The pressure switch is one of the most important components of the gas furnace. It connects directly to the inducer motor and regulates furnace operation by letting the appliance only run when there’s the right level of venting pressure from the draft inducer.
If it detects negative pressure, a sign that the inducer motor has malfunctioned, it will shut down the furnace to prevent back-drafting (more on back-drafting shortly).
Additionally, the pressure switch protects the furnace itself by shutting it down if a mechanical malfunction is detected. As such, you must keep the switch in the best condition at all times. Otherwise, you’d be exposed to significant risk.
Read on to learn where the furnace pressure switch is located so you can watch and troubleshoot it more easily.
What Does a Pressure Switch Do on the Furnace?
To better understand how the pressure switch works, you need first to understand the role of the draft inducer motor.
The inducer motor, or draft inducer motor, is a blower system that facilitates the venting of exhaust gases out of the furnace. Combustion gases are waste gases such as carbon dioxide, carbon dioxide, and sulfur dioxide, poisonous in high concentrations.
Due to the need to completely remove these gases from your home, the inducer fan is the first component that comes on when you turn on your furnace. In most furnaces, it runs for 60 seconds before anything else happens.
Allowing it to run for 60 seconds before heating commences creates a negative pressure or draft that purges the heat exchanger of waste gases.
Hopefully, you can now see why a safety mechanism is important to track the inducer fan and control its operation. That’s what the pressure switch does.
How it Works
The pressure switch is an open switch that closes under a pre-defined negative pressure value, i.e., a pressure below atmospheric pressure. It connects to the inducer motor housing via rubber or vinyl tubing.
If the inducer motor is operating normally, the pressure switch will quickly detect the correct pressure level within the system. But, more importantly, the pressure will force the switch to close, thus complete the relay/circuit. As soon as the relay circuit completes, the rest of the furnace will kick on.
However, if there isn’t enough negative pressure to close the circuit, the pressure switch will stay open. Thus, the furnace cannot run.
If the pressure switch fails to close the first time, the furnace will shut down softly and allow you to try starting it again. However, if you try 3-5 times without success, the control board will stop sending voltage to the inducer motor, essentially “locking out” the furnace.
It will also lockout if the inducer motor has been running for a pre-defined period (which varies depending on the manufacturer) and the pressure switch hasn’t closed.
You’ll know that your furnace has locked out from a pressure switch issue if the red light on the control board flashes a fault code 31. Fault code 31 indicates that the pressure switch didn’t close.
Fortunately, you can fix most pressure switch issues yourself or even bypass the switch to light the furnace anyway. However, you first need to locate the switch.
Where is the Pressure Switch on the Furnace?
The pressure switch is located near the inducer motor and directly connects to the inducer housing. It’s a disc-shaped component with a diaphragm that pulls inside or out depending on whether or not the spinning inducer motor has created a vacuum inside.
If there’s no vacuum inside, the switch will still be open. However, if there’s a vacuum inside, it pulls out at the diaphragm and closes the switch.
However, it would help if you learned more about the different pressure switch types to identify your switch.
- Single vs. Dual vs. Modulating Pressure Switches
There are three main types of pressure switches – single-stage, two-stage, and modulating switches. Single-stage models are explicitly designed for single-stage furnaces, two-stage models for two-stage furnaces, and modulating switches for modulating switches.
- Conventional vs. Condensing Pressure Switches
Pressure switches can also be conventional or condensing units. Conventional pressure switches are standard pressure switches found in regular furnaces. Meanwhile, condensing pressure switches are found in condensing furnaces.
The main difference is that conventional pressure switches have just one connecting hose, connecting the switch to the body of the draft inducer motor housing.
On the other hand, condenser pressure switches have two hoses. The first one senses pressure at the draft inducer/burner enclosure, while the second focuses on venting pressure at the condensate collector box.
It’s much easier to locate the pressure switch when you know the type of switch on your furnace.
Locating the Pressure Switch
To access the pressure switch, switch off the power to your furnace and turn off the gas supply. Then remove the main access panel on the front of the furnace.
The pressure switch is located near the draft inducer motor. It has a nylon hose (or two, depending on the type of furnace) and two wires running from it to other components within the furnace.
How to Check the Pressure Switch on a Furnace
Pressure switches can remain effective for a long time. However, as with other relay systems, the pressure switch can get stuck or break down altogether.
If your furnace has malfunctioned and the signs point to a bad pressure switch, you can perform various tests to verify the problem.
Test with a Monometer
The first way to test a pressure switch is to use a specialized pressure switch tester. A pressure switch tester is a monometer with an internal air pump and a light indicator. The light will come ON when the pressure switch closes.
Although most homeowners don’t have pressure switch testers, nearly every HVAC expert uses one as they offer a more straightforward way to test pressure.
Use a Tee to Test
In the absence of a specialized tester, you can use a tee. First, install the tee into the switch’s vacuum tube and run a vacuum tube to a monometer. It’s a simple setup. Then, place voltmeter leads across the switch’s terminals.
From there, energize the pressure switch and record the voltages. Pressure switches operate at 24V. If the level drops below this value, the switch will not work. If it’s 0V, the switch is likely damaged.
Test the Resistance
You can also test the functionality of the switch by measuring the resistance. First, disconnect the wires to the pressure switch by pulling the wire connectors off the switch terminals. Then, set your multimeter to resistance (ohms). From there, touch each tester probe to one of the wires.
The multimeter should read zero or very close to zero, indicating no resistance or very little resistance, if any. If the reading is higher, the switch has failed and must be replaced.
How to Bypass the Pressure Switch on a Furnace
Yes, you can bypass the pressure switch! It’s strongly prohibited given the pressure switch’s role in regulating the venting of exhaust gases. Bypassing the switch may allow dangerous gases to flow back into the house without a way to tell.
However, bypassing the switch may also be the only way to confirm – beyond doubt – that your pressure switch is damaged. If the furnace works when you remove the pressure switch from the equation, you can confidently conclude that you have a bad pressure switch.
The good news is that bypassing the switch is as easy as directly touching the two wires connecting to the switch. Disconnect them from the switch and touch them directly. Then see if the furnace works. If it does, the pressure switch is bad.
Remember that bypassing the pressure switch is very dangerous. Therefore, you should only ever bypass the switch temporarily, such as during troubleshooting.
How to Tell if Your Pressure Switch is Bad
Of course, testing, as we’ve discussed above, is the best way to tell if you have a bad pressure switch. If the switch fails only or all of the tests, it’s probably broken. You can also bypass the switch to confirm if it’s bad.
However, several other signs and checks may also signal a bad or failing pressure switch. These include;
- Furnace blows cold air: If your furnace runs but only puts out cold air, there may be no heating because the pressure switch is open.
- Flashing error codes: Most furnaces have onboard LED systems that flash in pre-defined patterns to signal various issues. These codes can tell you if the pressure switch is damaged. Most furnaces flash error code 31 to signify a bad pressure switch.
- No “clicking” sound during ignition: Finally, you can also listen to the standard “click” sound when starting the furnace. The sound usually occurs when the pressure switch closes. Therefore, if it doesn’t close, the “click” will be absent. It could be a sign of a bad switch.
The pressure switch is a critical component of the modern furnace. As such, it must be in perfect working condition to enjoy safe and reliable heating. Knowing its location is the first step to taking the best care of the switch.