Ignitors: Silicon Carbide vs Nitride

The short version

A hot surface ignitor (HSI) glows well past 1,800°F to light the burners in a gas furnace. Two materials are in the field: silicon carbide and silicon nitride. They look different and they fail differently, but resistance and warm-up time vary by model rather than by material, and the published ranges overlap. Work from the material instead of the part number and you test against the wrong specification. The furnace can then fail to light on the next call for heat. Voltage is a separate check: both materials come in more than one rating, so the voltage depends on the part number rather than the material.

Robertshaw lists an open ignitor as a sign of a cracked silicon carbide element. An aging element has a higher resistance than it had when new. A meter will show that change and a glance at the burner will not.

Identify the exact part number before you test, and test before you replace.

What a hot surface ignitor does

The hot surface ignitor replaced the standing pilot light on most residential gas furnaces built since the early 1990s. When the thermostat calls for heat, the control board runs the inducer motor to establish draft, then energizes the ignitor.

Autoignition temperature is the temperature at which a gas ignites without a spark or external flame. The element heats above the autoignition temperature of the fuel. The National Oceanic and Atmospheric Administration (NOAA) puts that temperature near 999°F for liquefied natural gas and 842°F for propane, and Linde’s propane safety data sheet agrees at 450°C. NOAA notes that liquefied natural gas has no standard composition, so published autoignition temperatures vary. A hot surface ignitor reaches well past either temperature.

After the warm-up period, the control board opens the gas valve. Gas flows across the hot element and ignites on contact. A flame sensor then proves combustion by measuring a microamp signal through the flame. On Robertshaw’s 780 series the board turns the ignitor off several seconds after the valve opens, not when the sensor proves flame.

The trial for ignition is the window after the gas valve opens during which the control board waits for a flame signal. If the board does not detect flame within that window, it closes the gas valve and retries. White-Rodgers publishes the sequence for one control, the 50A65-5165 integrated furnace control: a 5-second trial for ignition, a 60-second interpurge to clear unburned gas, and two retries before system lockout. The manual lists a 300-second lockout time, and the control resets itself after an hour.

Warm-up time varies by model rather than by material. Robertshaw’s guide for the Norton silicon carbide ignitors gives 34 seconds for the 201 and 17 seconds for the 271, both timed to a minimum 1,800°F at 102 volts. The silicon nitride Norton 601 reaches that temperature in 5 seconds, but White-Rodgers rates its 21D64-2 nitride upgrade kit at 17 seconds minimum. The difference matters because the control board’s warm-up delay has to match the ignitor installed.

Appearance alone shows little. In its universal kit instructions, Resideo tells the technician to run a cycle and watch the installed ignitor: if it glows, the problem is not the ignitor. If it does not glow, Resideo says to unplug the leads and confirm 120 VAC at the harness during warm-up. A resistance reading outside the specification for a new part points at a cracked element, but the reading shows nothing about whether gas reached the burner. Check the gas side against the furnace manufacturer’s procedure.

The two types

Silicon carbide (SiC) was the original hot surface ignitor material in residential furnaces. The element is a flat or spiral piece of rough, speckled ceramic. HVAC School describes the surface as “rough-looking” and “speckled,” and Fox Family Heating writes that bringing a finger and thumb together “even somewhat quickly” is force enough to break the carbide tip.

Silicon nitride (Si3N4) is the newer material, and HVAC School says it is more common in newer gas furnaces. The element is a round rod or a flat strip with a smooth surface. HVAC School describes these elements as “round rods or flat strips of metal with a smoother surface.” Color varies: Resideo calls the element in its universal nitride kit black. Kyocera describes the construction as “ceramic materials with built-in heating elements,” rather than the ceramic itself carrying the current. HVAC School says silicon nitride ignitors “heat up more quickly than silicon carbide igniters, use less energy, and last longer; they hold less heat and don’t wear out as quickly as a result.”

Silicon carbide is fragile; handle it by the ceramic base, not the element. Silicon nitride is substantially more durable than silicon carbide, but the same habit is good practice.

Whether skin oils actually damage silicon carbide is debated. In a 2009 informational guide for its Norton ignitors, Robertshaw calls the body-oil contamination belief “untrue.” Avoiding bare-hand contact is still field practice. The brittleness of the element is not in dispute. Neither is the need to keep contaminants off a surface that Robertshaw lists at 2,400 to 2,600°F at 120 volts for the Norton 201 and 271.

Silicon nitride lasts longer than silicon carbide. White-Rodgers says nitride ignitors typically outlast carbide ignitors by two to seven times. Service life also depends on voltage, cycling and contamination.

Most residential furnaces run their ignitors at 120 volts. Some run them at 80 volts from a reduced-voltage output on the control board. Voltage does not follow from the material: White-Rodgers sells 80 volt silicon nitride ignitors as direct replacements for Amana, Lennox, Rheem and Trane furnaces, and its 50A65-5165 control requires one. Silicon nitride models are also sold at 24 volts, though 24 volt models are uncommon in residential forced-air equipment.

Property	Silicon carbide	Silicon nitride
Appearance	Rough, speckled	Smooth; color varies
Shape	Flat element or spiral	Round rod or flat strip
Warm-up time	Norton 271: 17 sec; Norton 201: 34 sec	Norton 601: 5 sec; HotRod 21D64-2: 17 sec min
Cold resistance (new)	Norton 271: 40 to 75 ohms; Norton 201: 45 to 400 ohms	Norton 601: 50 to 300 ohms
Fragility	Brittle; handle by the ceramic base	Substantially more durable
Common voltage ratings	120V (Norton 201, 271)	24V, 80V or 120V

The figures above come from named models, not from material-wide ranges.

Testing an ignitor

Turn off power to the furnace before you touch the ignitor or its wiring, and let the element cool. The ignitor circuit runs at 120 volts on most furnaces and 80 volts on some. Treat either as a shock hazard.

Resistance. Disconnect the ignitor leads from the wiring harness or terminal block. Use auto-ranging, or pick a range above the model’s published maximum resistance. A new Norton 201 reads as high as 400 ohms, so a 200 ohm range shows OL (overload) on a good ignitor. Measure across the two leads and compare the reading to the manufacturer’s specification for that part number.

A reading of OL means the meter is over range. With a range that covers the model and the leads off the harness, an OL reading means the element is open, which Robertshaw lists as a sign of a crack. Replace the ignitor. A reading above the manufacturer’s specified range for a new ignitor does not always mean replacement, because resistance rises with use. The replacement threshold is roughly double the original room temperature resistance (RTR) at installation. Indoor Comfort Marketing’s HSI troubleshooting reference states that threshold directly: “the RTR will be higher on a used igniter; the resistance should be no more than double the original resistance at installation.”

Norton’s 201, a silicon carbide model, reads 45 to 400 ohms new and 90 to 800 ohms used. Norton’s 271, also silicon carbide, reads 40 to 75 ohms new and 80 to 150 ohms used. Norton’s 601, a 120 volt silicon nitride model, reads 50 to 300 ohms new. Those used spans are not a pass mark on their own. Robertshaw’s rule is twice that individual ignitor’s resistance at installation, so a 271 that started at 40 ohms is due for replacement at 80 ohms, not 150. Robertshaw publishes the new ranges and the double rule; the used spans come from Indoor Comfort Marketing.

Visual inspection. Look for cracks. Robertshaw also lists a buildup of white silica dust around a bright spot as a sign of a crack. On silicon carbide, a hairline crack across the element is enough to open the circuit. If the meter still reads OL, trust the meter over your eyes. Silicon nitride is less prone to visible cracking, but damage to the ceramic body or the exposed element is still worth checking.

Amp draw. Position a clamp meter around one ignitor lead with the furnace de-energized, then restore power and read the current during the warm-up cycle. For a flexible current probe, Fluke says to de-energize the installation or wear suitable protective clothing, and not to apply or remove the probe on live wires. The clamp goes around one lead, not the pair. Do not move the clamp or touch the connections while the circuit is live. Indoor Comfort Marketing’s 4.75 amp maximum applies to the high-current carbide models: Robertshaw lists 4.25 to 4.75 amps for the Norton 201 and 271 but 0.4 to 1.0 amps for the 601. Treat a low current alongside a high resistance as a sign of an aging element only after confirming the applied voltage.

When the ignitor is not the problem. A furnace that lights and then shuts down within a few seconds is failing to prove flame, not failing to ignite. Check flame current in microamps, along with grounding and the sensing circuit, before replacing the ignitor.

Why ignitors fail

Physical damage. Silicon carbide elements crack from impact, or from the thermal cycling of repeated heat-ups and cooldowns over years. Robertshaw warns that cracks come from hitting the tip or dropping the ignitor. An element cracked through reads OL and does not heat.

High supply voltage. Indoor Comfort Marketing’s reference puts the burnout threshold at approximately 132 volts and states that supply voltage above 125 volts reduces ignitor lifespan. A supply voltage that runs consistently high shortens the element’s life before any visible defect appears.

Excessive cycling. An oversized furnace satisfies the thermostat quickly, shuts down, and starts again minutes later. Each restart is a thermal cycle on the ignitor. Fox Family Heating says that excessive cycling shortens ignitor life and that “an improperly sized unit is going to cause all kinds of problems.”

Contamination. Fox Family Heating lists sheetrock dust, condensation, dirt, rust and fiberglass as contaminants that settle on the element.

Propane. Fox Family Heating reports field observations of faster degradation on propane: its author writes that he has seen ignitors standing in a propane flame lose the top half of the carbide tip after three to five years. Propane burns hotter at the burner than natural gas.

Faulty control board. A board that never de-energizes the ignitor leaves the element hot well past its warm-up window, which runs from 5 seconds for the Norton 601 to 34 seconds for the 201.

Replacing one type with the other

Universal silicon nitride kits cover the silicon carbide ignitors their cross-reference lists. The White-Rodgers HotRod (part number 21D64-2) is a 120 volt silicon nitride kit that HVAC School describes as covering more than 170 factory part numbers. It ships with multiple bracket configurations to adapt the mounting.

Verify three things before installing a replacement:

Voltage. Confirm the voltage rating on the original ignitor and match it. An 80 volt ignitor receiving 120 volts will burn out immediately and may damage the control board. A 120 volt ignitor receiving 80 volts will not reach ignition temperature. The voltage rating may be on the ignitor body or its label; when it is not, work from the furnace’s factory part number.

Warm-up timing. A control board’s gas valve delay is built around the warm-up time of the ignitor the furnace shipped with. Do not assume the nitride replacement is faster: White-Rodgers rates the 21D64-2 HotRod at 17 seconds minimum, the same warm-up time as a Norton 271 carbide ignitor, and the HotRod EX at 8 seconds. Check the kit’s warm-up time against the control board literature for the furnace, and change the board’s timing only where that literature calls for it.

Element position. HVAC School notes that surface area is the most important design factor: the element needs enough area in the gas stream to ignite the fuel reliably. Silicon carbide ignitors sometimes have a larger surface area than their silicon nitride replacements, so after installing the bracket, verify the element sits where gas crosses it.

Resideo’s universal kit gives the checkout in two stages. In the first stage, power on with the gas supply still off, run a call for heat, and confirm the element heats and clears the surrounding parts. In the second stage, open the gas, run another call for heat, and confirm the appliance “lights properly and stays lit once the actual gas flow starts.” Repeat the call for heat twice more to prove the light-off is reliable. If the warm-up delay is too short, the gas arrives before the element is ready, and the result can be a failed light or a rough one.

Common mistakes

Check the resistance reading against the manufacturer’s specification for the exact part number, not a memorized range
Verify the voltage rating on the ignitor before ordering a replacement
Handle silicon carbide ignitors by the ceramic base rather than the element
Check flame current if the furnace lights and then shuts down, since that behavior points to flame sensing rather than ignition
Confirm the control board’s warm-up delay matches the replacement ignitor’s warm-up time

Don’t

Condemn an ignitor before you have measured its resistance against the part’s specification, the applied voltage at the harness, or both
Install a 120 volt ignitor in an 80 volt circuit or the reverse
Judge an ignitor by how bright it looks, since apparent brightness shows nothing about the temperature reached in the allowed warm-up time
Skip the cross-reference when substituting an aftermarket part, since bracket position affects flame contact and lead length affects fit and routing
Leave the ignitor circuit energized while handling the ignitor or disconnecting its wiring

Frequently asked questions

Can I replace a silicon carbide ignitor with silicon nitride?

Usually you can, when the kit’s cross-reference lists the application. Resideo warns that “some appliances may have igniter applications that are beyond the capabilities of this kit.” The voltage must match: verify the original ignitor’s voltage rating and use a replacement at the same voltage. Do not assume the replacement is faster, either. White-Rodgers rates the 21D64-2 at 17 seconds minimum, the same warm-up time as a Norton 271 carbide ignitor.

How do I tell which type is installed?

A rough, speckled flat or spiral element is silicon carbide; a smoother round rod or flat strip is silicon nitride. Do not go by color alone: Resideo calls the nitride element in its universal kit black. Check for a part number on the body or a label near the ceramic base; if there is none, identify the type from the furnace’s factory part number and a kit cross-reference.

What resistance reading means the ignitor should be replaced?

An OL reading on a disconnected ignitor, on a range that covers the model, means an open element, which Robertshaw lists as a sign of a crack. Replace the ignitor. A reading that exceeds roughly double the resistance that ignitor had when it was new also calls for replacement. There is no typical range to lean on: Robertshaw lists 40 to 75 ohms for the Norton 271 and 45 to 400 ohms for the 201, both silicon carbide.

Why does the furnace lock out after three attempts?

The board tries a set number of times, commonly three, with an interpurge between attempts, then locks out to stop gas going into an unlit heat exchanger. The cause may be a weak ignitor, a closed manual gas valve, a failed gas valve, or a flame sensing problem.

Is the skin oils warning real for silicon carbide?

The statement people quote comes from a 2009 Robertshaw informational guide for its Norton ignitors, not from a Norton bulletin. In that guide, Robertshaw calls the body-oil contamination belief untrue. The question is not fully settled. Either way, hold the ignitor by the ceramic base, which covers the contamination question and the breakage risk at once.

Glossary

Autoignition temperature: the temperature at which a gas ignites without a spark or external flame. NOAA lists 999°F for liquefied natural gas and 842°F for propane, and notes that composition varies.
Control board: the electronic board in a furnace that manages the ignition sequence, monitors safety switches, and controls the gas valve and blower motor.
HSI: hot surface ignitor. A ceramic element that heats past 1,800°F to ignite gas at the burner. The Norton 201 and 271 reach 2,400 to 2,600°F at 120 volts.
Interpurge: the period between ignition attempts during which the inducer motor runs to clear unburned gas from the combustion chamber and venting.
Lockout: a safety state in which the control board stops attempting ignition after a set number of failed trials. Most residential boards reset after a timed delay or on a power cycle.
OL: overload, also written as over range. A multimeter reading indicating resistance beyond what the meter can read, meaning the circuit is broken or the range is too low. On a disconnected ignitor measured on an adequate range, it indicates an open element, which can result from a crack.
RTR: room temperature resistance. The resistance of the ignitor measured at ambient temperature with power off. Technicians use it to evaluate element condition against the manufacturer’s specification.
Silicon carbide (SiC): a ceramic compound used as the resistive heating element in older hot surface ignitors. The ceramic itself heats when current passes through it.
Silicon nitride (Si3N4): a ceramic compound used as the body of newer hot surface ignitors. Kyocera describes the construction as ceramic with a built-in heating element; the exact element varies by manufacturer.
Trial for ignition: the window after the gas valve opens during which the control board waits for a flame signal before closing the valve.

Drafted with AI assistance and reviewed by the author.

← All posts