Furnace Igniters: Hot Surface vs Spark and Failure Signs

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When a gas furnace runs its inducer, pauses, and then never lights, the igniter is one of the first parts a technician checks. Most furnaces built in the last few decades no longer have a pilot burning all the time. Instead, each heating cycle starts with an electronic ignition step, and a furnace igniter that is cracked, weak or slow can leave a house cold even though everything else works. This guide explains the types of igniters, where ignition fits in the furnace startup sequence, the symptoms of a failing igniter, why the element should never be touched with bare fingers, how it relates to other safety controls, and what replacement typically costs. Gas and electrical diagnosis belongs with a qualified HVAC technician; the goal here is to help homeowners and buyers understand what they are hearing and what an inspector or technician is likely to report.

Part of our Heating, Cooling & Ventilation guide — start there for the full picture →

What a Furnace Igniter Does

The igniter provides the heat or spark that lights the gas at the burners at the start of each heating cycle. Once the control board confirms the burners are lit, the igniter shuts off until the next call for heat. It does not keep the flame going, and it does not prove the flame exists; that is the job of a separate flame sensor.

Residential gas furnaces commonly use one of two approaches:

• Hot surface ignition (HSI). A ceramic element heats until it glows bright orange, hot enough to ignite gas on contact. This is the most common type in current residential furnaces.
• Intermittent pilot or direct spark. A spark electrode lights either a small pilot, which then lights the main burners, or the main burners directly. Many intermittent pilot systems produce a rapid ticking sound while sparking.

Older furnaces with a standing pilot that burns continuously use a different flame-safety part, covered in our thermocouple guide. Those furnaces have no igniter in the modern sense.

Hot Surface Igniter Types: Silicon Carbide vs Silicon Nitride

Hot surface igniters come in two main materials, and the difference matters when a replacement is chosen.

Silicon carbide

Silicon carbide igniters are the older design, often recognizable as a flat, zigzag or U-shaped dark gray element. They heat well but are brittle. Thermal cycling gradually changes their resistance, and they can crack from a bump, from heat stress, or simply from age. Many furnaces from the 1990s and 2000s shipped with this type.

Silicon nitride

Silicon nitride igniters are typically a small rounded or flat-tipped element and are generally considered more durable and more resistant to physical shock. Many newer furnaces use them, and universal replacement kits often use nitride elements. Some operate on standard line voltage, while others are designed for a lower voltage supplied by the control board, so a replacement must match the furnace’s specification or be part of a kit designed for that model. Installing an igniter rated for the wrong voltage can burn it out quickly or keep it from getting hot enough.

Spark and intermittent pilot systems

Spark ignition systems have their own failure points: a cracked ceramic insulator, an electrode gap that has drifted, a worn ignition cable, a corroded ground, or a pilot orifice partly blocked by debris. A furnace that ticks steadily but never lights may have a spark that is too weak or too far from the gas, or it may have a gas supply problem. This sound is different from the single clicks of relays on a control board.

The Furnace Startup Sequence

Ignition is one step in a chain of checks, and a failure anywhere upstream can look like an igniter problem. A typical sequence on a modern induced-draft gas furnace goes roughly like this:

1. Call for heat. The thermostat closes the heating circuit and signals the control board.
2. Inducer motor starts. A small blower begins pulling air through the heat exchanger and pushing combustion products out the vent. Our furnace inducer motor guide covers noises and failure signs.
3. Pressure switch closes. The switch confirms the inducer is creating adequate draft. If it does not close, the board will not move on to ignition. A blocked vent, a cracked hose, a clogged condensate trap or a weak inducer can all prevent this step; see the furnace pressure switch guide.
4. Igniter warms up. On HSI furnaces the element glows for a period set by the control board, commonly somewhere in the range of several seconds to under a minute. On spark systems the electrode starts sparking.
5. Gas valve opens. Gas flows to the burners and lights off the glowing element or spark.
6. Flame is proven. The flame sensor must detect the flame within a few seconds, or the board closes the gas valve. A dirty sensor is a classic cause of short cycling; see our furnace flame sensor guide.
7. Blower starts. After a short delay to let the heat exchanger warm up, the main blower begins moving air through the house.

Because of this order, the timing of the failure gives useful clues. If the inducer runs but the igniter never glows, the pressure switch circuit is a likely suspect along with the igniter itself. If the igniter glows but the gas never lights, the gas supply or gas valve may be at fault. If the burners light and then shut off after a few seconds, the flame sensor or its ground is more likely than the igniter.

Signs of a Bad Furnace Igniter

Common symptoms include:

• The inducer runs, but no orange glow appears through the sight glass or burner opening. With the pressure switch confirmed closed, this points toward an open igniter, a broken wire or connector, or no output from the board.
• The igniter glows dimly or only on part of the element. A weak igniter may not get hot enough to light gas reliably, so ignition succeeds on some cycles and fails on others.
• A visible crack, white spot or burned area on the element. Cracks often appear before a complete failure.
• Repeated ignition attempts followed by a lockout. Many control boards try several times, then lock out for a period, often an hour on many models, before trying again. The board’s status light frequently flashes a code indicating ignition failure. The code chart is usually printed on the furnace door.
• Heat that works intermittently, especially on the first cold mornings of the season, when a marginal igniter is pushed hardest.

Technicians typically confirm an igniter problem by checking for voltage to the igniter during the ignition step and measuring the element’s resistance against the manufacturer’s range with power off. A resistance far outside the expected range, or an open circuit, means replacement. A good igniter with no voltage points toward the control board or wiring.

Lockouts and resets

Switching a furnace off and on resets most lockouts, and a homeowner can reasonably try that once. Repeatedly resetting a furnace that keeps locking out is not a fix. Each failed cycle has a reason, and it may be a safety-related one, such as a blocked vent tripping the pressure switch or a high-limit condition. A furnace that locks out more than once should be looked at by a technician.

Handling an Igniter: Don’t Touch the Element

Hot surface igniters are delicate. Manufacturers and replacement-part instructions commonly warn against touching the ceramic element with bare fingers, because oils and salts from skin can create hot spots that shorten its life or cause it to crack. Replacement igniters are typically handled by the ceramic base or mounting bracket only, and are installed without bending or forcing the element against the burner.

Other handling points technicians watch for:

• Power off first. The igniter circuit may carry line voltage, and the furnace should be shut off at its service switch before the burner compartment is opened.
• Correct position. The element must sit at the correct distance from the burner per the manufacturer’s instructions. Too close and it can be damaged; too far and it may not light the gas.
• No vacuuming or brushing the element. Even light contact during cleaning can crack an old silicon carbide igniter.
• Matching replacement. An OEM or listed universal replacement rated for the furnace’s voltage and mounting.

Igniters, Tune-Ups and Other Safety Controls

An igniter often fails at the worst time, which is why many technicians inspect it during routine service. ENERGY STAR’s maintenance checklist recommends annual pre-season check-ups, with the heating system checked in the fall, and lists items such as tightening electrical connections and measuring voltage and current on motors, checking controls for proper and safe operation, checking the starting cycle so the system starts, operates and shuts off properly, and, for heating, checking gas connections, gas pressure, burner combustion and the heat exchanger. Our furnace tune-up guide covers what a good service visit includes and what it costs.

Ignition problems are sometimes mixed up with other safety shutdowns:

• High-limit trips. If the furnace lights normally, runs for a while, then shuts the burners off while the blower keeps running, overheating from a clogged filter or blocked returns may be tripping the furnace limit switch. That is an airflow problem, not an ignition problem.
• Rollout switches. Some furnaces have flame rollout switches near the burners. A tripped rollout switch can indicate a blocked flue or heat exchanger problem and warrants a technician’s attention.
• Heat exchanger concerns. Flame that lifts, rolls or changes when the blower starts can be a sign of heat exchanger trouble. Our guide to a cracked heat exchanger explains how that is evaluated.

Carbon monoxide still matters

An igniter failure itself usually just means no heat. But a furnace that is being serviced for ignition problems is also a good moment to confirm safe venting. CDC describes carbon monoxide as an odorless, colorless gas, lists gas- and oil-burning furnaces among household sources, and recommends having the heating system, water heater and other gas-burning appliances serviced by a qualified technician every year, along with battery-operated or battery back-up CO detectors near every sleeping area.

Simple Checks Before Calling a Technician

A few basic checks can save a service call or at least give the technician useful information. None of them involve opening the gas valve or handling the igniter.

• Thermostat. Confirm it is set to heat, the setpoint is above room temperature, and its batteries are not dead.
• Power and switches. Furnaces have a service switch that often looks like a light switch near the unit or at the top of the basement stairs. Check that it is on and that the breaker has not tripped.
• Furnace door. Many furnaces have a door safety switch that cuts power if the blower compartment door is not seated properly.
• Filter. A badly clogged filter does not stop ignition directly, but it can trip the high limit and cause short cycling that looks like an ignition problem.
• Status light. Note the blink pattern on the control board through the sight window before cycling power, since resetting may clear the stored code.
• Exterior vents. On high-efficiency furnaces with plastic vent pipes, check that the intake and exhaust terminations outside are not blocked by snow, ice or debris, a common winter issue along the Front Range.

If those check out and the furnace still fails to light, it is time for a technician. Note when the problem happens and what you hear or see, as that history speeds up diagnosis.

Repair or Replace: Typical Igniter Costs

Igniter replacement is one of the less expensive furnace repairs, which is part of why it is a common first step. General 2026 estimates:

• Igniter part: roughly $20 to $80 for many universal and OEM hot surface igniters, with some brand-specific parts costing more.
• Professional replacement: commonly in the low hundreds of dollars including diagnosis and labor, often roughly $150 to $400 depending on the furnace, access, and whether it is an after-hours call.
• Spark ignition module or intermittent pilot assembly: often more than a simple HSI element, particularly if the control module is replaced.
• Control board: if the board is not powering the igniter, a replacement board can cost considerably more than the igniter itself.

These figures vary across the Denver metro, Colorado Springs, and smaller Front Range communities, and are not quotes. Some technicians carry a spare igniter on service calls during heating season, which can shorten repairs.

On an older furnace, an igniter is rarely the deciding factor in replacement, because it is inexpensive. The bigger questions are age, heat exchanger condition, repair history, efficiency and repeated breakdowns. ENERGY STAR notes that certified gas furnaces in northern regions are up to 18 percent more energy efficient than baseline models, which can factor into a replacement decision. Our guide to furnace replacement cost walks through those trade-offs.

Not the same as a gas stove igniter

Searches for igniters also bring up kitchen ranges. Gas oven and range igniters are different parts with different designs, ratings and failure patterns, even though some oven igniters also glow. A furnace igniter will not fit or work in a range, and vice versa. Our guide to the gas stove igniter covers clicking burners and ovens that will not heat.

What Home Inspectors Look For

ASHI’s Standard of Practice directs inspectors to open readily openable access panels and inspect installed heating equipment, vent systems, flues and chimneys, and distribution systems, and to describe the energy source and heating system. The same standard does not require inspectors to inspect heat exchangers or determine the adequacy of combustion air components.

In practice, an inspector usually runs the furnace through normal thermostat controls and observes whether it starts, lights and delivers heat. If the furnace does not ignite, locks out, short cycles or shows signs such as scorching, soot or rust in the burner area, the report typically recommends evaluation by a licensed HVAC contractor rather than naming a specific failed part. Inspectors also note missing CO alarms and venting defects that can affect safe operation.

Our guide to heating and cooling inspections gathers related articles on furnaces, AC equipment and venting, and the main resource on hiring a home inspector explains how to choose an inspector and read the report.

References

• Heating and Cooling Maintenance Checklist — ENERGY STAR
• ENERGY STAR Certified Furnaces — ENERGY STAR
• Carbon Monoxide Poisoning Basics — Centers for Disease Control and Prevention
• ASHI Standard of Practice for Home Inspections — American Society of Home Inspectors

Want a furnace’s ignition, venting and safety controls checked before you buy in the Denver metro or along the Front Range? Reach out through our contact page to connect with a vetted local inspector.