The furnace pressure switch is a critical part of gas furnace systems. It’s one of the safety primary safety controls and is solely responsible for preventing the backdraft of combustion gases into your home. In doing so, it protects you and your family from potential poisoning.
Unfortunately, though, the furnace pressure switch too can malfunction and even become entirely defective. When this happens, lighting the furnace becomes a challenge. The burner will not come ON at all, no matter how many times you try. The furnace may even go into lockout.
We feel it’s beneficial for every gas furnace owner and user to learn at least a few things on how the pressure switch works. This way, you can perform essential diagnostics if you encounter pressure switch issues in the middle of the night. Knowing what could go wrong and the best way to respond is also important for your safety.
What’s a Pressure Switch on a Furnace?
The furnace pressure switch is a safety component designed to protect the home and its occupants from dangerous backdrafts. The backdraft of combustion gases, i.e., when combustion gases flow back into the house, can cause gas poisoning and even asphyxiation. Therefore, the pressure switch has to prevent any backdraft.
So, how does it do this? It’s very simple. The pressure switch works alongside the draft inducer motor – the fan system responsible for venting exhaust gases from the furnace and out of the house.
It connects directly to the inducer box via a vacuum hose and closes or remains open in response to the pressure value in the draft inducer system.
In a furnace operating normally, the airflow due to the draft inducer fan’s rotation causes a negative pressure within the inducer box, forcing the pressure switch to close, thus allowing the rest of the furnace to run.
However, if there isn’t enough negative pressure, i.e., if the airflow isn’t strong enough to generate sufficient “pull,” the pressure switch, which is open when the furnace is OFF, will remain open.
An open pressure switch keeps a crucial electrical circuit within the furnace open, meaning the furnace cannot even start, let alone run.
Where is the Pressure Switch on a Furnace?
The pressure switch is located near the draft inducer motor of gas-powered forced-air furnaces. It’s a disc-shaped device that’s easily identifiable once you remove the furnace’s front cover.
You’ll know you’ve found the pressure switch when you come across a round-shaped device with two wires and either one or two hoses connecting to it. Standard/conventional furnace pressure switches have just one hose leading to the body of the draft inducer motor.
Meanwhile, the pressure switches in condensing furnaces have two hoses. The first senses the pressure at the draft inducer or burner enclosure, while the second sense proper venting at the condensation collector box.
The majority of furnaces have just one pressure switch. However, two-stage systems have two while modulator furnaces have three. Whichever the design and the number of units, the switches are all located in the same area – adjacent to the inducer motor.
How to Know if My Pressure Switch is Bad (Symptoms)
Although one of the most common signs is a furnace that won’t start no matter how hard you try, the furnace can refuse to start for many other reasons, including flame sensor-occasioned lockout and a malfunction igniter.
So, you need to check a few more things before confidently saying that the pressure switch is defective. You may need to test the switch to prove, beyond doubt, that it’s malfunctioned.
Let’s begin with telltale signs before discussing how to test your pressure switch to prove that it’s defective.
A defective pressure switch displays the following systems;
1. No “Click” During Ignition
If you’ve listened carefully before, you’ll have realized that the furnace produces a “clicking” sound when the unit starts. It’s usually the gas valve opening. So, what does this have to do with the pressure switch?
Well, the gas valve would never open if the pressure switch is open. If the valve opens, you’re 100% sure that the pressure switch is OK and operating normally.
On the other hand, the gas valve hasn’t opened if you don’t hear the click during startup. One of the main reasons the gas valve on a furnace will refuse to open is a malfunctioned pressure switch.
2. Furnace Refuses to Start Completely
If your furnace doesn’t come ON at all, it might signify that you have a faulty pressure switch. As we’ve already mentioned, the furnace may also refuse to start for many other reasons. So, you may need further tests. However, often it’s either a bad ignition system or a “lockout.”
Furnace “lockout,” specifically a hard lockout, is a self-induced state in which the furnace becomes unresponsive to user input until you fix the underlying issue. A common cause of “lockout” is repeatedly starting a furnace with a faulty pressure switch.
3. Furnace Blows Room-Temperature Air
Suppose the furnace starts, presumably while in “soft lockout,” you will only get room-temperature air from your vents – not warm air. In addition, the air may feel slightly colder than room temperature because of the breeze.
This is usually a sign that the inducer fan is running, but there’s no heating in the furnace. Again, a common reason here is a malfunctioned pressure switch.
Most likely, the fan is blowing too slowly, or the pressure isn’t getting to the pressure switch. Either way, it’s a common sign that the pressure switch isn’t working.
4. Cracks or Holes on the Hose
The hose on the pressure switch connects directly to the draft inducer motor and is solely responsible for transferring the negative pressure from the inducer box to the pressure switch. Thus, if the hose is compromised, the pressure switch may not work effectively.
But, given that the regular pressure switch hose is made of nylon, it’s natural for the hose to get cuts and holes over time, primarily due to natural wear and tear. These cuts can “leak” air and thus compromise pressure inside the switch, leading to inaccurate performance.
How Do You Test a Pressure Switch?
After verifying one or more of the four issues above, you must test the pressure switch with a multimeter to determine whether the device is broken. Here’s how to proceed;
- A flashlight
- Pressure switch hose (possibly)
- Turn off the power and gas: Turn off the power by switching off the switch located near or on the furnace. If there’s none, switch it off at the breaker. Once done, turn off the gas supply at the pressure valve.
- Remove the main panel: Lift the main panel from the furnace. You may need to unscrew a few screws.
- Inspect the hose: Is it securely connected at both ends? Does it have cracks/holes? Is it obstructed anywhere? Make sure to correct all the above issues before proceeding to the next step. Check the hose port, too, to ensure it’s clean with no debris.
- Set it up for resistance testing: Disconnect the wires from the pressure switch by pulling off the wire connectors at the terminals. Then touch the multimeter probes to one of the pressure terminals, one to each terminal.
- Take the resistance reading: The test should read zero (0) ohms or very close, indicating no resistance. If the reading is higher, the switch is compromised and must be replaced.
- Take the next steps: If the switch is damaged, replace the hose and main panel and call an HVAC pro to replace it. If it’s not damaged, check the igniter.
What Happens if the Pressure Switch Goes Bad?
A bad pressure switch can cause all kinds of issues. Typically, the furnace will not come on no matter how many times you try. And if you try too many times, the furnace may go into lockout.
What Causes a Furnace Pressure Switch to Fail?
The pressure switch can fail for many reasons. However, the most common causes of failure are as follows;
- A clogged hose port
- An obstructed flue
- A raptured or struck diaphragm
- A damaged/cracked hose
- The switch has water in it.
How Long Does a Furnace Pressure Switch Last?
The furnace switch can last many years with regular maintenance. Indeed, many people only replace the switch once or twice over the life of the furnace. Given that an average furnace lasts 15 years, we can say that the switch should last 5-7 years with good maintenance.
When Should I Replace My Furnace Pressure Switch?
You only need to replace your furnace pressure switch if it’s completely damaged or becomes too unreliable. For instance, if you have to reset the switch every two weeks, it might be better to fix the underlying problem. A replacement only costs $75 to $250.
How Do You Bypass a Pressure Switch?
- Disconnect the two wires attaching to the pressure switch.
- Turn ON the furnace and wait for the draft induce to start up.
- Touch the two wires to jump the furnace. This step closes the switch.
Remember that jumping the furnace pressure switch is risky. Thus, it should only be done temporarily, not as a permanent solution.
Now you know how to diagnose a faulty furnace pressure switch and perform a basic test to check whether your pressure switch is damaged. Don’t hesitate to speak to an HVAC technician if you need further advice.