How does a boiler work?

In many large buildings or a group of large buildings, boilers are an essential component. Boilers are typically used to heat buildings or water, they are also sometimes used for process power or even generating electricity.

Thus, there are many different types of boilers that come in different sizes, shapes, colors, fuels, and everything else. The types range from watertube boilers to firetube boilers to the small fin-based boilers which are commonly found in smaller buildings and homes. Despite the type, they all operate on one principle: to boil water to create steam (or, in some cases, hot water.)

Kewanee produced firetube boilers for over a century, in both firebox and scotch-marine varieties.

Interactive Map: click on a part of the boiler to jump to the paragraph describing that part.

kewanee front

1974 kewanee

The Origin of Firetube Boilers

The first steam locomotives and steamboats used boilers to produce the energy necessary to operate. In a large percentage of the cases, these boilers were very early examples of the firetube boilers.

Firetube boilers are the most common in commercial applications, especially heating and process applications. However, other types of boilers (such as the watertube boiler) do exist. Kewanee Boiler produced exclusively firetube boilers.

Components of a Boiler

This section discusses components essential for operation of the boiler itself. System components, such as condensate pumps, expansion tanks, and water treatment systems have been left out.

Boiler Controls

In a firetube boiler, one of the first and foremost systems in the control system. It is what starts the burner, and monitors the burners and sensors and safety switches to ensure the boiler is operating safely at maximum efficiency. In the old days, boiler explosions were very common. It wasn't just because boilers were much more common - it was also caused by the archaic safety mechanisms used.


The "fire-eye" is a type of control which continuously monitors the flame, and controls the ignition and modulation of the burner. This is a crucial component of the control system. A build-up of combustibles (such as natural/LP gas) can be very dangerous, and can result in an explosion. Therefore, the control will shut off fuel supply to the burner if the flame is lost or the burner fails to ignite.

Low Water Cut Off (LWCO)

A LWCO is the safety mechanism which ensures the boiler will quit firing if the water level inside the boiler becomes too low. If the water levels get too low or the boiler is fired while empty, the crown sheet (the piece of metal separating the combustion chamber and water chamber) will glow red hot. Even a drop of water can result in an explosive release of steam. A failed (stuck closed) or jumped LWCO can turn any boiler or water heater into a ticking time bomb. The cause of many deadly boiler explosions are a failed or jumped LWCO. Due to the importance of LWCOs, most boilers have two LWCOs - a main LWCO and a failsafe LWCO.

Temperature & Pressure Relief Valve

The temperature and pressure relief valve does exactly what you think it would do: it relieves a pressure buildup in the boiler in case of a closed steam pipe or blockage in the steam pipe, or another failure. As with any pressure vessel, a buildup of pressure which exceeds the pressure the boiler can withstand can result in a violent explosion. There are also gauges on all boilers (steam and hot-water) which display the pressure and water temperature inside the boiler.

Cycling Control

In many older boilers with older burners and control systems, the fire-eye system is a separate unit from the cycle controller. The cycle-controller, before there were microprocessors, was very similar to a dryer timer. There was a spinning mechanism which would complete a circuit either energizing or deenergizing a part of the burner system at specific times. These systems were replaced by much more reliable systems implementing microcontrollers.

Cycling controls are a vital part of the boiler. It is what stops the burner in the case of an issue, such as a lack of water in the boiler or an increase in pressure. This can stop the boiler from reaching critical levels which can cause it to explode. (Before control systems, boiler explosions were common place. Not only because boilers were more prevalent, but because the mechanism which stops the burner wasn't there. Solid fuels, such as coal and wood, were also dangerous because they would cause a runaway situation where the fire could not be quickly extinguished in an emergency situation.)

The Burner

The burner is another crucial part of the boiler. Without any heat, there would be no way to boil the water inside to produce the hot water or steam. Before natural gas, oil, and LP gas were used in boilers, most people had to manually stoke coal into the boiler - which was a physically and mentally exhausting task, as well as having to be around the hot boiler constantly. After automatic (or mechanized) coal stokers were developed, natural gas and oil soon became the norm.

The burner consists of a blower to blow combustion air into the combustion chamber, a gas train, and the control system. We've already discussed the control system above, but what about the parts of the burner itself and the gas train?

Combustion Air Blower

Unlike the induced draft blower found in most home furnaces, burners attached to boilers create a positive draft in the combustion chamber. This provides the air needed for combustion. In majority of cases, the burner gets its combustion air directly from the boiler room.

In many parts of the United States, regulation requires a vent fan inside of a boiler room so that it can draw outside air for combustion.

Gas Train

The gas train is the mechanism which delivers oil and/or gas to the burner. For gas units, this typically includes two or three gas valves, including one for the pilot. The gas train also typically includes manual valves for shutting the gas off by hand in case of an emergency or the boiler is being worked on.

Oil systems are a little more complicated, requiring pumps and preheaters in order to pump the oil to the burner and allow it to be properly atomized.

Fireside vs. Waterside

In a firetube boiler, there are two sections. The fireside includes the combustion chamber and tubes which the hot combustion products travel through to heat the water. The fireside also includes the collection boxes. The waterside, as you can probably already guess, is the part of the boiler which contains the water and steam. The fireside's combustion chamber is separated from the waterside by the crown sheet.

In a three-pass boiler, the combustion products circle twice through tubes, and once through the combustion chamber. This increases the amount of heat that is transferred from the fireside into the water, thus increasing the efficiency.

3-pass boiler diagram

Scotch Marine vs. Firebox

There were two types of firetube boilers which are commonly found in the field, both of which Kewanee produced. These two variations are firebox and scotch marine designs. I will also explain what a "packaged" boiler is.

Firebox Boilers

The boilers shown in the interactive map above are both firebox boilers. One (the left; a grey 1968 American Standard Kewanee boiler) is a Kewanee Type C, while the other (the right) is a Kewanee Type M. Firetube boilers are known for their square-ish shape with a "dome" at the top. (A similar shape to that of a school bus.)

Most firebox boilers are rated for lower steam/hot water pressures than their Scotch Marine counterparts. Thus, firebox boilers are typically found in heating applications.

Scotch Marine Boilers

scotchmarineScotch marine boilers, on the other hand, are almost always circular and long. They are commonly found in larger commercial buildings and almost always are rated for a higher output than their firebox counterparts.

Scotch Marine boilers are typically found in applications where a higher steam output (above 15 PSI) is required, such as process steam and power production. Thus, you'd more likely find a scotch marine boiler in a factory or power plant.

Packaged Boilers

The Type C boiler is not a packaged boiler. A packaged boiler has all of the ancillary devices in one assembly - or package. The Type C has an ignition control mounted remotely, with oil pumps and water pumps remotely located as well. Non-packaged boilers were common before the 1970s, when controls were still bulky.

The Type M boiler is a packaged boiler. All components of the boiler - including the ignition control - is mounted in one "package." Packaged boilers are easier to install, as installation consists of connecting the electrical and plumbing to the boiler.

Other Types of Boilers

In addition to firetube boilers, there are many other types of boiler which shares one thing in common: their job is to heat water.

Watertube Boilers

In heavy industrial and power plant applications, watertube boilers are the norm. They are typically large, bulky, and oddly-shaped in comparison to their firetube peers.

Watertube boilers operate in an opposite manner to firetube boilers. While a firetube boiler consists of a vessel full of water and tubes running through the vessel containing the hot flue gasses, a watertube boiler has water running through the tubes and the open space is the combustion chamber.

Watertube boilers have many pros, as well as many cons. Watertube boilers are more efficient in most cases, safer, and can be run at higher temperatures. However, firetube boilers are still more popular because they can handle a higher steam demand. Furthermore, firetube boilers can quickly adjust to a sudden surge in steam demand, while a watertube boiler would take time to adjust to the change in demand.

Condensing Boilers

condensingThe firetube and watertube boiler design has been around for years. For instance, the firetube boiler was the first type of boiler that found its home in steam engines.

However, in recent decades, a new type of boiler has emerged. The condensing boiler is a type of high-efficiency boiler that has added components inside in order to exchange as much heat out as possible. The average efficiency of a standard condensing boiler is about 95%, although some are more efficient.

Fin-Tube Boilers

In residential applications, firetube and watertube boilers are often way too big and would take up too much space. Therefore, fin-tube boilers are typically found in small commercial applications and some residential applications.

Fin-tube boilers are much different from their firetube and watertube counterparts. A fin-tube boiler's combustion system- which is typically similar to what you'd find in a standard gas furnace - heats up "fins" which surround a tube. This tube serves as a heat exchanger to heat the water.

Fin-tube boilers are popular because they come in a wide variety of sizes. You can purchase them from small sizes for residential applications in your own home, all the way up to larger models that heat entire buildings.


In residential applications, why have two systems for heating and water heating when you can combine those into one system? The combination boiler ("combo-boiler") has solved that problem.

Found in International locations (such as European nations) for decades, the combo-boilers are making their way into homes around the United States. These boilers are small and can't fit onto a wall - resembling a tankless water heater. However, these boilers don't just heat potable water, but they also heat water for heating.

Output (Heating)

While a boiler can produce steam or hot water for a variety of purposes (such as "process power", dry/steam cleaning, etc.), the most common is for heating.

A boiler will feed steam or heated water throughout the building. However, different buildings have different ways to use the steam/heated water to heat rooms.


The most common sign that you have a steam heating system is through radiators. There are two-types of radiators: fan-forced radiators and standard radiators.

Standard radiators are very simple. They consist of a pipe which feeds into "fins." These fins produce heat, which radiates from the radiator. (Hence the name.) Due to their simple design, they were very common before the 1960s. Depending on the placement in the room, they can cause uncomfortable "hot spots" and "cold spots" in the room.

Fan-Forced Radiators and Unit Heaters

Fan-forced radiators use a blower to force air through a coil inside of the cabinet. These are more advanced as they typically have a control system and heat an entire room better. Fan-forced radiators differ from fan coils as they are not ducted. Fan-forced radiators are very similar to unit heaters, which are also (typically) fan forced and mounted high on a wall or on the ceiling.

In-Floor Loops

In recent years, placing a loop of piping below the surface of the floor has become a new trend in heating. Instead of relying on (unsightly) radiators placed around the room, a coil placed below the flooring produces the heat.