What Does Pressure Switch Do In Furnace

Is your furnace acting up, leaving you shivering in the cold? One common culprit behind a furnace refusing to ignite or repeatedly shutting down is a faulty pressure switch. This seemingly small component plays a critical role in ensuring your furnace operates safely and efficiently. Let's dive into what a pressure switch does and how you can troubleshoot it, empowering you to potentially resolve the issue yourself or, at least, understand the problem better before calling a professional.

Understanding the Pressure Switch

Before we get into troubleshooting, let's clarify what the pressure switch actually *does*. In simple terms, it's a safety device that verifies the furnace's exhaust system is working correctly before allowing the burners to ignite. Think of it as a "proof-of-draft" sensor.

Here's the breakdown:

When your furnace starts, the inducer motor (a small fan) kicks on. This motor's job is to pull combustion gases out of the furnace and vent them outside.

The inducer motor creates a slight negative pressure within the venting system. This negative pressure is what the pressure switch "senses."

If the pressure switch detects the correct negative pressure, it closes an electrical circuit. This closed circuit signals to the furnace control board that it's safe to ignite the burners.

If the pressure switch doesn't detect the correct pressure (or loses it during operation), it stays open or opens, preventing the burners from igniting or shutting them down if they're already lit. This prevents dangerous combustion gases from leaking into your home.

So, a faulty pressure switch, even if the venting is fine, can prevent your furnace from working properly. It's a crucial safety measure!

Common Symptoms of a Pressure Switch Problem

Here are some telltale signs that your furnace might be suffering from a pressure switch issue:

Furnace won't ignite: The furnace goes through its startup sequence (inducer motor runs), but the burners never light.

Furnace ignites briefly, then shuts down: The burners light for a short period, but then the furnace shuts off, often displaying an error code related to pressure or draft.

Furnace cycles on and off frequently (short cycling): The furnace turns on and off more often than it should, indicating an intermittent issue with the pressure switch.

Error codes: Your furnace control board might display an error code specifically related to the pressure switch or a draft failure.

Inducer motor runs constantly: The inducer motor might run continuously without the burners ever igniting, indicating the pressure switch is not closing.

Troubleshooting the Pressure Switch: A Step-by-Step Guide

Before you start, always turn off the power to your furnace at the breaker box. Safety first! Also, be aware that working with gas appliances can be dangerous. If you are not comfortable with any of these steps, call a qualified HVAC technician.

Step 1: Visual Inspection (No Tools Required)

Start with a thorough visual inspection. Look for:

Obvious obstructions in the venting: Check the vent pipes outside your house for any blockages like bird nests, leaves, or snow. Clear any obstructions you find. This is a very common cause of pressure switch problems.

Disconnected or damaged vent pipes: Make sure all vent pipes are securely connected and free from cracks or holes. Even a small leak can affect the pressure.

Condensation buildup: Excessive condensation can accumulate in the vent pipes and block the airflow. Check for signs of water buildup.

Condition of the pressure switch itself: Look for any visible damage to the pressure switch, such as cracks, melted plastic, or disconnected wires.

Hoses connected to the pressure switch: Check the small rubber or plastic hoses connected to the pressure switch for cracks, kinks, or disconnections. These hoses are critical for transmitting the pressure signal.

Step 2: Checking the Hoses (No Tools Required, But Be Careful!)

The hoses connected to the pressure switch are often the source of problems. Follow these steps:

Carefully disconnect the hoses from the pressure switch and the inducer motor or vent pipe. Be gentle; they can be brittle.

Inspect the hoses for cracks, kinks, or obstructions. Even a tiny pinhole leak can prevent the pressure switch from working.

Blow through the hoses to check for blockages. If you can't blow air through them easily, there's likely a clog. A small piece of wire or a pipe cleaner can sometimes be used to clear obstructions, but be very careful not to damage the hose.

Reconnect the hoses securely. Make sure they are firmly attached to both the pressure switch and the inducer motor/vent pipe.

Step 3: Listening to the Pressure Switch (No Tools Required)

With the power restored (briefly!), listen carefully when the furnace starts. You should hear a faint "click" from the pressure switch as it closes. If you don't hear a click, it might indicate the switch is faulty or not receiving the proper pressure signal.

Important: Only run the furnace briefly for this test. If the pressure switch is truly faulty, allowing the furnace to run with an impaired safety system can be dangerous.

Step 4: Testing the Pressure Switch with a Multimeter (Basic Tools Required)

This step requires a multimeter, a basic electrical testing tool. If you're not familiar with using a multimeter, it's best to skip this step and call a professional.

Here's how to test the pressure switch:

Set your multimeter to the continuity setting (it usually looks like a sound wave symbol or an arrow pointing to a vertical line).

Disconnect the wires from the pressure switch.

Place one multimeter probe on each of the pressure switch terminals.

With the furnace off, the multimeter should show "OL" or an open circuit. This indicates the switch is open (as it should be when there's no negative pressure).

Now, with the furnace running and the inducer motor operating, the multimeter should show a closed circuit (a reading of 0 ohms or a beep). This indicates the switch is closing properly when it senses the correct negative pressure.

If the multimeter doesn't show these readings, the pressure switch is likely faulty and needs to be replaced.

Step 5: Cleaning the Inducer Motor (Basic Tools Might Be Required)

A dirty inducer motor can struggle to create sufficient negative pressure. While this requires a bit more disassembly, it's a relatively straightforward task for a mechanically inclined homeowner.

Turn off the power to the furnace at the breaker box.

Locate the inducer motor. It's typically a small, round fan motor attached to the vent pipe.

Carefully disconnect the wires from the inducer motor.

Remove the inducer motor assembly. This usually involves loosening a few screws or bolts. Take pictures as you disassemble it so you remember how to put it back together.

Clean the inducer motor fan blades. Use a brush or vacuum cleaner to remove any dust, dirt, or debris. A buildup of debris can significantly reduce the motor's efficiency.

Reassemble the inducer motor and reconnect the wires.

Turn the power back on and test the furnace.

When to Call a Professional

While these troubleshooting steps can help you identify and potentially fix a pressure switch problem, there are situations where it's best to call a qualified HVAC technician:

You're not comfortable working with electricity or gas. Safety should always be your top priority.

You suspect a more complex issue. If you've tried these troubleshooting steps and the furnace still isn't working, there might be a problem with the control board, the gas valve, or another component.

You're not sure what you're doing. It's better to be safe than sorry. A qualified technician can diagnose the problem accurately and make the necessary repairs.

You smell gas. If you smell gas, evacuate your home immediately and call your gas company or 911.

The pressure switch is difficult to access. Some furnaces have pressure switches located in hard-to-reach areas, making it difficult and potentially dangerous to work on them.

You have a newer, high-efficiency furnace. These furnaces often have more complex systems, and troubleshooting them can be challenging.

Replacing the Pressure Switch (Proceed with Caution!)

If you've determined that the pressure switch is faulty and you're comfortable with DIY repairs, you can replace it yourself. However, exercise extreme caution and follow these steps:

Find the correct replacement pressure switch. Make sure it's the exact same model number as the old one. You can usually find the model number on the switch itself.

Turn off the power to the furnace at the breaker box.

Disconnect the wires from the old pressure switch.

Disconnect the hoses from the old pressure switch.

Remove the old pressure switch. It's usually held in place by a few screws or clips.

Install the new pressure switch.

Reconnect the hoses to the new pressure switch.

Reconnect the wires to the new pressure switch. Pay close attention to the wiring diagram on the old switch or the furnace itself.

Turn the power back on and test the furnace.

Important Note: After replacing the pressure switch, monitor the furnace closely for a few days to make sure it's working properly. If you notice any unusual behavior, call a qualified HVAC technician.

By understanding the role of the pressure switch and following these troubleshooting steps, you can potentially save yourself a service call and get your furnace back up and running. Remember, when in doubt, always consult with a professional!