A faulty gas valve is one of the most common causes of a furnace that won't heat. We stock replacement gas valves for Lennox, Carrier, Trane, Nordyne, and most major brands — including Honeywell and White-Rodgers OEM units and compatible replacements.

A faulty gas valve is one of the most common causes of a furnace that won't heat. We stock replacement gas valves for Lennox, Carrier, Trane, Nordyne, and most major brands — including Honeywell and White-Rodgers OEM units and compatible replacements.

A faulty gas valve is one of the most common causes of a furnace that won't heat. We stock replacement gas valves for Lennox, Carrier, Trane, Nordyne, and most major brands — including Honeywell and White-Rodgers OEM units and compatible replacements.

A faulty gas valve is one of the most common causes of a furnace that won't heat. We stock replacement gas valves for Lennox, Carrier, Trane, Nordyne, and most major brands — including Honeywell and White-Rodgers OEM units and compatible replacements.

The flame sensor is a small metal rod mounted inside the burner assembly that confirms ignition has occurred after the gas valve opens. When the furnace calls for heat, the gas valve opens and the ignitor fires. Once a flame is established, the flame sensor detects it through a process called flame rectification, passing a small electrical current through the flame to verify combustion. If the control board does not receive this confirmation signal within a few seconds, it shuts the gas valve as a safety measure and the furnace goes into lockout. A dirty or degraded flame sensor is one of the most common causes of a furnace that starts, runs briefly, and then shuts off. Flame sensors are inexpensive, straightforward to replace, and available for virtually every major furnace brand including Lennox, Carrier, Trane, Nordyne, Armstrong, Heil, and Rheem.

Capacitors in a furnace system provide the electrical boost needed to start and run the blower motor. The start capacitor delivers a surge of current to get the motor turning from a standing start, while the run capacitor maintains a consistent current supply during normal operation. Some furnace blower motors use a single dual-run capacitor that handles both functions. A failing capacitor is a common cause of a blower motor that runs slowly, hums but does not start, or fails to run at all. Capacitors degrade over time and are particularly susceptible to heat, which shortens their useful life in furnace applications where ambient temperatures in the mechanical cabinet are high. Capacitors are rated by microfarad (uF) and voltage, and it is important to match these ratings to the original specification on the motor nameplate. We stock furnace capacitors across a wide range of ratings compatible with Lennox, Carrier, Trane, Nordyne, Rheem, Heil, and other major brands.

The draft inducer motor, also called the inducer blower or combustion blower, is responsible for pulling combustion gases through the heat exchanger and exhausting them out of the flue before and during the heating cycle. On most modern furnaces, the inducer motor runs first when the furnace calls for heat, before the gas valve opens, to purge any residual gases from the heat exchanger and establish proper draft. The pressure switches on the furnace monitor the inducer's operation and will prevent the gas valve from opening if the inducer is not running correctly. A failing inducer motor is a common cause of pressure switch faults, no-heat calls, and unusual noises during furnace startup. Draft inducer motors are brand and model-specific components, and matching the correct motor to the furnace model is critical for proper operation and pressure switch compatibility. We stock inducer motor assemblies for Lennox, Carrier, Trane, Nordyne, Armstrong, Heil, Rheem, and other major brands.

Pressure switches are safety devices that monitor the operation of the draft inducer motor and verify that proper airflow through the heat exchanger is established before allowing the gas valve to open. When the inducer motor runs, it creates a negative pressure in the flue system that closes the pressure switch, signaling the control board that combustion airflow is confirmed. If the pressure switch does not close, the furnace will not advance to ignition. Common causes of pressure switch faults include a failed inducer motor, a blocked flue or condensate drain, a cracked heat exchanger, or a failed pressure switch itself. Because multiple issues can trigger a pressure switch fault, diagnosis should confirm the root cause before replacing the switch. We stock single-port and dual-port pressure switches for all major furnace brands, and pressure switches should be matched by brand, model, and the pressure rating marked on the original switch.

The ignitor is responsible for lighting the gas burners when the furnace calls for heat. Most furnaces manufactured since the 1990s use a hot surface ignitor (HSI), which is a fragile ceramic or silicon nitride element that heats to ignition temperature when voltage is applied. Older furnaces may use a spark ignitor or a standing pilot system. Hot surface ignitors are one of the most commonly replaced furnace components because they are physically fragile, subject to thermal stress from repeated heating and cooling cycles, and contaminated easily by oils from handling. A cracked or failed ignitor is one of the most common causes of a furnace that runs the inducer motor and opens the gas valve but fails to light. Silicon nitride ignitors are more durable and longer-lasting than the older silicon carbide style and are the preferred replacement on many platforms. We stock OEM and quality replacement hot surface ignitors for Lennox, Carrier, Trane, Nordyne, Armstrong, Heil, Rheem, and most other major furnace brands.

Limit switches are safety devices that monitor furnace temperature and shut off the burners if the heat exchanger exceeds a safe operating temperature. Every furnace has at least one primary limit switch, and many have multiple limits monitoring different areas of the heat exchanger and plenum. If the furnace overheats due to restricted airflow, a blocked filter, a failed blower motor, or a closed supply register, the limit switch opens and shuts down the burners to prevent heat exchanger damage or fire. Some limit switches reset automatically once the temperature drops; others require manual reset. Repeated limit switch trips are a diagnostic signal that something is causing the furnace to overheat, and the root cause should be addressed before the limit switch itself is replaced. High-limit switches are also used on furnaces as rollout switches, which monitor for flame rollout outside the combustion chamber and provide an additional safety shutoff. We stock limit switches and rollout switches across a wide range of temperature ratings for all major furnace brands.

Furnaces contain low-voltage fuses, typically rated at 3 to 5 amps, that protect the control board and the transformer from damage caused by short circuits in the low-voltage thermostat wiring. When a thermostat wire shorts against a metal surface or another wire, the resulting overcurrent will blow the fuse rather than damaging the control board. A blown furnace fuse is a common finding when a furnace has no response at all to a thermostat call, and it is often the first thing to check before suspecting a failed control board. Furnace fuses are standard automotive-style blade fuses available in several common amperage ratings. It is important to replace a blown fuse with one of the same rating rather than a higher value, and if the replacement fuse blows immediately, the underlying short circuit in the wiring must be located and corrected. We stock furnace fuse kits and individual fuses for all major brands and recommend keeping spares on hand, particularly in commercial and multi-unit settings.

The burner assembly is the component in a gas furnace where fuel and air are mixed and combustion occurs. Most residential and light commercial gas furnaces use either inshot burners or ribbon burners arranged in a row across the bottom of the heat exchanger. The burners receive gas from the manifold and mix it with combustion air drawn in by the draft inducer before ignition. Over time, burners can corrode, warp, develop rust scale that disrupts the flame pattern, or become clogged with debris, all of which can cause uneven combustion, delayed ignition, or flame rollout. Burner assemblies also include the manifold, orifices, and in some cases the gas valve mounting bracket as an integrated assembly. Replacement burner assemblies must be matched to the specific furnace model to ensure correct orifice sizing for the fuel type (natural gas or propane), proper fit in the heat exchanger, and compatible manifold connections. We stock burner assemblies and individual burner components for Lennox, Carrier, Trane, Nordyne, Armstrong, Heil, Rheem, and other major brands.