Gas Furnace Sequence of Operation

The gas furnace sequence of operations is a significant component to understand for homeowners, HVAC technicians, and anyone interested in heating systems.

By comprehending these operations, one can ensure the proper operation of the gas furnace, leading to a comfortable and warm home during the heating season.

This guide will explain the sequence of operations in detail, highlighting the role of major components such as the gas valve, pressure switch, flame sensor, and control board.

What is the Sequence of Operations for a Gas Furnace?

A gas furnace operates in a systematic sequence, ensuring the safety and efficiency of the heating cycle. Here’s a general sequence of operation:

  1. Thermostat Call: The thermostat calls for heat, which signals the control board to begin the heating cycle.
  2. Inducer Motor: The control board activates the inducer motor, drawing fresh air into the combustion chamber.
  3. Pressure Switch: The pressure switch ensures that the inducer motor is functioning correctly, confirming adequate air flow to the combustion chamber.
  4. Hot Surface Ignitor or Spark Igniter: Once the pressure switch is activated, the control board signals the hot surface ignitor or spark igniter to initiate the ignition process.
  5. Gas Valve: After ignition, the control board opens the gas valve, allowing gas to flow into the combustion chamber.
  6. Burner Flames: The ignited gas creates burner flames that heat the heat exchanger.
  7. Flame Sensor: The flame sensor confirms the presence of the flames, ensuring the gas valve remains open.
  8. Blower Motor: The blower motor circulates heated air through the furnace cabinet and into the home.
  9. Safety Controls: The furnace utilizes safety controls like the fan limit control, safety switch, and safety device to prevent overheating and other potential hazards.

What is a Furnace Door AC?

Furnace Door AC is not a term typically used in the HVAC industry. If you meant Furnace Door Switch, it’s a safety device that cuts the power to the furnace when the access door is removed.

It helps to protect individuals from accidental contact with the electrical components of the furnace.

What Starts First on a Furnace?

The first part of a furnace that starts is the inducer motor. Triggered by the control board after a call for heat from the thermostat, it’s a critical component of the heating systems as it pulls fresh air into the combustion chamber.

How Should a Furnace Run?

A furnace should run smoothly, efficiently, and safely. It should achieve the preset temperature set on the thermostat, maintain consistent heat throughout the heating season, and operate without causing any safety concerns.

The fan control should engage to circulate air throughout your home, and the air filter should be checked and replaced regularly for optimal air flow.

Annual checks by the utility company or a professional HVAC service provider are recommended to ensure proper furnace routine and operation.

Understanding the Components and Their Roles

Blower Motor

The blower motor is a major component that circulates warm air through the furnace cabinet and into your home. It’s usually triggered once the heat exchanger has sufficiently warmed up.

Gas Valve

The gas valve is a safety device that controls the flow of gas into the combustion chamber. It only opens when the hot surface ignitor or spark ignitor has initiated the ignition process.

Pressure Switch

The pressure switch confirms that the inducer motor is functioning correctly, ensuring there is adequate airflow into the combustion chamber.

Flame Sensor

The flame sensor ensures the presence of flames in the combustion chamber, keeping the gas valve open.

Control Board

The control board is like the brain of the furnace, controlling the sequence of operations, from the inducer motor to the gas valve and the blower motor.

Inducer Motor

The inducer motor, which starts first on a furnace, is responsible for drawing fresh air into the combustion chamber.

Annual Check and Maintenance

Regular maintenance, including an annual check, is crucial for the smooth and efficient operation of the furnace.

This involves checking the air filter, inspecting the heat exchanger for cracks, and ensuring that the electrical components, including the control board, are functioning properly.


Understanding the gas furnace sequence of operations allows homeowners to appreciate the complexity of their heating systems.

From the inducer motor that starts the process, the role of the control board, to the vital safety features such as the pressure switch and gas valve, each component plays a critical role in providing warmth and comfort.

Regular checks and maintenance ensure the longevity and efficient operation of the furnace, keeping your home cozy and warm during the cold season.

Whether you have a single-stage, two-stage, or an electric furnace, understanding their sequence of operations is key to proper operation and routine maintenance.

About Roy Cohen

Roy Cohen has a burning passion for heating, cooling, and ventilation. He aims to help you save money on expensive repairs and bring you the best HVAC products. He has years of experience behind him in HVAC repair and garage maintenance.

You can find Roy at his LinkedIn or his email: [email protected]

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