How Does A Furnace Pressure Switch Work? Type & Components

In this blog post, we will know in detail the working of the furnace pressure switch.

What Are Furnace Pressure Switch?

Furnace pressure switches are the safety devices that are installed in the furnace system to limit exposure to carbon monoxide.

How Does A Furnace Pressure Switch Work

Functions Of Furnace Pressure Switch

Furnace maintenance means understanding the pressure switch. It’s a safety device that keeps harmful gasses like carbon monoxide away from your home. 

Here’s what you need to know:

Furnace Pressure SwitchMonitors air pressure and lets furnace start
Inducer Motor and Fan BladeCreates negative air pressure in heat exchanger chamber
Pressure Diaphragms (2)One detects negative pressure, other detects positive pressure

Components of the Furnace Pressure Switch

To understand the components of the furnace pressure switch, which plays a crucial role in the operation of your home furnace.

You need to know about the collection chamber, inducer motor, diaphragm, draft inducer, and inducer fan. 

The Collection Chamber

The pressure switch of a furnace is an essential part for safety and efficiency. It has a collection chamber, which gathers air pressure from the inside. 

This chamber has two parts – the diaphragm and the switch mechanism. The diaphragm is a flexible membrane that reacts to pressure changes. 

When the pressure reaches a certain level, it moves the diaphragm, triggering a switch. This signal shuts down or turns on furnace functions.

The Inducer Motor

The inducer motor is a key component when it comes to furnace pressure switches. It clears exhaust gasses from the furnace flue, preventing a buildup of pressure in the furnace and making sure it runs safely and efficiently.

The inducer motor also provides the initial airflow which ignites the burner and starts the heating process. 

It draws air, creating a draft that sucks in gas. Once lit, the flame sensor detects heat in seconds.

Typically, the inducer motor works fast and makes a lot of noise. However, newer models are designed to save electricity and run much quieter.

The Diaphragm

The diaphragm is a key part of the furnace pressure switch. It acts as a flexible separator between two sections. 

It expands or shrinks when there are changes in air pressure, to turn on and off the contact mechanism.

The thickness and material of the diaphragm impact its sensitivity towards pressure changes. This makes it essential for furnace operation. 

Quality materials like stainless steel or brass are used for the diaphragm, as the material should be strong. These materials are also resistant to corrosion and wear, caused by regular use.

The hard or soft properties of different types of diaphragms influence their reaction time. This needs to be chosen based on the furnace’s specifications.

The Draft Inducer

The Draft Inducer is a quiet and efficient component designed to regulate furnace pressure, extract flue gasses, and vent them outside. It also prevents any harmful fumes or carbon monoxide from entering your living spaces.

The Inducer Fan

The Inducer Fan creates suction to draw out exhaust gasses. Its size varies based on furnace size and model. 

It’s usually made of metal for durability. Cleaning and lubrication can extend its lifespan. But, if not maintained or damaged, dirt and debris can cause blockage and malfunction.

How Does the Furnace Pressure Switch Work?

To understand how a furnace pressure switch works, let me give you some insights. 

In this part, we’ll cover some of the central concepts surrounding the operation process, safety features, negative pressure, reasons for failure, and back drafting. 

Operation Process

Operating a furnace pressure switch needs attention to detail. 

Here’s the 4-step process:

Gradual startWhen you turn the furnace on, the pressure switch waits for the blower motor to reach its designed airflow speed.
Pressure readingWhen the motor is at speed, it senses the induced draft blower’s pressure level and checks if it’s within the calibration range.
Signal transmissionIf it’s all good, it sends a signal to open the gas or oil supply valves and produces heat.
Safety featureIf the intake air is insufficient, the pressure switch cuts off fuel supply until safe conditions return.

Safety Feature

Chilly winter nights demand a furnace to keep our homes warm. Pressure switches are key to ensuring everything runs smoothly, safely and efficiently.

  • Pressure switches detect potential blockages in the piping system which might impede airflow.
  • If there’s inadequate airflow, the switch will signal the furnace to shut down.
  • It also acts as an ignition lock to avoid overheating.
  • These safety features protect from carbon monoxide build-up.

Negative Pressure

Negative pressure in a furnace is lower than atmospheric pressure. This happens when there’s not enough air, which affects the furnace’s performance. 

Therefore, understanding negative pressure is important for keeping the furnace safe and efficient.

Negative pressure is needed to take combustion gasses out of the combustion chamber. This prevents them from entering the room. 

A pressure switch monitors the pressure inside the furnace. If it drops too low, the switch will shut off the furnace.

Reasons for Failure

The furnace pressure switch is really important for a furnace to work. But it can break down for various reasons.

Like blockages in the tubing, a broken motor or blower, and bad wiring connections.

Clogged filters can also stop the switch. They form when dirt, debris, and pet hair build up over time, blocking air flow.

Sometimes, the switch just gets old and stops working properly. Like any machine, it wears down and can’t give accurate readings. That can lead to it breaking down.

To avoid this, you need to keep your furnace in good shape. Regular maintenance and replacing old parts can keep it running for years. 

HVAC contractors offer services to help with furnace repair and maintenance around the world. Also, back drafting can shut down your furnace, like ghosting after a bad Tinder date.

Back Drafting

Back drafting is a term for air movement going in the opposite direction of where it’s supposed to. 

This can happen when combustion air is blocked, creating negative pressure. This makes flue gasses go back inside the building, leading to high levels of carbon monoxide.

To avoid back drafting, a furnace pressure switch is used. It monitors the pressure and, if it’s too low or high, turns off the burner. This stops combustion and prevents back drafting.

Adjusting the pressure switch correctly is important for optimal performance. If there are any concerns, call a technician.

Importance of the Furnace Pressure Switch

To ensure that your furnace operates safely, efficiently, and in order, using a furnace pressure switch is essential. 

Preventing Harmful Gasses

Safety is key when it comes to heating your home. The furnace pressure switch acts as a safety net, preventing toxic gasses like carbon dioxide from entering your home. 

It monitors ventilation, making sure enough air is flowing through the burner to stop carbon monoxide buildup.

If the switch is faulty or damaged, it could cause death from toxic fumes. So, it’s vital to inspect and maintain it annually to keep your household safe.

Ensuring Efficiency

The efficiency of a furnace depends on various factors. One of the most vital components is the pressure switch. It monitors air pressure to make sure your furnace runs at its best. 

This switch helps avoid safety hazards such as gas leaks or combustion issues, plus it saves energy and reduces utility bills.

It’s essential to maintain and check your pressure switch regularly. Broken or poorly maintained switches can lead to ineffective heating or dangerous gas leaks. 

Regular maintenance can help catch any underlying problems early and ensure optimal efficiency.

To keep your furnace in top shape:

  • Replace the air filter regularly.
  • Seal duct leaks.
  • Keep the unit clean.
  • Schedule annual tune-ups with professionals.

Ensuring Safety

The furnace pressure switch has a major role in safety. It keeps an eye on the airflow in the furnace. 

If any obstruction or restriction is there, it shuts the furnace down. This stops dangerous gasses like carbon monoxide from building up, which can be deadly for humans.

Without the switch, there’d be no way to spot airflow issues. This could lead to accidents, and many homes have had them. 

The pressure switch also helps with energy efficiency and lower bills. It makes sure the system runs well, without air loss or waste.

Types of Furnace Pressure Switches

To gain more insight into furnace pressure switches, let’s delve into the different types available. 

Mechanical Pressure Switches

Mechanical pressure switches are activated by a mechanical mechanism and work based on changes in air pressure.

When looking to buy one for your furnace or industrial processes, make sure it is within your budget and meets the job requirements.

Think about accuracy, response time, operating temperature range and voltage ratings. Compare different options before selecting one to prevent a bad decision.

Electronic Pressure Switches

When it comes to furnace pressure switches, electronic pressure switches are popular. They use electronic sensors to detect pressure changes in a furnace system. 

Let’s explore the types of electronic pressure switches and their features!

Differential Pressure Switches

Detects differences in air or gas pressures before switching on or off. 

Benefits: Smooth functioning and avoids malfunctions due to incorrect pressure levels.

Vacuum Pressure Switches

Detecting negative air or gas pressures. 

Benefits: Solve issues related to air conditioning and heating systems which require accurate vacuum measurements.

Temperature Switches

Leverage temperature readings to control furnace operations. 

Benefits: Keep the furnace from overheating by monitoring the temperature levels.

Electronic pressure switches have other perks, like reducing downtime, improving energy efficiency, and providing more accurate controls. 

Common Problems with Furnace Pressure Switches

To troubleshoot common problems with your furnace pressure switch, with its switch being an important safety feature.

Obstructions in the Flue Pipe

Furnace pressure switches can be tricky when it comes to flue pipe blockages. Leaves, birds’ nests, and other debris can easily clog the exhaust duct and make switches fail.

Stay ahead of the problem, Inspect and clean your flue pipe often. Plus, make sure your home has proper ventilation; this will help keep harmful gasses away.

Debris in the Diaphragm

Debris build-up in a furnace pressure switch diaphragm can cause problems. 

Blockages stop the switch from detecting air pressure changes correctly, resulting in inaccurate readings. This can cause the unit to start intermittently or, sometimes, not at all.

Damage to the diaphragm itself can also occur. Low pressure can cause components like internal vacuum bleeds to become clogged with dust or other particles, reducing their effectiveness. 

Regular cleaning and maintenance is recommended to keep the furnace running smoothly.

Negative Air Pressure in the Home

Maintaining a balanced airflow in your home is very important. Negative air pressure can cause discomfort like drafts and difficulty breathing.

It can also damage heating elements, like furnaces’ pressure switches. These switches control the air into and out of the furnace. 

Negative air pressure makes them think there isn’t enough airflow, and the furnace may shut down.

Cold air can come in through small openings like around doors and windows. This causes a draft and hot/cold spots. The performance of the furnace’s pressure switch will be affected, and it may shut down.

Troubleshooting Tips for Furnace Pressure Switches

To troubleshoot furnace pressure switch problems, with its vital safety feature, you must investigate the root cause of the issue. 

Checking for Obstructions

It’s essential to give your furnace’s pressure switch a check for obstructions. This could be caused by dirt, rust, or soot accumulating in the tube.

Follow these 6 steps to get it done:

  1. Turn off the furnace and disconnect power supply
  2. Unfasten tubing from pressure switch and blow into it
  3. If air doesn’t flow freely, remove obstruction with a long wire or brush
  4. Carefully reattach tubing to prevent damage
  5. Connect power and restart system
  6. If problem still persists, contact a pro HVAC technician

Inspecting the Diaphragm

The furnace pressure switch is a must-have for a safe and efficient heating system. To spot issues, inspecting its diaphragm is key. 

Do the following:

  1. Cut the power to the furnace.
  2. Take off the cover to access the pressure switch.
  3. Carefully remove the diaphragm from its housing.
  4. Look for wear, tears, or cracks on both sides.
  5. Replace the entire pressure switch if damage is found.

For best results, check your furnace manufacturer’s manual for instructions. This way, you won’t accidentally damage any parts and troubleshoot properly.

Checking the Circuit and Terminal Connections

Troubleshooting furnaces? Check the circuit and terminal connections. Here’s how:

  1. Turn off the power supply to the furnace before the inspection.
  2. Check wires on the pressure switch are secure – use a multimeter or test light to check the voltage running through each connection.
  3. If there is no voltage, tighten these connections with needle-nosed pliers or screwdrivers.

Testing and Replacing the Furnace Pressure Switch

To test and replace the furnace pressure switch, you need to follow a step-by-step process with two solutions:

  • Testing the switch 
  • Replacing the switch. 

Testing the Switch

Testing the switch will give you insights into its mechanical and functional components while replacing the switch is an option if it fails to meet the necessary conditions. 

  1. Turn off, unplug, and identify the switch.
  2. Then, remove the tubing and plug the pressure tap with your finger or thumb.
  3. Measure voltage with a multimeter and record the readings on each wire.
  4. If there’s no voltage, replace the switch. But remember: consult your manual beforehand and leave testing to the professionals.
  5. Finally, when replacing the switch, get one that fits your system – different brands have different specifications. Time to switch things up and replace that faulty furnace pressure switch!

Replacing the Switch

If your furnace pressure switch is acting up, replace it. Secure efficient heat and avoid system failure by following these 6 steps:

  1. Switch off the power supply.
  2. Scan the furnace panel for the switch.
  3. Unplug any wires from the faulty switch.
  4. Install a new switch in the panel where the old one was.
  5. Reattach any wires that were disconnected in step 3.
  6. Turn on the power and check if the furnace functions properly.

Furnace Pressure Switches and Your Home’s Heating System.

Furnace pressure switches are key safety devices that protect your home from dangerous gasses. 

They monitor air pressure to guarantee sufficient suction to remove exhaust fumes from the heat exchanger. 

If the switch senses negative air pressure, it’ll shut off power to the furnace and avoid carbon monoxide entering your house.

It’s important to comprehend how furnace pressure switches work, as they can be a frequent source of issues in your heating system. 

Causes of failure include obstruction in the flue pipe, a problem with the inducer fan or motor, or back drafting caused by the wind outside the building envelope.

Furnace pressure switches are of two main types: mechanical pressure switches, which use diaphragms to detect changes in air pressure, and electronic pressure switches which use a meter or terminal on a circuit board to spot changes in air pressure. 


To conclude, furnace pressure switches are essential for any heating system. Knowing their operation and upkeep requirements can help make sure your home stays heated safely and efficiently for many years.