To make steam, you need water and combustion. To create combustion, you need fuel, you need an ignition source, and you need air. Lots of air. But if you want efficiency and safety, that air has to be delivered at specific amounts and pressures to mix with the fuel, and to provide draft to the stack. To operate a boiler safely, you also need air to perform the crucial functions of purging combustion chambers during shutdown or restart. That’s where air switches come in.
WHAT AIR SWITCHES DO
At their core, air switches are designed to detect changes or differences in air pressure. This is important in the boiler world because proper airflow is crucial to maintaining the proper air/fuel mixture for efficient combustion. By monitoring air pressure, these switches can make sure enough air is flowing through, or detect a lack of air pressure caused by a blockage, a clogged filter, or a malfunctioning damper. This detection is known as “proving”, because it allows the switch to provide proof of airflow or lack of proper airflow.
HOW AIR SWITCHES WORK
Air switches can use several different types of technology to detect the presence of – or change in – proper air pressure.
Diaphragm Air Switches use a flexible membrane that moves when a difference in air pressure occurs on either side of the membrane. The membrane is connected to a mechanical linkage which also moves when the membrane moves. As it moves, the mechanical linkage either completes or interrupts an electrical switch.
Piston or Bellows Air Switches use a movable piston or a bellows that moves or changes position as air pressure changes. The piston or bellows is connected to a microswitch that will either complete or interrupt an electrical circuit. Their construction makes piston or bellows air switches a little more robust than diaphragm switches, allowing them to be used in areas with higher pressures, greater pressure differentials, or dirty or oily air streams.
Thermal Air Switches have a heating element that is kept at a constant temperature. As air flows over the element, it changes the temperature and, in turn, the resistance of the element. This change in current can be detected by the boiler control system and interpreted as a change in air movement or pressure. Thermal air switches are extremely rare in the boiler industry, but they are out there.
Vane or Paddle Air Switches have a physical paddle or vane mounted in the air stream that moves as airflow changes. As the vane or paddle moves, it moves a mechanical linkage connected to an electrical switch that either completes or interrupts an electrical circuit.
HOW AIR SWITCHES ARE USED
Air switches serve a wide range of purposes in a boiler system, which is why you’ll find them in multiple locations.
Combustion Air Proving: Air switches ensure that the proper amount of air is flowing to the burners to maintain the ideal fuel/air mixture so that fuel won’t be wasted, or that the boiler’s fire side won’t run too lean.
Combustion Pressure Proving: Air switches are used to prove flame on boilers using the pressure generated by combustion – generally in pulse-based combustion.
Draft Proving: In order to maintain the draft that keeps exhaust gases flowing to the stack, air must be flowing in sufficient quantities and must maintain a specified pressure.
Purge Airflow Proving: During the purge cycle, airflow must be sufficient to remove any combustion byproducts from the fire side. Air switches, therefore, help ensure that the combustion chamber will be properly purged for a safe restart.
Blocked Intake Sensing: Airflow switches make sure that the air intake is capable of delivering enough air to the burners. If a blockage occurs, the air switches alert the control system and the boiler operator to check for a blockage.
Combustion Air Filter Flow: By measuring the pressure before and after the filter in the air stream, an air switch can ensure that the filter has sufficient air moving through it to maintain the boiler manufacturer’s specified air volume and flow.
High Furnace Pressure: Furnace pressure can increase significantly under unstable combustion conditions. This switch can be used to shut down a unit accordingly.
DIFFERENT TYPES OF SWITCHES
Because air serves different purposes depending on where it is in the boiler system, air switches are available in multiple configurations that are optimized for the role they serve in the boiler.
Automatic Reset Switches like the ones found in combustion proving will automatically reset themselves as the boiler’s burners start and stop during normal operation. That way, boiler operators don’t have to go in and manually reset the air switches during each firing cycle.
Manual Reset Switches prevent restarts during unsafe or inefficient conditions, requiring the boiler operator to manually reset them and confirm that conditions are right for proper operation before restart.
Single Port Switches are used for pressure proving. Single Port Switches are ideal for areas in the boiler that have a higher range of air pressure fluctuation.
Two-Port or Differential Switches are used for areas in the boiler that have lower pressure differentials or air flow, because they are more sensitive than single-port switches. They can also be used to verify flow by checking the pressure drop across a restriction such as a burner diffusor.
CHECK THE RANGE
While there are a wide range of air switches available, they aren’t interchangeable. For example, a sensitive dual-port switch is not suited for areas of higher pressure or differential. That’s why it’s always crucial to check the range of the switch you’re installing or replacing, to make sure it’s rated for the conditions in which it will be operating.
KNOW THE TERMS
In the boiler world, and in WARE’s BoilerWAREhouse.com online parts site, you’ll find several common acronyms and labels that indicate how an air switch operates, or where it’s used in a boiler system.
DFS stands for Differential Flow Switch, indicating a switch that measures the air pressure difference (differential) between two points.
FS stands for Flow Switch, meaning the switch checks for air flow.
JD-2 switches are extremely popular switches that are adjustable, allowing them to be fine-tuned to the specific boiler in which they are used.
AFS stands for Air Flow Switch, one of the world’s most popular manufacturers of air switches. They are usually found in air flow proving applications, and are also used to monitor fan or blower operation.
SML stands for Switch Manual Lock, meaning the switch must be reset manually after it breaks or trips.
SMR, which stands for Switch Manual Reset, is similar to SML switches in that the switch must be reset manually before boiler operation can continue.
If you have any questions or concerns about the way your air switches are operating, or if you’re not sure what kind or type of switch to use for a specific application, the experts at WARE are always standing by to help. Our boiler techs and parts experts are all highly trained, and know which air switches your boiler needs to operate properly and efficiently. Reach out to us directly. We’re always here to help.
FAQs: Air Switches in Boiler Systems
Q: What does an air switch do in a boiler system?
A: An air switch monitors airflow or pressure changes to ensure safe and efficient combustion. It proves airflow during burner operation, detects blockages, and helps maintain proper draft, purge cycles, and filter performance.
Q: What are the different types of air switches used in boilers?
A: Common types include diaphragm, piston or bellows, vane or paddle, and thermal air switches. Each is suited to specific airflow conditions or pressure ranges within a boiler system.
Q: What’s the difference between single-port and differential air switches?
A: Single-port switches measure pressure at a single point, ideal for high-pressure areas. Differential (two-port) switches compare air pressure between two points and are used in low-pressure or sensitive proving applications.
Q: What is combustion air proving, and why is it important?
A: Combustion air proving ensures that adequate air is flowing to the burners to support efficient fuel combustion. Air switches confirm this airflow to prevent unsafe ignition or fuel waste.
Q: What does a purge airflow switch do?
A: It ensures that combustion byproducts are properly purged from the chamber before a restart, preventing dangerous ignition conditions.
Q: What are JD-2 air switches, and where are they used?
A: JD-2 air switches are adjustable differential flow switches commonly used in combustion airflow proving. They’re versatile, reliable, and used across many boiler types.
Q: What is the difference between a manual reset and automatic reset air switch?
A: Manual reset switches require operator intervention after tripping, while automatic reset switches re-engage automatically during normal burner cycles.
Q: How do I choose the right air switch for my boiler?
A: Always check the pressure range and application type—high vs. low pressure, proving vs. shutdown, single vs. dual port. Not all switches are interchangeable.
Q: What does AFS mean on an air switch?
A: AFS stands for Air Flow Switch, a common label and manufacturer name. AFS switches are used for airflow detection and fan/blower monitoring.
Q: Where can I get help selecting the right air switch?
A: WARE’s BoilerWAREhouse has a full inventory of air switches, and our trained technicians and parts experts can help you find the right match for your application. Visit the parts store or contact us.