The Switch is On: Air Pressure Switches and Their Role in Boiler Systems
Boilers are a fantastic harmony of air, fire, and water. Each element works with all the others to convert fuel into usable steam. To do that reliably and efficiently, though, you have to keep each of these elements in sync with all of the others, and one of the key pieces of equipment in a boiler that helps do that is the air switch.
You Just Got Drafted
The combustion cycle inside a boiler’s furnace follows a specific pathway that’s known as the “draft”. The draft is best defined as the flow of gases into, through, and out of a boiler’s furnace. This draft needs to keep moving for two reasons. First, the draft introduces fresh oxygen in the form of outside air into the combustion chamber. Second, the draft’s movement or flow is what takes the combustion gases out of the furnace and up into the stack.
Pressure Is On
Air and other gases don’t just move on their own. They respond to pressure, with a natural inclination to move from an area of high pressure to an area of lower pressure. That’s why soda cans explode, that’s why tires go flat, and that’s why the boiler’s draft flows in only one direction. Or, rather, it’s supposed to. Air pressure switches make sure the draft continues to flow correctly by measuring the pressure in the particular area of the boiler.
Come On In
The first place you’ll find an air switch on most boilers is in the windbox. We aren’t talking about your father-in-law, though. A boiler’s windbox is a manifold that directs the air into a boiler’s furnace in a specific way that creates turbulence in the air to help it mix better with the fuel and therefore burn more efficiently.
The air pressure switch located in the windbox is there to confirm the conditions are correct for safe burner firing. When it’s time to fire the burners, outside air is driven into the windbox by a fan. As the fan starts to blow, it increases the pressure in the windbox. This in turn triggers the switch, which tells the safety systems that the air is flowing correctly for combustion.
Out You Go
There are also pressure switches located in the boiler’s stack on firebox-type boilers, to confirm that the pressure is where it’s supposed to be to keep the draft moving in the right direction. Remember, it’s the differential pressure that keeps pulling the draft along. Hot exhaust gases are not as dense as incoming air, so the pressure will naturally be lower in the stack.
Inside The Switch
Pressure switches come in many varieties depending on where they’re designed to work. Some use mechanical bellows or a piston that moves back and forth with pressure changes to open and close a circuit. Some are solid-state and use electronic sensors to detect pressure changes. Pressure switches also come in low and high varieties. The low-pressure switches trigger when the pressure drops below a set point, while the high-pressure switches activate when the pressure goes beyond a certain parameter. Together, they form the guard rails that keep the boiler’s draft moving as it should.
If you need replacement air switches or need to have yours tested as part of your routine maintenance, WARE has a wide selection of switches, and we have the know-how to inspect and install them. If there’s anything you need, just let us know.