A furnace pressure switch is a safety device that monitors the airflow in your furnace’s venting system. It ensures your furnace turns off if there are venting problems that could lead to a dangerous buildup of carbon monoxide. Knowing how to test a furnace pressure switch can help you diagnose furnace issues and determine if a faulty switch needs to be replaced.
What is a Furnace Pressure Switch and How Does it Work?
A furnace pressure switch, also called an air pressure switch, is usually located near the draft inducer fan. This switch has a tube that connects it to the furnace’s venting system. It measures the difference in air pressure between the inside of the furnace and the outside vent.
Here is how a furnace pressure switch works:
- When the furnace turns on, the draft inducer fan starts moving air through the heat exchanger and out the venting. This creates a negative pressure inside the furnace.
- The negative pressure pulls air through the pressure switch’s tube. This trips the switch and sends power to the gas valve and igniter.
- Once the furnace ignites and heats up, the rising hot air continues moving out the vent, maintaining the negative pressure.
- If the vent becomes blocked or restricted, the negative pressure is lost. This causes the pressure switch to open. When it opens, it cuts power to the gas valve and furnace components, shutting the system down safely.
The pressure switch serves as a fail-safe to prevent dangerous furnace operation if airflow becomes obstructed. Testing the switch is necessary to determine if it’s working correctly or needs replacement.
Signs Your Furnace Pressure Switch May Be Faulty
Some common signs your furnace pressure switch may not be working properly include:
- Furnace short cycling – The furnace turns on but shuts off after a few minutes before reaching the set temperature. Then it repeats this on/off cycling.
- Furnace won’t stay lit – The furnace ignites but shuts down after a minute or two. This continuous igniting and shutting down is called “lockout”.
- Dirty filter shutoff – The furnace works fine with a clean filter but goes into lockout when the filter gets dirty.
- Obstructed venting – Signs of vent blockage like corroded pipes or a birds nest in the outdoor vent terminal.
- No LED status light – On some furnace circuit boards, the LED doesn’t display normal operation codes.
- Draft inducer runs continuously – The draft inducer fan keeps running even when the furnace should be off.
Any of these issues point to a problem with the furnace’s airflow that likely involves a faulty pressure switch. Testing the switch is the next logical troubleshooting step.
How to Test a Furnace Pressure Switch
You’ll need a multimeter to test a furnace pressure switch. Follow these steps:
Turn Off Power to the Furnace
Locate the furnace circuit breaker or main power switch and turn it OFF for safety.
Locate the Pressure Switch
The pressure switch is usually mounted on the side or top of the furnace cabinet near the draft inducer motor. It has a plastic tube connected from its barbed fitting to the furnace’s venting.
Locate the pressure switch near the draft inducer
Detach the Switch’s Tube
Gently remove the plastic tube from the pressure switch’s barbed fitting. Taking it off allows you to test the switch independently from the furnace venting.
Set Multimeter to Continuity Mode
Turn your digital multimeter to continuity mode. This is indicated by a diode symbol or a setting like CONT, BEEP, or AUDIBLE.
Test the Pressure Switch Contacts
Touch your meter probes to the two terminals on the pressure switch. These are labeled NO and COM.
- NO stands for “normally open”.
- COM is “common”.
With the tube detached, the contacts should show no continuity in this position. The multimeter display should show OL, infinity Ω, or no beep/tone.
Use a multimeter in continuity mode to test the NO and COM contacts
Reattach the Plastic Tube
Carefully push the plastic tube back onto the barbed fitting. Make sure it seals tightly.
Test Switch Continuity Again
Keep your meter probes touching the NO and COM contacts. Gently suck on the end of the plastic tube and slowly release. This should pull the pressure switch closed.
At some point while sucking or releasing, you should hear a beep tone or see a zero reading if the contacts close properly. If no change occurs, the switch likely needs replacing.
Note: You can also blow into the tube while testing. The key is to create a pressure difference.
Here’s a video demonstrating the testing process:
Check the Status LED
Some pressure switches have an LED light that indicates status. The LED being off may also indicate a bad pressure switch.
Turn Power Back On
Once you’ve finished testing, turn the furnace circuit breaker or switch back ON.
Determine if Replacement is Needed
If the multimeter shows the switch contacts are opening and closing properly when pressure is applied, the pressure switch itself is likely okay. The problem may be with the venting system or draft inducer fan.
If the pressure switch shows no continuity change during your testing, it will need replacement. Only install an identical replacement pressure switch.
What if the Pressure Switch Passed the Test?
If your test confirms the furnace pressure switch is working correctly, the problem lies elsewhere. Here are some other things to investigate:
- Blocked furnace vent – Examine the entire venting system for obstructions. Look for crushed/disconnected pipes or nests in the outdoor vent terminal that could block airflow.
- Faulty draft inducer – The draft inducer fan may be defective. Check for power to the inducer and test its capacitors. Replace if needed.
- Improper vent configuration – Improper vent slope, too many elbows, or wrong vent diameter can impact airflow. Inspect the venting layout.
- Dirty filters – Very dirty filters cause increased restriction in the furnace. Replace the filter if needed and test operation again.
- Flame sensor – A faulty flame sensor can cut power to the furnace prematurely. Inspect and clean the flame sensor.
- Gas valve – Make sure the gas valve receives power and opens to allow fuel flow. Replace it if defective.
- Circuit board – Furnace control boards can go bad and cause intermittent operation. Test system components for power and ground during operation to isolate control board issues.
Carefully inspecting the furnace for vent restrictions, component failures, or losing power can help identify the actual problem if the pressure switch tests okay.
What if the Pressure Switch Failed the Test?
If the multimeter shows the pressure switch contacts do not close properly when you apply pressure, it indicates the switch needs replacing. Here are a few tips for replacing a bad furnace pressure switch:
- Match the replacement switch style exactly. There are different designs for various furnace models.
- Note the pressure rating and temperature range of the original before buying the new one.
- Label wires before disconnecting and note the NO and COM positions.
- Apply a small amount of thread seal tape to threaded switch port fittings.
- Insert the plastic vent tube fully over the barbed fitting.
- Secure any electrical covers removed to access the switch.
- Turn power back on and test furnace operation through several cycles to confirm normal operation.
Replacing the pressure switch with a proper new part should resolve furnace problems if venting restrictions and other failures have been ruled out.
FAQs About Testing a Furnace Pressure Switch
What if I don’t have a multimeter?
You can do a basic test without a multimeter by disconnecting the plastic tube and watching the furnace try to ignite. If it won’t start without the tube attached, the switch itself is likely ok. If it ignites fine without the tube, the switch probably needs replacing.
My furnace runs fine with the door panel off but goes into lockout when closed. What’s wrong?
This points to insufficient combustion air or a restriction preventing proper airflow through the blower section. Dirty filters, blower motors, ductwork, and venting issues can contribute to this. Check for obstructions and proper termination of the intake and exhaust vents.
Do I need to turn off power to the air handler unit too?
If the furnace system has a separate air handler, you only need to turn off the furnace power. Power can remain on to the air handler/blower which has its own circuit breaker. Only the furnace itself needs disconnected during pressure switch testing.
How can I tell which wire is common on the pressure switch?
Unfortunately wire colors vary by manufacturer, so you can’t rely on color alone. Use the continuity test to identify which terminal remains closed when pressure is applied. That will be the COM terminal.
What temperature does the pressure switch need to handle?
Most residential furnace pressure switches are rated for temperatures between -20°F to 180°F. Make sure your replacement switch meets or exceeds the temperature limits of your original one.
How often should the pressure switch be replaced?
Furnace pressure switches can operate for many years when working properly. Only replace them if testing shows they are defective or fail to open/close contacts properly. Don’t replace as routine maintenance.
Conclusion
Knowing how to test a furnace pressure switch is an important diagnostic skill that can help identify airflow issues and determine if replacement is needed. Use a digital multimeter to check if the switch contacts open and close properly in response to pressure changes on the barbed tube fitting. If the pressure switch tests faulty, replacing it with an exact equivalent part should solve furnace problems related to intermittent operation, short cycling, or failure to ignite. If the switch tests okay, focus troubleshooting on the furnace venting system, inducer fan, gas valve, and other components that influence proper airflow and combustion. Learning to test and replace a malfunctioning pressure switch can allow you to quickly restore safe and reliable furnace operation and heating comfort.
How Furnace Pressure Switches Work
Before learning how to test a furnace pressure switch, it helps to understand what it does and how it operates. Here is an overview of how furnace pressure switches work:
Monitor Air Pressure
The primary job of the pressure switch is to monitor the difference in air pressure between the inside of the furnace combustion chamber and the venting system.
Have an Air Tube
The switch connects to the furnace interior via a plastic tube. This tube transmits the internal air pressure to the switch.
Have Electrical Contacts
The pressure switch has electrical contacts that are either open or closed depending on the air pressure.
Control Gas Valve Power
The switch controls power to the gas valve and furnace ignition components. It only allows gas flow if internal pressure is correct.
Open Contacts When Pressure is Lost
The contacts open to cut power if airflow becomes restricted and internal pressure increases. This prevents gas flow in unsafe conditions.
Close Contacts When Pressure is Normal
When the draft inducer creates proper negative pressure, the switch contacts close to allow gas valve operation and ignition.
Provide Safety Shutoff
The main purpose is to provide a safety shutoff if venting problems occur that could lead to dangerous carbon monoxide accumulation.
Require Correct Pressure to Operate
Proper airflow and negative pressure must be present for the furnace to light off and continue running safely. The pressure switch ensures this.
Indicate Airflow Obstructions
If the contacts open unexpectedly, it often indicates an airflow restriction like blocked vents, faulty draft inducer, or very dirty air filters.
Understanding how the pressure switch, air tubing, and electrical contacts interact is useful when diagnosing and testing furnace operation. The switch plays an important role in monitoring safe airflow conditions.
Steps for Testing a Furnace Pressure Switch
Testing a furnace pressure switch with a multimeter is a fairly straightforward process. Here are the basic steps involved:
Turn Off Power
Shut off power to the furnace at the main breaker or switch for safety when working on components.
Locate the Switch
Find the pressure switch near the draft inducer fan. It will have a plastic tube attached.
Remove the Tube
Gently detach the plastic tube from the barbed fitting on the pressure switch. This isolates it from the furnace.
Set Multimeter
Set your digital multimeter to read continuity. Usually indicated by an audible beep or diode symbol.
Check Continuity
Touch the meter probes to the COM and NO switch terminals to check for continuity. It should show open.
Reattach the Tube
Push the plastic tube back onto the pressure switch fitting. Make sure it’s sealed on tight.
Check Continuity Again
Keep probes on the terminals and change the air pressure by gently sucking on the tube. The switch should close at some point as pressure changes.
Check Status Light
If the switch has an LED light, make sure it comes on with normal operation after reattaching the tube.
Determine if Replacement is Needed
If the contacts didn’t open and close properly during testing, the pressure switch likely needs to be replaced.
Following these basic steps will allow you to accurately test whether the furnace pressure switch is functioning correctly or needs to be replaced with a new part.
What Tools are Needed to Test a Furnace Pressure Switch?
Testing a furnace pressure switch only requires a few basic tools:
Digital Multimeter
A digital multimeter capable of reading continuity is the most important tool needed. Either a battery-powered or plug-in type will work.
Probe Tips
Pointed probe tips allow easier contact with the switch terminals for accurate testing.
Tube Removal Tool
Needle-nose pliers or channel-lock pliers can help grip and detach the small plastic tube from the pressure switch fitting.
Flashlight
A flashlight helps illuminate the furnace interior to locate the pressure switch and air tubing in the dark space.
Screwdriver
A basic screwdriver allows removal of the furnace access panel to get to the pressure switch.
Gloves
Gloves keep your hands clean while working inside dirty furnace cabinets. Cloth gardening gloves are ideal.
Safety Glasses
Wearing basic eye protection is always a good idea when working around furnace components.
Manometer (Optional)
A digital manometer can measure the exact pressure inside the furnace if desired. But it’s not needed for basic go/no-go pressure switch testing.
The main necessities are a digital multimeter capable of continuity testing and some pliers to detach the small plastic tube from the pressure switch temporarily. With these basic tools, you can easily test a furnace pressure switch.
How to Replace a Faulty Furnace Pressure Switch
If testing determines the pressure switch is defective, replacing it with a new part is the solution. Here are the steps for safely replacing a bad furnace pressure switch:
Turn Off Power
Shut off electric power to the furnace at the breaker or disconnect switch.
Remove Access Panel
Take off the furnace access panel to get to the pressure switch.
Label Wires
Label the wire positions on the pressure switch or take a photo for reference.
Detach Wires
Disconnect the wires from the faulty pressure switch.
Remove Mounting Screws
Remove any screws holding the pressure switch in place.
Detach Tubing
Gently remove the plastic tubing from the pressure switch fitting.
Install New Switch
Mount the new pressure switch in the same location as the old one.
Attach Tubing
Push the tubing onto the barbed fitting on the new pressure switch.
Reconnect Wires
Reattach the wires to the proper terminals on the replacement switch.
Check Furnace Operation
Turn on power and test the furnace through several heating cycles to verify normal operation before replacing the access panel.
Installing an exact replacement pressure switch and connecting all tubes and wires properly should resolve furnace problems caused by a faulty switch.
Wiring a Furnace Pressure Switch
The furnace pressure switch wiring is very simple, but it’s important to connect the wires correctly when installing a new switch.
2 Terminals
Most furnace pressure switches have two terminals:
- NO – Normally Open
- COM – Common
Wire Colors
The wire colors vary depending on furnace manufacturer:
- Black wire → Common terminal
- White wire → NO terminal
- Red wire → NO terminal
- Yellow wire → NO terminal
Connecting Wires
Connect the common wire from the furnace control board to the COM terminal on the pressure switch:
- This wire stays powered constantly.
Connect the control wire that loses power when switch opens to NO terminal:
- This wire de-energizes to stop gas flow if venting is restricted.
Refer to the wiring diagram on the furnace to verify correct wire connections. Polarity matters, so make sure power-in and power-out wires go to proper terminals.
Check Continuity
Double-check for continuity between COM and NO terminals when switch closes before reinstalling the furnace access panel.
Where is the Furnace Pressure Switch Located?
Locating the pressure switch is the first step in testing it. Here is where to find the furnace pressure switch:
Near Draft Inducer
The pressure switch is typically mounted near the draft inducer fan housing on the side or top of the furnace.
Look for Air Tube
Identify it by the clear plastic tube attached to the