Getting the most from your Semmes, AL furnace hinges on understanding it. Start that understanding with the basic parts that make up your furnace and that commonly have issues.

Registers, Vents and Air Filter

The parts of the furnace that control airflow into and out of the system are often the most neglected. The return registers and supply vents allow for airflow through the system. These need the proper clearance, at least 6 inches for supply vents, to allow proper circulation.

In addition to the vents and registers, your furnace depends on the air filter to keep airborne contaminants out. Keep in mind this will require periodic changing, typically every 30 to 90 days for 1- and 2-inch filters.

Circulating Fan & Blower Motor

The circulating fan and its attached blower motor create the air movement through the system. It draws air through the return registers, through the furnace, and then pushes it back out through the supply vents.

To keep working effectively, the circulating fan requires regular cleaning and balancing, which is part of annual maintenance. The blower motor requires lubricating if it isn’t sealed. Your maintenance technician will also test the motor to ensure that it’s spinning at the proper speed.

Burner & Burn Chamber

The burn chamber contains the furnace burner, which is similar to the burner on a gas grill. The chamber contains the gaseous fuel and the toxic exhaust created while the fuel burns.

The burner is adjustable so that a technician can alter the air-to-fuel ratio to create an efficient burn. Efficiently burning fuel will emit a blue flame with no flickering yellow and will produce little soot.


The furnace has an ignition system that lights the fuel at the beginning of each heating cycle. Older furnaces use a standing pilot light, which is a small flame that’s constantly burning. The pilot light ignites the fuel at the beginning of a heating cycle.

Most modern furnaces use either an intermittent pilot or a hot surface igniter. Intermittent pilot lights use an electric spark to light the pilot at the start of a heating cycle. Hot surface igniters are usually ceramic and glow red hot at the beginning of the cycle to light the burner.

Heat Exchanger

The heat exchanger is part of the exhaust system that transfers heat from the hot exhaust to the circulating air. The exhaust travels through the tubes of the heat exchanger while the circulating air moves over those tubes. This allows it to effectively transfer heat without ever risking mixing the exhaust and circulating air.

A cracked heat exchanger is one of the more serious furnace repairs you may face. This kind of damage allows toxic carbon monoxide to leak into your home, risking your family’s safety. Overheating is one of the more common causes of a cracked heat exchanger.

Flame Sensor

You’ll find the flame sensor in the burn chamber, and is one of the key safety mechanisms in your furnace. Its job is to ensure that there’s a flame when there’s fuel flowing into the chamber. This is essentially a heat sensor that ensures a minimum temperature.

The flame sensor can become dirty as the system runs, making it difficult for it to properly register heat. To prevent this, a maintenance technician will clean it and test it as part of routine maintenance.

Upper Limit Switch

Where the flame sensor ensures the proper minimum temperature, the upper limit switch keeps it from overheating. Most furnaces have this switch located above the burners and below the heat exchanger.

The upper limit switch can wear over time, making it register higher temperatures than are actually present. Your maintenance technician will check this sensor to make sure that it’s working properly.

Keep all of the parts of your furnace in good working order. Call to schedule your furnace maintenance appointment with one of the NATE-certified technicians at Air Specialty today.

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