Low Gas Pressure Switches- Everything You Need to Know

In the complex world of boiler systems, safety is of paramount importance. One vital component that helps ensure the safe operation of a boiler is the low gas pressure switch. While it may appear a simple device, it plays a critical role in monitoring gas pressure and safeguarding boiler operations. This article aims to unravel the workings, purpose, and nuances of boiler systems' low gas pressure switches.

Note: It is important to note that this article does not qualify you to perform any procedures or service a boiler and burner. It is recommended to consult with a licensed professional boiler company for proper service procedures. Incorrect operation of a boiler and burner can result in severe property damage, injury, or even death.

This article is for informational purposes only. It should not be used as a real-world application guide.

What is a low gas pressure switch?

A low gas pressure switch is a device that includes a diaphragm connected to a mechanical switch of electrical contacts. The device may also have an adjustable setpoint. The low gas pressure switch, or LGPS, continually monitors the gas pressure before the main shutoff valves on the boiler gas train. Suppose the gas pressure drops below the switch's setpoint. In that case, the diaphragm will activate the electrical switch contacts and cause the circuit to open, shutting down the boiler.

Identification

It can be challenging to tell them apart when identifying a low gas pressure switch among other pressure switches, such as an air pressure switch. If the low gas pressure switch is not mounted on a burner gas train, it's just a regular pressure switch with no specific labeling to identify it as a low gas pressure switch. To determine which type of pressure switch you are dealing with, refer to the manufacturer model number and cross-reference it with the manufacturer manual. The manual will provide information about the switch's functionality and action. For low gas pressure switches, the switch will change state when the pressure drops below the setpoint. Only when the pressure switch is mounted on a gas train with the intent of monitoring for a pressure drop below the setpoint does it become a low gas pressure switch.

Diagram of a gas train showing the location of the low gas pressure switch

Location on Gas Train

The gas train of a boiler/burner is all of the gas piping after the first equipment isolation valve. Think of all the gas piping that would be included in a packaged boiler when it arrives new; that section of gas piping is the gas train, which includes all the automatic block valves, pilot gas piping, pressure switches, and gas control valve. The low gas pressure switch will always be located before the automatic block valves on the gas train. If the low gas pressure switch were located after the automatic block valves on the gas train, when the burner shut down and the automatic block valves closed, there would be no pressure downstream of the valves. The lack of pressure would cause the low gas pressure switch to open every time the burner shut down; what a nightmare that would be!

Remember, the low gas pressure switch will sense the gas pressure available to the gas train's inlet under normal conditions. The switch's whole purpose is to ensure the burner only tries to start when the minimum gas pressure is available.

Why is a LGPS Needed?

The purpose of a low gas pressure switch is to prevent the burner from running below the manufacturer's rated minimum pressure. The manufacturer develops this gas pressure setting through engineering and real-world testing. However, only some burners/boilers will have a specified minimum gas pressure setting. In that case, there are a couple of methods to find out where to set the switch setpoint, which will be explained later.

So, what happens when the gas pressure drops below the recommended minimum for the burner? Here are a couple of things

1. The burner can run higher than standard excess air, reducing combustion efficiency. A burner should always run at the highest efficiency level while maintaining safe combustion.
2. If the gas pressure is low, it can cause the burner not to receive enough fuel. As a result, the combustion of the flame can be altered and create dangerous levels of carbon monoxide. This gas is toxic and can be deadly for people near the boiler.
3. A delay or interruption in fuel pressure can cause the flame almost to extinguish. However, the burner flame safeguard still has not detected a loss of flame. Then, an immediate restoration of full gas pressure can cause ignition of the flame at a higher firing rate. That may cause an explosion in the combustion chamber.
4. Components in and around the burner may be damaged. The flame can burn back or sit too close to the combustion head. The improper flame combustion can overheat the metal, causing damage.

Does the LGPS have to be manual or automatic reset?

There are two main general types of pressure switches when used for low gas pressure monitoring. The first type is an automatic reset switch. When the gas pressure drops below the setpoint, the pressure switch electrical contacts will change state and open the electrical circuit. When the gas pressure rises above the setpoint + differential of the pressure switch, the electrical contacts will change state again and close the circuit, allowing the burner to start if there is a call for heat.

Low gas pressure switch showing the manual reset button and the pressure range.

The second kind of pressure switch is a manual reset pressure switch. This type of switch works the same way an automatic reset switch does, except for one difference. When the gas pressure rises past the setpoint + differential of the pressure switch after it has dropped, the electrical contacts will not change state and close the circuit without manual intervention from a person. A reset button or lever has to be pressed for the electrical contacts to close and let the burner start again.

If you are still determining what type of pressure switch is needed, use a manual reset switch. The more complicated answer after a low gas pressure event is the boiler/burner is required to be manually reset locally by a qualified operator. It has to be reset manually to determine the cause of the low gas pressure and ensure the boiler/burner is safe to start again. Depending on the wiring of the burner flame safeguard, an automatic reset LGPS can be used, but the burner flame safeguard needs to lock out when the LGPS detects low gas pressure.

Regulatory Compliance

All low gas pressure switches need to be UL-listed in the United States. Underwriters Laboratories is an independent testing organization that tests components to verify the component will do what the manufacturer says it will. If you want to learn more, you can read all 116 pages of UL standard 353. When ordering a low gas pressure switch, the pressure switch should be marked and labeled with the UL stamp.
Switches may also be required to be FM (Factory Mutual) approved. Check with your local jurisdiction's laws and codes.

When is a LGPS required?

Low gas pressure switches are not required on all boiler applications. Below are the requirements per ASME CSD-1 for installing a low gas pressure switch on the gas train. Remember, only some states in America have adopted ASME CSD-1.

Atmospheric natural draft per CSD-1

1. Greater than 400,000 BTU/HR but less than or equal to 2,500,000 BTU/HR.
   1. Not required for ignition systems with pilots.
   2. Required for direct ignition and hot surface ignition systems. Need to cause safety shutdown and lockout.
2. Greater than 2,500,000 BTH/HR but less than or equal to 5,000,000 BTU/HR.
1. Required and need to cause safety shutdown and lockout.
3. Greater than 5,000,000 BTU/HR but less than 12,500,000 BTU/HR
   1. Required and need to cause safety shutdown and lockout.

Power Gas Burners, Mechanical Draft Atmospheric Burners, and Pulse Combustion Systems per CSD-1.

1. Greater than 400,000 BTU/HR but less than or equal to 2,500,000 BTU/HR.
   1. Not required for ignition systems with pilots.
   2. Required for direct ignition and hot surface ignition systems. Need to cause safety shutdown and lockout.
2. Greater than 2,500,000 BTH/HR but less than or equal to 5,000,000 BTU/HR.
   1. Required and need to cause safety shutdown and lockout.
3. Greater than 5,000,000 BTU/HR but less than 12,500,000 BTU/HR
   1. Required and need to cause safety shutdown and lockout.

Here are some more requirements from other independent agencies.

UL Requirement- Any burner 2501 MBH or above needs to have a low gas pressure switch.
FM Requirement- Any burner 2500 MBH or above needs to have a low gas pressure switch.

It is important to check with local jurisdictions as it is not a one size fits all approach. Unless you are specifying and building the gas train, the equipment manufacturer will have already selected the proper components for the gas train. If you have more questions about it, the manufacturers will have people able to help.

Installation

Most low gas pressure switches will have a 1/4" NPT connection, but some switches have varying connection sizing, such as 1/2" NPT. On special occasions, when a manufacturer has designed gas valve components to work with pressure switches, the pressure switch may only use an o-ring connection to the gas valve body to help save space and make wiring the switch more accessible.

When installing the low gas pressure switch, ensure a pressure test port is available to test the same gas pressure the switch will be sensing. Most gas valve bodies will have pressure taps available for measuring gas pressure before the valve, so keep that in mind if there is no pressure tap available on the piping near the switch.

Bottom of low gas pressure switch showing the pressure connection.

LGPS Case Construction Material

Manufacturers offer low gas pressure switches with plastic, steel, or aluminum cases. What is the difference between the switches? There is not much, but you need to look at the details. For example, the Crossroads Controls model LGP-A pressure switch has an all aluminum case construction. The pressure switch has a vent connection and can handle a max pressure of 10 PSI. Compare that to the Crossroads Controls LGP-G pressure switch, which has a mix of plastic and aluminum as the construction material. The switch is ventless and has a lower max operating pressure of 2 and 6 PSI, depending on the switch setpoint range.

Installing a vent pipe on the low gas pressure switch may be required in some areas, while in others, it may be unnecessary, so a ventless switch may be chosen instead. Neither construction style is superior to the other; it is the responsibility of the person choosing the pressure switch to determine what specifications are needed and select the appropriate pressure switch model.

Mounting Position

Make sure to refer to manufacturer manuals for individual pressure switches. Not all pressure switches can be mounted in any position. Some manufacturers, such as Honeywell, recommend that the C6097A model pressure switch be mounted with a horizontal diaphragm for the most accurate pressure switch actuation pressure vs. the setpoint setting.

Make sure to also keep in mind where the electrical connection to the pressure switch is. Manufacturers may recommend mounting the pressure switch horizontally with the electrical connection bottom side down to help prevent dust and moisture from entering the inside of the device.

Operating Environment

When selecting a low gas pressure switch, you must consider the environment in which the pressure switch will be installed. Is the air full of moisture and wet contaminants that can get inside the pressure switch and corrode the electrical contacts? Is the mounting point of the switch high vibration? Is the pressure switch installed in an environment where an explosion proof rating is needed? Is the installation outside and needs to be shielded from the sun?

All these considerations must be considered before selecting the pressure switch. Most manufacturers offer a typical pressure switch, but if you look into options that can be ordered, you can customize the pressure switch for the end user's needs.

Closeup of pressure switch electrical terminals

Electrical Wiring

Most low gas pressure switches will have four electrical terminals to terminate wires: common, normally open, normally closed, and ground. Some pressure switches will include two sets of contacts, but the other set of contacts is not required for use when wiring a low gas pressure switch. Below is a wiring diagram for a low gas pressure switch. Understand that the normal state of a pressure switch is like when you take it out of the box. No gas pressure is acting on the pressure switch, so the electrical switch contacts will be between the common and the normally closed. The electrical contacts change state only when pressure exceeds the setpoint + differential setting. Above that setting, the common and normally open terminals will have continuity.

The pressure switch may also include a small light inside the case called a pilot light. The pilot light will be wired to the electrical terminals of the pressure switch to help identify the switch state. It should not be relied on when troubleshooting the switch, as the lights can burn out. The small pilot lights can be ordered in different colors; the colors do not mean anything and are chosen purely on user preference.

Low gas pressure switch with cover off. Arrows showing where the manual reset button and the pilot light are.

If your burner abides by CSD-1 standards, any non current carrying enclosure must be grounded. However, if a pressure switch is grounded by its installation, it is not required to have a separate ground wire. When selecting a ground wire, ensure the insulation color is green and can be with or without a yellow stripe.

The wire size to the low gas pressure switch needs to be rated for the load but cannot be less than 18 AWG. Usually, the wiring to the pressure switch will be 14-16 AWG. The color of the current carrying wires can be anything you want except white and green. White insulation wires commonly signify neutral wires in electrical. Since gas pressure switches are electrical switches and not a load, there is no neutral wire to the switch; the wire on the common and normally open terminal will be hot, carrying line voltage with the same potential when the switch is made.

Wiring diagram for a low gas pressure switch. The current wires will be landed on the red terminals; common and normally open.

Units of Pressure

In the USA, most low gas pressure switches are going to be in inches of water column or pounds per square inch. The incoming gas pressure will determine the correct units to select the switch range. Make sure to not select a range that is way higher than needed like 1-8 PSI range and the incoming gas pressure is only 1 PSI. A 12-60 IN WC range would be a better selection.

Setting the LGPS Setpoint

The set point of the LGPS is essential to the correct operation of the switch. Set too low, and the LGPS will allow the gas pressure to drop below the burner's recommended minimum settings. Set the setpoint too high; the burner may experience nuisance shutdowns from the LGPS. Here are three different ways the LGPS setpoint setting can be selected.

1. Consult manufacturer's data. The manufacturer of the boiler/burner sometimes will specify what the setpoint of the LGPS should be.
2. If the boiler/ burner manufacturer does not specify a setpoint, the setpoint can also be set at 50% of the boiler/burner's main manifold gas pressure.

Field testing and setting of the setpoint can also be performed. This is the most accurate and preferred method. Connect a manometer to read the same gas pressure as the LGPS. Insert a combustion analyzer into the boiler stack to read a combustion analysis. When the main flame of the burner is firing and the combustion analyzer has settled, slowly shut the manual gas valve on the inlet of the gas train upstream of the LGPS. Closing off the valve will lower the available gas to the burner, and the combustion will increase in excess air. An alternate method is if the LGPS is installed downstream of a pressure regulator. The regulated pressure can be reduced instead of closing the manual gas valve. This method will lower the pressure in a more controlled manner. Monitor the combustion readings until you hear or see the flame become unstable or until the combustion analyzer reads high levels of carbon monoxide; usually, above 200 PPM is a good place to stop. When this happens, note the gas pressure reading of the manometer. The LGPS setpoint should be set at this reading. This process can be repeated to check the results. The higher the LGPS setpoint is, the sooner the burner will shut down. The goal is to achieve the highest setting without causing nuisance shutdowns.

Testing Operation of a LGPS

Per CSD-1, it is recommended to test the function of the LGPS at least once a month. Check with the equipment owner; they may have different requirements exceeding the recommended testing interval. Below are a couple of different ways the operation of an LGPS can be tested. There is no one correct method, but the best test is the one that simulates real-world conditions the closest.

1. The operating pressure and switching point must be within the setting range of the hand wheel. Install a manometer or pressure gauge to monitor the gas pressure near the low gas pressure switch connection. Note the gas pressure when the burner runs at its lowest firing rate. Check the burner manufacturer's required minimum regulated gas inlet pressure and verify that the gas pressure is above the setting. Increase the setpoint setting until the setpoint exceeds the gas pressure reading on the manometer. The switch should trip, and the burner shut down. If the switch is an auto reset type, turn the setpoint setting back to the original setting. Reset the burner flame safeguard alarm. If the switch is a manual reset type, turn the setpoint setting back to the original setting. Reset the burner flame safeguard. The burner should alarm again on low gas pressure because the manual reset on the switch was not pressed. You always want to verify the switch latches when tripped if it is a manual reset style. Reset the low gas pressure switch and reset the burner flame safeguard alarm.
2. The LGPS can be removed from the equipment and tested using a calibrated gauge or process instrument calibrator. This is the most accurate method to determine the function and accuracy of the switch. It is the most time-intensive method and requires the boiler to be locked out, so it is not recommended for regular testing intervals. When the LGPS is installed back on the equipment, the function of the pressure switch still needs to be tested to ensure the equipment shuts down when gas pressure drops below the setpoint.
3. When the boiler or burner is firing on the main flame, turn off the upstream manual gas valve on the gas train. This action will result in the burner running out of gas, causing an alarm on flame failure. Furthermore, the LGPS will trip because the pressure in the gas train has been reduced to zero. Although this method is the most basic for testing purposes, it may not be the most accurate when determining and testing the trip point for an LGPS.

Popular Manufacturers of Pressure Switches

• Ashcroft
• Cleveland Controls
• Crossroads Controls (Acquired Antunes pressure switch product line in 2017)
• Dwyer Instruments
• Dungs Combustion Controls
• Honeywell
• Kromschroeder (Owned by Honeywell)
• Siemens

Understanding the role and importance of the low gas pressure switch is crucial for boiler systems' safe and efficient operation. The pressure switch is just one component out of many that work together to help prevent accidents and damage to the equipment. The switch is not intelligent, though; it is up to you to ensure it is installed, wired, and set correctly to help provide the necessary protection to the gas train. If unsure about anything with the switch installation and operation, ask questions. Refrain from assuming, because the LGPS has been installed and others have checked it, that it is correct and functions appropriately.

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