Furnace pressure switches serve as a safety feature on your furnace. It is one of the components that work together to ensure your furnace’s peak performance and safety. Under different conditions, high shock and vibration, the pressure switch give stability control.
This article is a guide to the features of the furnace pressure switch. Also, I will give you some helpful and detailed procedures for finding the right furnace pressure switch replacement and how to replace it.
What is a Pressure Switch?
A pressure switch is a built-in safety device located near the motor in a furnace. If it detects negative pressure from the draft inducer motor, it turns off the furnace to protect your home. It then sends the exhaust fumes back to the system to prevent a dangerous backdraft.
Components of a Pressure Switch
The following are the main components of your furnace’s pressure switch:
- Diaphragm – Functions as a pressure sensor. It is often made of a malleable material, which is pressure-sensitive.
- Adjustment spring – The adjustment spring is used to increase and decrease the pressure switch’s reset and trip point settings. The reset point is for the falling pressure, and the trip point is for the rising pressure.
- Automatic on/off lever – Activates or turns off the pressure switch. It may be a knob instead of a lever in some circumstances, but it does the same thing.
- Electrical contacts – Allows an external power source current to flow across the motor. The contacts are made from a conductive material of 90%silver and 10% nickel.
- Terminals – Connect the contacts to an external power source. They are the points where incoming and outgoing power leads are connected to the pressure switch.
How Does a Pressure Switch Work in a Furnace?
The furnace operates in the following sequence of events.
- Once power transfers to the furnace control board, the thermostat signals the call for heat. Then the inducer motor kicks on.
- Once the inducer motor is on, the pressure switch has to test and confirm if the inducer motor is operating correctly.
- If the inducer motor is okay, the pressure switch will signal the ignitor to activate the gas valves.
- Once ignited, gas valves pour flame pours across burners to get them lit, and the flame sensor proves all burners are lit.
- Finally, the fan blower will then blow the heat into your home through the duct, thereby heating your home.
Factors to Consider When Selecting a Pressure Switch
Below, you will find some factors to consider when getting a new pressure switch for your furnace.
As you know, the most important thing for any product when it comes to compatibility is the brands. Like many devices, the furnace’s pressure switch comprises different components, and a specific brand makes those components. So, when buying a pressure switch, make sure you find the dealer that carries the same brand as your furnace’s pressure switch.
The pressure switch must be able to withstand the highest operating pressure. Make sure to get the one strong enough to withstand the pressure of your furnace.
Each furnace’s pressure switch has a specific temperature range. Make sure you get the same maximum and minimum temperature range.
Electrical and mechanical are the two types of pressure switches. An electric pressure switch is more expensive but has more control over the settings, like pressure setpoint and hysteresis, than a mechanical pressure switch.
Hysteresis is the difference between the switch point and the reset point. If the reset point is set too high, the switch will remain engaged for an extended period.
If the setpoint is too short, the switch will often transition between on and off states.
It is possible to adjust the hysteresis option in an electronic pressure switch, but manufacturers usually preset a mechanical pressure switch.
Type of Media
The type of media should be compatible with the material used for the housing and seal of the pressure switch. NBR (nitrile butadiene rubber) is compatible with air and hydraulic/machine oil. EPDM (ethylene propylene diene monomer rubber) is suitable when the medium is water.
Common media used with pressure switches are:
- Hydraulic oil
- Heating oil
- Type of pressure switch
Types of Pressure Switches
There are two main types of pressure switches:
- Mechanical (electromechanical)
- Electronic (solid-state)
Mechanical Pressure Switch
On mechanical pressure switches, the diaphragm is in charge of opening or closing the circuit when the pressure in the medium increases or reduces to a certain level. Once the pressure reaches its setpoint, the diaphragm transfers the force to the sensing part of the pressure switch.
The sensing part will either open or snap close once it makes contact with the pressure from the diaphragm. Once the pressure reduces, the connections will return to their normal state with no need for a power supply. The sensing parts might include a Bourdon tube or a piston.
Here we discuss the pros and cons of the mechanical pressure switch.
- Generally cheaper than electronic pressure switches.
- They have the ability to work with auxiliary power
- Capable of handling large voltage than electronic pressure switches
- They are passive devices; no power source is needed
- Lower longevity with high switch rate applications
- Unless gold contacts are present, regulating low-power contact signals is ineffective.
- Some mechanical pressure switches have the change screw hidden beneath the DIN plug connector. So, setting the point pressure is not on site.
Electronic Pressure Switch (Solid State)
The electronic pressure switch operates on electronic sensors. It controls simple tasks by opening and closing the electronic switches.
Many solid-state pressure switches include analog signals proportional to the pressure. As a result, they shut or open the contacts and convey the actual pressure value. Electronic pressure switches also have the benefit of being flexible.
Electronic pressure switches come with a switch point, output signal, delay period, hysteresis, and various other features to fit your needs.
Here we discuss the pros and cons of the Electronic pressure switch.
- Ideal for applications requiring a high switching rate
- Adjusting the setpoint is simple
- High precision and consistency
- It requires a power source to function.
- It cost more than a mechanical pressure switch.
What Causes a Pressure Switch to Fail?
Usually, there are a few causes for your pressure switch to fail. Below I list the usual culprits.
- Old – After years of service, a pressure switch may start to become less sensitive, causing wrong readings.
- Dirty Pressure Switch – When there’s dirt, debris, or particles in the pressure switch, it will stop working.
- Damaged Diaphragm – If the diaphragm has holes or cracks, this will indeed cause pressure switches to fail.
- Stiffened Diaphragm – A coated or stiffened diaphragm can limit the pressure switches’ ability to work effectively.
What Happens When the Pressure Switch in a Furnace Fails?
The system will go into “safe mode” if the control board senses a malfunction in the pressure switch. After the safe mode has been activated, the pressure switch will try a couple of times to make it work, and if it doesn’t, it will start to bring cold air into your home. The cold air indicates that your furnace is faulty and needs to be checked immediately.
When Can I Replace my Furnace Pressure Switch?
You can only replace the furnace pressure switch if it malfunctions, i.e., after troubleshooting. If the pressure switch is just dirty or jammed, it is advisable to fix it before considering replacement. Gently remove the switch from the inducer fan blower, then blow it on one end to remove trapped dirt.
What to Do if the Blower Fan Does Not Turn On?
If the blower fan does not turn on, you get a pressure switch error code. Remove a wire from the pressure switch. This means that the pressure switch was closed if the blower fan started to run. If this happens, a simple tap on the shoulder may open a stuck-closed pressure switch, allowing the blower to turn on. This also indicates the pressure switch is faulty and needs a replacement with a new one.
Could the Connected Hoses be the Problem?
Hoses attached to the furnace pressure switch must be in good working conditions, clear of obstructions, and linked on both ends. You can reconnect loose hoses. Clean out any debris from clogged hoses. Or replace any cracked hoses. If the hose ports have cracks, replace the pressure switch you notice a crack on the hose ports.
Testing a Furnace Pressure Switch
Below is how to detect if a pressure switch is faulty and needs replacement.
Using an Ohmmeter to Test Pressure Switch Pressure
- Place the ohmmeter lead on the pressure switch terminals.
- The pressure switch works fine if the ohmmeter reads zero or close to zero!
- A high reading means it has failed.
Using a Voltmeter to Test Pressure Switch Voltage
- Place the voltmeter on the two terminals and ground to check the voltage. The reading should be between 24 and 28 volts of DC. If it is less or more than this range, your pressure switch is faulty.
How to Replace the Furnace Pressure Switch – Step-by-Step Guide
- Turn off the power – Replacing the pressure switch with the power ON is risky. You can be electrocuted. First, turn off power to the furnace before replacement.
- Turn off the gas – Accessing the furnace’s interior parts with the gas supply turned on is risky. It increases the risk of gas poisoning and fire. So, cut the gas supply by turning off the gas valve. You can tell that the valve is OFF when the lever is perpendicular to the gas line/pipe.
- Remove the combustion chamber cover – The top part of the combustion chamber houses the combustion chamber cover. To remove it, you may need to unscrew a few screws.
- Locate the pressure switch – The pressure switch in most furnaces is a disc-shaped metal component connected to a hose. In most furnace systems, there is only one, while some have two or even three. For example, two-stage pressure switches in dual-stage furnaces have two pressure switches.
- Disconnect the wires – Wires run from pressure switches to the control board and other furnace sections. Disconnect the wires at the switch’s front.
- Disconnect the hose – As stated before, a standard furnace has one pressure switch and one connecting hose. But the dual-stage models have two or more hoses. Make sure to disconnect all the hoses.
- Unmount the old switch – The process will vary depending on whether the new pressure switch has a bracket. You can remove the old one with the bracket if the new one has a bracket. But, if it doesn’t come with a bracket, only remove the old switch, leaving the bracket in place. You need to unscrew and lift off the device(s) in both cases.
- Secure the new sitch in place – Slide a new switch onto the motor and screw it into place. It is essential to make sure the new pressure switch is tightly placed. You don’t want it hanging.
- Reconnect the wires and tubes and replace the panel – For the new pressure switch, begin with the last one you removed, the hose, and work your way back to the combustion chamber panel.
The furnace should work if you purchased the correct switch and followed all the steps above.
Question: Which Pressure Switch Type is the Best?
Answer: Here are some advantages to the two main types of pressure switches. You can decide which one is best for you based on your furnace. Solid-state pressure switches have a longer life, higher precision, and resilience to shock and vibration. At the same time, electromechanical switches can switch more significant currents and are not voltage-dependent.
Question: How to Adjust a Pressure Switch?
Answer: In a mechanical pressure switch, turn the nut clockwise to increase the switch point and counterclockwise to lower it. There is a keypad used to regulate an electric pressure switch.
Question: How to Test a Pressure Switch?
Answer: Connect the multimeter to the pressure switch’s terminals after unplugging the pressure switch. The multimeter should read an open circuit for an open switch. Turn on the course to permit the fluid pressure to enter the pressure switch. Replace the switch if the multimeter does not read 0 ohms. An open switch is the polar opposite of a closed switch.
Question: What Does a Pressure Switch Do?
Answer: A pressure switch is a safety device. It monitors the fluid pressure in the system. Depending on the pressure level, it either opens or closes a circuit.
Question: How to Tell if a Pressure Switch is Bad?
Answer: If the fluid is leaking, the pressure is too low, causing pressure to fluctuate. This will make the switch display the wrong initial pressure.
Question: What is the Difference Between a Pressure Switch and a Pressure Sensor?
Answer: Pressure switches operate using electrical switches with a preset pressure level. But pressure sensors read and convert it to an electrical signal.
Question: What is the Difference Between a Pressure Transmitter vs. a Pressure Switch?
Answer: Let’s clear up any confusion you might have about the difference between a pressure switch and a pressure transmitter.
A pressure switch, like a pressure transmitter, has a transducer. The pressure switch’s output is digital on/off signal. There are two possible states for a digital signal: on and off.
The output of a pressure transmitter is an analog electrical voltage or a current signal. The transducer output pressure range is 0 to 100%. So the main difference is a pressure switch uses a digital signal for its transducer output, and a pressure transmitter uses an analog signal.
A pressure switch is a safety device inside the furnace that detects negative pressure from the draft inducer motor. This mechanism keeps you, your loved ones, and your property safe as it sends the dangerous and explosive exhaust fumes back to the system.
The inducer motor should be the first to turn on when a furnace starts a new cycle. The pressure switch is an open switch that closes when there is negative pressure. The rest of the furnace’s operation will continue when the pressure switch closes. If the pressure switch does not close, the system will enter safe mode.
You need to make sure that your pressure switch is in top condition with all this in mind. Above I provide some tips on examining and troubleshooting your pressure switch. But at the end of the day, you should replace the pressure switch for various reasons, such as aging or a damaged diaphragm. To help you out, I provide nine straightforward tips on replacing it.
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