Furnace Pressure Switch

Furnace Pressure Switch and Its Benefits: Ensuring Safe and Efficient Operation

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A furnace pressure switch is a vital component of modern heating systems, contributing to the safe and efficient operation of your furnace. This small device plays a crucial role in monitoring the pressure within the furnace and ensuring that the combustion process is occurring as intended. In this article, we’ll explore what a furnace pressure switch is, how it works, its benefits, and its significance in maintaining a reliable heating system.

Understanding the Furnace Pressure Switch

A furnace pressure switch is a safety device installed in furnaces, particularly in high-efficiency and condensing models. It’s designed to monitor the air pressure within the furnace’s combustion chamber. This pressure is a key indicator of the combustion process, ensuring that the furnace is operating correctly and preventing harmful conditions such as backdraft or incomplete combustion.

How Furnace Pressure Switch Works

Here’s an overview of how a furnace pressure switch works:

  1. Basic Functionality:
    • A furnace pressure switch is a safety device designed to ensure proper airflow within the furnace system.
    • It monitors the pressure in the venting system or combustion chamber to verify that the system is operating correctly.
  2. Components of a Furnace Pressure Switch:
    • Diaphragm: The pressure switch contains a flexible diaphragm that reacts to changes in pressure within the furnace.
    • Switch Mechanism: Attached to the diaphragm is a switch mechanism, typically a set of contacts that open or close based on the movement of the diaphragm.
    • Tubing or Ports: The pressure switch is connected to the furnace’s venting system through tubing or ports that allow air pressure to be measured.
  3. Operating Principle:
    • When the furnace starts up, the inducer motor (which helps to create airflow) activates, drawing air through the venting system.
    • As the inducer motor runs, it creates a negative pressure or suction within the venting system.
    • The negative pressure causes the diaphragm in the pressure switch to flex or move, which in turn actuates the switch mechanism.
    • When the pressure reaches a preset level indicating proper airflow, the switch contacts close, allowing the furnace’s ignition sequence to continue.
    • If the pressure falls below the preset level, indicating a blockage or insufficient airflow, the switch contacts remain open, preventing the furnace from igniting and potentially causing a safety hazard.
  4. Safety Features:
    • Furnace pressure switches often have redundant safety features to ensure reliable operation.
    • They may include multiple sets of contacts for added redundancy, ensuring that the furnace shuts down if any part of the pressure switch fails.
    • Some pressure switches also have a manual reset feature to prevent the furnace from restarting automatically after a safety shutdown, requiring a technician to investigate and address the issue.
  5. Integration with Furnace Control System:
    • The pressure switch is an integral part of the furnace’s control system, working in conjunction with other safety devices and sensors to ensure safe and efficient operation.
    • It communicates with the furnace’s control board, which monitors the status of the pressure switch and other components to control the furnace’s operation.

Understanding how a furnace pressure switch works is essential for troubleshooting issues and ensuring the safe and efficient operation of your furnace system. Regular maintenance, including inspection and cleaning of the pressure switch and venting system, can help prevent problems and prolong the lifespan of your furnace.

Benefits of a Furnace Pressure Switch

  1. Safety Assurance: The primary benefit of a furnace pressure switch is its role in ensuring safe furnace operation. By monitoring pressure levels, it prevents scenarios like backdrafts or incomplete combustion, which can release harmful gases like carbon monoxide into your living space.
  2. Efficiency Enhancement: A properly functioning pressure switch contributes to efficient combustion. When the combustion process is optimized, it results in cleaner burning and better energy efficiency, which can lead to lower energy bills.
  3. Protection Against Damage: A malfunctioning furnace pressure switch can indicate issues within the combustion system. Detecting problems early can prevent further damage to the furnace components and extend the lifespan of your heating system.
  4. Compliance with Regulations: Furnace pressure switches are often required by building codes and safety regulations. Ensuring that your furnace is equipped with a functional pressure switch helps you comply with these standards.
  5. Early Warning System: If the pressure switch sends a signal to the control board due to abnormal pressure readings, it acts as an early warning system that alerts you to potential issues with your furnace before they become more serious.

Significance in Efficient Heating Systems

In high-efficiency and condensing furnaces, where combustion is carefully controlled for optimal efficiency, the role of the pressure switch becomes even more critical. These furnaces rely on precise air-to-fuel ratios to achieve maximum efficiency. A pressure switch ensures that the combustion process is maintained within the specified parameters, contributing to the furnace’s overall efficiency.

What is the Importance Of a Pressure Switch in Your Furnace?

The pressure switch in your furnace holds significant importance for the safe and efficient operation of the heating system. Its role goes beyond simply being a component; it acts as a safeguard, ensuring that the combustion process is occurring as intended and preventing potentially hazardous conditions. Here’s why the pressure switch is crucial:

1. Safety Assurance: The primary function of a pressure switch in a furnace is to ensure safety. It monitors the air pressure within the combustion chamber and detects any deviations from the expected range. If the pressure goes beyond safe limits, the pressure switch sends a signal to the furnace control board, prompting actions to prevent unsafe conditions. This is particularly critical in preventing backdrafts or incomplete combustion, which could release dangerous gases like carbon monoxide into your living space.

2. Prevention of Harmful Conditions: A malfunctioning furnace can lead to serious consequences. Incomplete combustion or irregular pressure can create a buildup of hazardous gases or even lead to fires. The pressure switch acts as an early warning system, shutting down the furnace if it detects any unsafe pressure levels, preventing potential harm to your family and property.

3. Efficient Combustion: For your furnace to operate efficiently, the combustion process must be controlled and optimized. A functional pressure switch ensures that the air-to-fuel ratio is maintained within the proper parameters. This, in turn, promotes clean and efficient combustion, contributing to better energy efficiency and lower energy bills.

4. Protection Against Damage: Issues within the combustion system can lead to damage to furnace components. The pressure switch’s role in monitoring the combustion process helps detect problems early, allowing for prompt maintenance and repairs. This not only prevents further damage but also extends the lifespan of your heating system.

5. Compliance with Regulations: Many safety standards and building codes require the presence of a pressure switch in furnaces. Ensuring your furnace is equipped with a properly functioning pressure switch helps you comply with these regulations, ensuring the safety of your home and meeting legal requirements.

6. Early Warning System: The pressure switch doesn’t just protect your furnace; it protects you. If the pressure switch sends a signal due to abnormal pressure readings, it serves as an early warning system. This alerts you to potential issues with your furnace before they become more serious, allowing you to take proactive measures.

7. Enhanced System Longevity: By preventing unsafe conditions and promoting efficient combustion, the pressure switch contributes to the overall health and longevity of your heating system. A well-maintained furnace, equipped with a functional pressure switch, is more likely to have fewer breakdowns and a longer operational lifespan.

How To Replace a Furnace Pressure Switch?

Replacing a furnace pressure switch is a task that requires some technical knowledge and skill. If you’re not comfortable working with HVAC systems, it’s best to hire a professional technician to ensure the replacement is done correctly and safely. However, if you have experience and feel confident in your abilities, here’s a general guide on how to replace a furnace pressure switch:

Note: Before beginning any work, turn off the power to your furnace and follow safety guidelines to avoid accidents or injuries.

Materials Needed:

  • New pressure switch (make sure it matches the specifications of your furnace)
  • Screwdriver
  • Pliers
  • Wrenches (if needed)
  • Electrical tape
  • Wire connectors (if needed)
  • Multimeter (for testing)


  1. Safety Precautions:
    • Turn off the power to the furnace from the circuit breaker.
    • Wait for the furnace to cool down before starting any work.
  2. Access the Pressure Switch:
    • Locate the pressure switch. It’s usually near the burner assembly or connected to the inducer motor housing.
  3. Disconnect Wires:
    • Carefully remove the wires connected to the pressure switch terminals. Take a photo or label the wires to remember their positions.
  4. Remove the Old Pressure Switch:
    • Loosen and remove any screws or mounting brackets holding the pressure switch in place.
    • Gently disconnect any tubing connected to the pressure switch.
  5. Install the New Pressure Switch:
    • Position the new pressure switch in the same location as the old one.
    • Reconnect any tubing to the appropriate ports on the pressure switch.
  6. Connect Wires:
    • Attach the wires to the terminals on the new pressure switch according to your photo or labels.
    • Use wire connectors to ensure a secure and proper connection. Wrap exposed wires with electrical tape if needed.
  7. Secure the Pressure Switch:
    • Tighten screws or brackets to secure the pressure switch in place. Make sure it’s properly aligned and stable.
  8. Testing:
    • Turn on the power to the furnace.
    • Use a multimeter to check the continuity and voltage across the terminals of the pressure switch to verify its functionality.
    • Observe the furnace’s operation to ensure that the pressure switch engages correctly during startup.
  9. Final Checks:
    • Double-check all connections, tubing, and wiring to ensure everything is properly connected and secure.
    • Ensure that there are no leaks in the tubing or any obstructions that could affect pressure readings.
  10. Turn On the Furnace:
    • Once you’re confident that the pressure switch replacement was successful, turn on the furnace and monitor its operation for a complete heating cycle.

Remember, if you encounter any issues during the replacement process or if you’re unsure about any step, it’s better to seek professional help. Incorrectly installing a pressure switch can lead to safety hazards, furnace malfunctions, or even damage to the heating system.

Types of Furnace Pressure Switch

Furnace pressure switches come in various types, each serving specific functions within the heating system. Here are the common types of furnace pressure switches:

Single-Stage Pressure Switch:

This is the most basic type of pressure switch. It monitors the pressure within the combustion chamber and ensures it’s within safe parameters for ignition and operation. If the pressure falls outside the predetermined range, the switch sends a signal to the control board to interrupt the ignition process, preventing unsafe conditions.

Two-Stage Pressure Switch:

Two-stage pressure switches are commonly found in two-stage or multi-stage furnaces. These furnaces have multiple heating capacities, and the pressure switch ensures that the combustion process is optimized for each stage. It monitors pressure variations and sends signals to the control board to adjust the heating output accordingly.

Differential Pressure Switch:

Differential pressure switches are often used in high-efficiency or condensing furnaces. They monitor the difference in pressure between the inlet and outlet of the furnace’s heat exchanger. This type of pressure switch ensures that the combustion process is efficient, with proper airflow through the heat exchanger. If the pressure differential is too low, it can indicate issues like clogs or restrictions that might affect performance.

Induced Draft Pressure Switch:

Induced draft pressure switches are specific to furnaces with induced draft motors. These motors create a negative pressure within the combustion chamber to ensure efficient exhaust gas removal. The induced draft pressure switch monitors this negative pressure and verifies that the motor is functioning properly. If the pressure doesn’t meet the required threshold, the switch prevents ignition to avoid harmful conditions.

Vent Pressure Switch:

Vent pressure switches are used in high-efficiency furnaces with sealed combustion systems. They ensure that the furnace’s exhaust gases are properly vented to the outdoors. The switch monitors the pressure in the venting system and verifies that it’s within acceptable limits. If there are blockages or issues with venting, the switch prevents ignition to prevent unsafe exhaust buildup.

Flow Switch:

While not exclusive to furnaces, flow switches are used in some heating systems. They monitor the flow of fluids (such as water or glycol) through the heat exchanger. If the flow rate drops below a certain level, the switch can prevent ignition to avoid potential damage to the heat exchanger.

Each type of furnace pressure switch serves a specific role in ensuring safe and efficient operation. The type of pressure switch your furnace requires depends on the specific design and functionality of your heating system. If you’re not sure about the appropriate pressure switch for your furnace, consulting a professional HVAC technician is recommended.

Common Issues and Troubleshooting

Here are common issues with furnace pressure switches and troubleshooting steps:

  1. Failure to Activate:
    • Possible Causes:
      • Clogged vent pipe or intake/exhaust pipes.
      • Faulty inducer motor.
      • Defective pressure switch.
    • Troubleshooting Steps:
      • Check for blockages in the vent pipes and clear any debris.
      • Verify that the inducer motor is running properly and replace it if necessary.
      • Test the pressure switch using a multimeter to determine if it’s functioning correctly. If not, replace it.
  2. Failure to Deactivate:
    • Possible Causes:
      • Blocked flue or vent pipe causing excessive pressure.
      • Faulty pressure switch.
    • Troubleshooting Steps:
      • Inspect the flue and vent pipes for obstructions and clear them if necessary.
      • Test the pressure switch to ensure it’s opening when the inducer motor stops. Replace the switch if it’s faulty.
  3. Cycling On and Off:
    • Possible Causes:
      • Inadequate combustion air supply.
      • Clogged air filter.
      • Faulty pressure switch.
    • Troubleshooting Steps:
      • Check for obstructions in the combustion air intake and clear any blockages.
      • Replace the air filter if it’s dirty or clogged.
      • Test the pressure switch and replace it if it’s not maintaining a consistent reading.
  4. Obstructions and Blockages:
    • Possible Causes:
      • Debris or ice buildup in the vent pipe.
      • Blocked condensate drain line.
    • Troubleshooting Steps:
      • Inspect the vent pipe for obstructions and clear any debris or ice buildup.
      • Check the condensate drain line for blockages and clear them if necessary.
  5. Electrical Problems:
    • Possible Causes:
      • Loose or corroded electrical connections.
      • Faulty wiring or control board.
    • Troubleshooting Steps:
      • Inspect all electrical connections for tightness and corrosion. Tighten or clean connections as needed.
      • Test the wiring and control board using a multimeter to identify any faults. Replace components if necessary.
  6. Testing and Diagnosis Techniques:
    • Procedure:
      • Use a multimeter to test for continuity across the pressure switch terminals.
      • Apply suction to the pressure switch tubing to simulate operation and observe if the switch opens and closes as expected.
      • Compare pressure readings with manufacturer specifications to ensure proper operation.

If troubleshooting the pressure switch does not resolve the issue, it’s advisable to consult a qualified HVAC technician for further diagnosis and repair. Working with gas furnaces can be dangerous, so it’s essential to prioritize safety and seek professional assistance when needed.

Furnace Pressure Switch FAQS

What is a furnace pressure switch?

A furnace pressure switch is a safety device in heating systems that monitors the air pressure within the combustion chamber. It ensures safe and efficient operation by verifying proper combustion and preventing hazardous conditions like backdrafts or incomplete combustion.

Why is a furnace pressure switch important?

A pressure switch is crucial for safety. It prevents unsafe conditions that could lead to the release of harmful gases like carbon monoxide. It also contributes to efficient combustion, protects the heating system, and ensures compliance with safety standards.

How does a furnace pressure switch work?

The pressure switch constantly monitors the air pressure in the combustion chamber. If the pressure falls outside the predetermined safe range, the switch sends a signal to the furnace control board. This signal can lead to actions such as shutting down the burner to prevent unsafe conditions.

What happens if a pressure switch fails?

If a pressure switch fails, it might prevent the furnace from igniting or cause the furnace to shut down during operation. This is a safety feature to prevent unsafe conditions. It’s important to replace a malfunctioning pressure switch promptly.

Can I replace a furnace pressure switch myself?

Replacing a pressure switch requires technical knowledge of HVAC systems. If you’re not experienced, it’s recommended to hire a professional technician to ensure safe and proper replacement.

How often does a furnace pressure switch need replacement?

Pressure switches are durable components, but they can fail over time due to wear and tear. If you experience issues with your furnace’s operation or suspect a pressure switch problem, consult a technician for diagnosis and potential replacement.

Can a pressure switch affect heating efficiency?

Yes, a pressure switch affects heating efficiency by ensuring proper combustion. If the pressure switch fails, combustion might not be optimized, leading to inefficient burning and potential energy wastage.

Are there different types of furnace pressure switches?

Yes, there are different types of pressure switches, including single-stage, two-stage, differential, induced draft, and vent pressure switches. Each type serves specific functions within various furnace designs.

How do I know if my pressure switch is malfunctioning?

Common signs of a malfunctioning pressure switch include furnace startup issues, frequent shutdowns, error codes on the furnace display, or unusual noises during operation. If you suspect a problem, it’s best to have a professional technician diagnose the issue.

Can I bypass a pressure switch to keep the furnace running?

Bypassing a pressure switch is not recommended and can be dangerous. Pressure switches are designed to ensure safe operation. Bypassing them can lead to unsafe conditions, including potential carbon monoxide leaks.


A furnace pressure switch is an essential safety and efficiency component in modern heating systems. By monitoring combustion pressure and ensuring safe and optimal operation, it plays a critical role in preventing harmful conditions, protecting your heating system, and promoting energy efficiency. Regular maintenance, including checking and cleaning the pressure switch, is essential to ensure its reliable performance, allowing you to enjoy a warm and comfortable home during the colder months with peace of mind.

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