Surface vs Spark vs Standing Pilot

What it is

Every gas furnace needs something to light the gas. Over the decades that "something" went through three generations, and you'll still see all three in the field. Knowing which type you're standing in front of changes how you test it and what parts you carry.

The three families:

• Standing pilot — a small flame that burns 24/7, held safe by a thermocouple. Oldest design.
• Intermittent pilot (spark) — a pilot that only lights on a call, sparked by an electrode, proven, then the main valve opens.
• Hot surface ignition (HSI) — no pilot at all; an electric element glows hot enough to light the main burners directly. Most common on modern furnaces.

How it works

Standing pilot: The pilot flame heats a thermocouple, which generates a small DC voltage (millivolts) that holds open the pilot safety in the gas valve. On a call for heat, the valve's main section opens and the always-burning pilot lights the burners. If the pilot goes out, the thermocouple cools, the millivolt signal drops, and the safety drops the gas — that's the safety logic.

Intermittent pilot (spark): On a call, a spark igniter fires a high-voltage spark to light a pilot. A flame-sensing rod (or the spark electrode doing double duty) proves the pilot, then the board opens the main valve and the burners light off the proven pilot. The pilot only burns when there's a call — no standing flame wasting gas.

Direct spark ignition (DSI): Same idea but the spark lights the MAIN burners directly, no pilot. Less common on residential furnaces, more on some unit heaters and commercial gear.

Hot surface ignition: The board sends 120V (some are 80V) to a small igniter element. It heats to ~1,800–2,500°F and glows bright orange. The main valve opens, gas hits the glowing element, and the burners light. A separate flame rod proves the flame. Two element materials: older silicon carbide (the brittle gray-black "fork" — fragile, don't touch the element) and newer silicon nitride (more robust, often a flat blade or mini design).

In the field

Testing a standing pilot system:

• Pilot won't stay lit after you release the gas knob → thermocouple is weak or the connection is loose. A healthy thermocouple puts out roughly 25–30 mV open-circuit; under load (closed-circuit, in the valve) you want at least ~10 mV or so to hold the safety. Below that and the valve drops it.
• Check the thermocouple tip is bathed in the pilot flame, the fitting at the valve is snug (hand-tight plus a quarter turn — overtightening kills them), and the pilot flame is a crisp blue envelope around the tip.

Testing a spark/intermittent pilot:

• No spark → check the igniter cable, the electrode gap, the ground, and the ignition module.
• Sparks but no pilot → gas supply to the pilot, plugged pilot orifice.
• Pilot lights but main won't → flame not being proven; check the sense rod/ground.

Testing an HSI:

• Igniter doesn't glow → ohm it out cold. A silicon carbide igniter typically reads around 40–90 ohms; silicon nitride varies more by model. Open (infinite/OL) means a cracked element — replace it. Also verify the board is actually sending voltage to it; a dead element and a board not energizing the element look identical until you meter the output.
• Glows but burners don't light → gas/valve issue or igniter position relative to the burner.
• Lights but drops out → that's flame sensing, not the igniter (the igniter's job is done once flame proves).

Normal values & targets

• Thermocouple output: ~25–30 mV open-circuit; needs roughly ≥10 mV under load to hold.
• HSI supply voltage: 120V (line-voltage type) or ~80V on dedicated lower-voltage igniters — match the part.
• Silicon carbide HSI resistance: ~40–90 Ω cold (model dependent); OL = replace.
• HSI element temperature: glows around 1,800–2,500°F at full warm-up.
• HSI warm-up time before gas: ~15–45 sec per the board.

Common faults & what they mean

• Repeat HSI failures over a couple seasons — often a symptom, not just bad luck. Frequent on/off cycling (oversized furnace, thermostat issues) and line-voltage surges shorten element life. Also, anyone who handled the old element with bare fingers may have left oils that create a hot spot — always handle silicon carbide by the ceramic base.
• Standing pilot keeps going out — drafty install, dirty pilot orifice, or a dying thermocouple. Confirm the millivolt output before blaming the valve.
• Spark igniter cracks/carbon-tracks — high-voltage leak to ground; you'll often hear it snapping to the burner instead of across the gap. Replace the electrode/cable.
• HSI glows but cracks when it tries to light — thermal shock on a brittle silicon carbide element, sometimes from condensate dripping on it. Look for a venting/condensate issue if it keeps eating igniters.

Tech tips & gotchas

• Never touch a silicon carbide HSI element with bare hands. Skin oil bakes into a hot spot and fails the element early. Handle by the ceramic block only.
• Don't assume a non-glowing HSI is bad until you've confirmed the board is sending it voltage. Meter the igniter output on the board. A control that never energizes the element (because, say, the pressure switch never closed) leaves you chasing the wrong part.
• A "universal" silicon nitride igniter can replace many silicon carbide elements, but confirm the voltage and mounting before you slap one in.
• On standing-pilot units, if a customer wants reliability, the millivolt circuit and thermocouple are the usual weak link — carry thermocouples.
• Carbon on a spark/flame electrode acts like an insulator. A light cleaning sometimes restores a marginal pilot-proving or flame-sense circuit.

Safety / code notes

• The thermocouple/flame-proving safety is the heart of why these systems are safe — it shuts the gas if there's no flame. Never defeat it, jumper it, or wire around a flame-proving circuit to "get heat tonight."
• If you convert or replace an ignition component, match the appliance's listing and the gas valve type; mismatched igniters or modules can defeat the intended safety timing.
• Combustion-air and venting requirements still apply regardless of ignition type — follow the fuel-gas code combustion-air and venting sections for the appliance category.