On a modern gas furnace or boiler, the igniter lights the burners and the flame sensor proves that the flame is actually there. If either part fails, the control board shuts off the gas valve and the appliance goes into lockout. These are two of the most common no-heat calls in the field, and both are parts every heating tech should have on the truck in a few common styles.
This category covers hot surface igniters in silicon carbide and silicon nitride, in 120-volt and 24-volt versions, along with flame sensors and related ignition parts. Supco and other universal lines offer igniters with mounting brackets and connectors designed to cover many OEM applications, plus straight and bent flame sensor rods to match common furnaces.
Igniters wear out because they heat to glowing temperatures every cycle and eventually crack or burn open. Flame sensors fail more slowly, coating over with oxide and combustion deposits until the signal drops below what the board needs to keep the gas valve open. Both parts should be handled with care and installed by a qualified technician because gas appliances can create fire and carbon monoxide hazards when they are not set up correctly.
On a call for heat, the control board starts the draft inducer, proves venting through the pressure switch, then powers the hot surface igniter. After a warm-up period, the board opens the gas valve. Gas hits the glowing igniter and lights. Within a few seconds, the board must detect flame through the flame sensor, or it shuts the gas valve and retries or locks out.
Silicon carbide igniters are the older, widely used style, often flat or in a zigzag shape. They heat up relatively slowly and are brittle; touching the element with bare fingers can leave oils that create hot spots and shorten life, and a small bump can crack them. Most are 120-volt designs.
Silicon nitride igniters are more rugged, heat faster and generally last longer. Many are compact round or flat elements encased in a tough ceramic. They are made in 120-volt and lower voltage versions, including 24-volt and other voltages used by specific manufacturers. Never install an igniter at a voltage it is not rated for; a 24-volt igniter on 120 volts fails instantly, and a 120-volt igniter on a low-voltage circuit will not get hot enough.
A flame sensor is a metal rod positioned in the burner flame. The control board applies an AC voltage to the rod. Because the flame conducts more easily in one direction than the other, a small DC current flows through the flame to ground. This is called flame rectification, and the current is measured in microamps (µA). If the board sees enough microamps, it keeps the gas valve open. If not, it shuts down. Many residential furnaces operate with a few microamps of flame signal; the required minimum varies by manufacturer and should be taken from the equipment documentation.
Not every short-cycle is a sensor. Poor grounding, reversed line polarity, a weak burner flame or a dirty burner can all produce the same symptom. Check those before replacing the sensor.
| Part | Type | Strengths | Watch out for |
|---|---|---|---|
| Hot surface igniter | Silicon carbide | Widely used, broad OEM coverage | Fragile; slower warm-up; avoid touching element |
| Hot surface igniter | Silicon nitride | Rugged, fast heating, long life | Must match voltage and control compatibility |
| Hot surface igniter | 24V low voltage | Used by specific OEM systems | Never connect to 120V |
| Flame sensor | Single rod, flame rectification | Simple, inexpensive | Needs good ground and correct flame position |
HVAC365 carries Supco igniters and flame sensors because Supco has served the HVAC and appliance repair trade since 1945 and builds replacement ignition parts for broad OEM coverage. Product pages include voltage, material and cross-reference details so you can confirm the match before ordering, and these items ship in 3-5 business days.
Often yes, using a replacement that is designed as a conversion for that application. The voltage, mounting position and control compatibility must all be right. Follow the replacement's instructions.
Many residential furnaces run with a few microamps of flame signal, but the minimum required varies by manufacturer. Use the equipment documentation for the exact range.
The most common cause is a weak flame signal from a dirty or poorly positioned flame sensor. Grounding problems, reversed polarity and burner issues can also cause it.
It depends on the type, the number of cycles and handling. Silicon nitride igniters generally last longer than silicon carbide. Careful installation and avoiding contact with the element extend life.
Yes, cleaning often restores the signal. If the porcelain is cracked, the rod is damaged or the signal stays low after cleaning and grounding checks, replace it.
Common causes are the wrong voltage, touching the element during installation, a cracked element from handling or a control applying voltage incorrectly. Check voltage at the igniter and inspect the old part's location for flame impingement.
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