How a boiler operates

Many buildings use a boiler (or multiple boilers) to produce hot water or steam for heat, heating potable (tap) water or for process applications such as manufacturing, power generation, laundry, et cetera.

These boilers come in many different types, shapes, sizes, colors, fuels and more. Despite the many type of boilers, they all share a common purpose.

  1. Firetube boilers
    • Types of firetube boilers
    • Parts of a firetube boiler
    • How they operate
  2. Other types of boilers
  3. Output
    • Heating
    • Process

Firetube Boilers

Kewanee typically produced firetube boilers.

Firetube boilers are a type of boiler where the combustion products flow through tubes surrounded by water. This heats the water, either to a certain temperature for hot water operation – or transforms it into steam.

Firetube boilers are typically used for heating applications, though some are still used for some process applications. (Many process applications now use watertube boilers.)

Firetube boilers are perhaps the oldest type of boiler, since they were mainly used in early (and later) locomotives and steam engines.

Types of Firetube Boilers

There are two types of firetube boilers: firebox boilers and “Scotch Marine” boilers.

Firebox boilers are typically used for heating applications. These boilers are typically rated for lower steam and hot water pressures, typically around 15-30 psi maximum. Examples of firebox boilers produced by Kewanee include the Type C, Type M, and Type R models.

Scotch Marine boilers, on the other hand, are typically much larger and used for both heating and process applications. These boilers are typically long and cylindrical in shape. Scotch Marine boilers sometimes operate at a higher pressure than their firebox counterparts, sometimes up to 150 psi steam. Examples of Scotch Marine boilers produced by Kewanee include the Scottie Jr. and Classic series. Because their shape resembles the boiler found on a steam locomotive, Scotch Marine boilers are sometimes referred to as “locomotive boilers.”

Parts of a Firetube Boiler – Burner and Safety Components

Firetube boilers consist of many components that work together to produce hot water or steam safely and reliably. This page discusses the parts of the boiler “package” itself – it does not discuss ancillary system components, such as condensate pumps, expansion tanks, water treatment systems, etc.

Boiler Combustion Controls: The combustion controls for a boiler do a lot more than turn the burner on or off. The combustion controls are responsible for safely operating the boiler – shutting it down in the case of an issue or dangerous situation. The controls are connected to other components that monitor the boiler’s conditions – such as the low-water cutoff and Pressuretrols.

Combustion controls are also used tor timing purposes, such as to start the burner or shut it down. They also control the firing rate of the burner.

Most modern combustion controls are microprocessor-based. (Even more modern controls can be connected to a computer for temperature control or remote monitoring.) However, older boiler combustion controls relied on mechanical switches and vacuum tubes. However, both systems operate under the same simple principle – to monitor the flame and other boiler conditions and switch the burner off in case of an issue.

The combustion control panel on a 1966 Kewanee Type M boiler. The Fire-Eye controller is the red module on the right.

Fire-Eye System: The “eye” of the boiler’s combustion control system is the Fire-eye, sometimes also referred to as a flame scanner. This optical sensor is located near the burner and monitors both the pilot and main flame. When a flame is “scanned” (or sensed) the scanner will produce a small electrical current that is sent to the microprocessor or cycle switch. If the scanner loses sight of the flame, the current diminishes and the controls shut the boiler down.

In many cases, the current of the flame scanner is amplified by an amplifier for use with the combustion control system.

Low-Water CutOff (LWCO): The LWCO monitors the water level in the boiler. If the water level is low, it switches the burner off and – in some cases – automatically opens a valve to fill the boiler to the proper level. 

A LWCO is critical for safe operation of the boiler; boilers with too much water can result in a condition known as “water hammer” which can not only damage critical equipment connected to the boiler, but can also be a dangerous condition.

Water levels too low can create an even more dangerous situation. A boiler with no water that has a burner running can be a recipe for disaster. The burner can heat the crown sheet to extremely hot temperatures. Any drop of water can create an explosive release of steam, which often leads to the boiler rupturing. In some cases, this can completely destroy the building around the boiler – shooting parts hundreds or even thousands of feet or even a mile away.

Due to the critical safety issues that can be caused by running a boiler with too little water, most boilers have two LWCOs for redundancy – the main LWCO and an auxiliary LWCO.

In addition to LWCOs, all boilers have a water level gauge located near the LWCO to allow the operator to view the water level inside the boiler.

Temperature & Pressure Relief Valve: The TPRV prevents the boiler from building too much steam or pressure. If the boiler exceeds the rated open temperature or pressure of the TPRV, the valve will open – releasing the excess steam/pressure from the boiler.

If the pressure exceeds that of the boiler’s design rating, the boiler can fail – having disastrous consequences. Most smaller boilers used for heating applications are rated for 15-30 PSI, whereas larger boilers for large building complexes or process applications can be rated up to (and sometimes exceeding) 150 PSI!

As with the water level, boilers feature gauges that display the temperature and pressure of the boiler.

Pressuretrols: In addition to the TPV and pressure gauges, boilers feature a series of pressure switches called Pressuretrols. Most boilers have three Pressuretrol switches – a low-limit which starts the burner if the pressure drops too low, a high-limit which shuts the burner down when satisfied (the normal operating pressure is met), and an auxiliary high-limit which shuts the burner down if pressure creeps dangerously high. These switches are mainly used for control purposes. (Pressuretrol is a genericized trademark of Honeywell. These switches may be manufactured under or called different names.)

Three Pressuretrol switches located on a boiler behind the steam pressure gauge.

Burner Assembly: A modern boiler contains a burner assembly, which creates the jet-like flame that shoots into the combustion chamber of the boiler.

In the old days – prior to the common use of natural/LP gas and oil – boilers were typically fired using coal. Coal-fired boilers usually required someone to manually stoke the coal in the boiler, a grueling and dirty task. Eventually, automatic stokers were developed that automated this task.

However, by the middle of the 20th century, most locations had access to natural gas. Other locations had oil tanks used for heating oil, and some places had both. Therefore, burners used to fire the gas or oil started to become commonplace on boilers.

The burner assembly on boilers consist of many components – the aforementioned combustion control and flame scanner, a combustion air blower, a gas train, an ignition source, pilot flame and main flame.

An aftermarket Industrial Combustion gas burner mounted on a 1964 Kewanee scotch marine boiler.

Combustion Air Blower: The combustion air blower blows air into the burner, providing oxygen required for combustion.

Gas Train: The gas train is a series of valves that supply fuel to the burners. The gas train typically consists of three solenoid gas valves: two for the main burner, and one for the pilot burner. Gas trains typically have additional manual valves used to shut off gas supply in case of service or emergencies.

Oil supply systems are typically more complicated – featuring a series of pumps, valves and filters.

Ignition Source: An ignition source – typically a spark produced by a high-voltage transformer – ignites the pilot flame.

Pilot Burner: The pilot flame ignites first and is “proven” by the flame scanner before the main burner is activated.

Main Burner: The main burner is ignited by the pilot burner, and usually resembles a jet. This burner is what heats the boiler.

You may hear the terms steam trim and water trim in reference to these components. The trim are the waterside components of the boiler, such as the Pressuretrols, temperature and pressure relief valve, and the water gauge. These are usually different between steam and hot water models.

Fireside vs. Waterside

A firetube boiler can be split into two main sections: the fireside and the waterside. In most cases, both are passive, meaning they don’t have any operational parts inside of them.

Fireside: The fireside is any part of the boiler that has combustion gases (or a flame) inside of it. In other words, it’s the part of boiler that’s dry or doesn’t contain water. It consists of the combustion chamber or furnace (the area where the burner fires into), the tubes themselves for products of combustion to travel through, and the exhaust – where the products of combustion leave the boiler to exit the building through a chimney or stack.

Firetube boiler designs differ. Many Kewanee models, such as the Type C boiler, utilize a “three-pass” design. The three-pass design means the products of combustion circle around the boiler in three passes or “zones.” The first pass is the combustion chamber or furnace itself. The second pass is a middle row of tubes, and the third pass is a top row of tubes that head toward the exhaust outlet. However, some boilers only use a two-pass design, where the gases only go through one set of tubes for a second pass. Other designs use even more passes.

(An easy way to tell how many passes a boiler has is through the exhaust location. Boilers with the exhaust in the front often have an even amount of passes – usually two or four. Boilers with a rear exhaust often have an odd amount of passes – typically three.)

Waterside: On the flip side, the waterside of a boiler is any part of the boiler that is wet or contains water or steam.

The waterside and fireside are separated by a crown sheet. The crown sheet forms the top of the combustion chamber and the floor of the boiler tank.

Packaged or not?

To add to the confusion of boiler types, some boilers are referred to as “packaged boilers.” In a packaged boiler setup, all of the parts related to the boiler directly – the burner controls and gas train, for instance – are in one group or package. In other words, a packaged boiler is manufactured at the factory with its burner, controls and gas train already installed. Other boilers usually have “aftermarket” burners, controls and gas train installed on site.

The 1968 Type C boiler is an example of a “non-packaged” boiler. Its burner controls are mounted on a wall away from the boiler, and the burner itself is made by a different manufacturer and was not installed at the factory. On the other hand, the 1974 Kewanee Type M is an example of a packaged boiler – the burner, burner controls and gas train are all in one area and were likely installed or matched at the factory.

Please note that boiler rooms often extend beyond the boiler itself. Many boiler rooms feature water treatment systems, condensate pumps, tanks, reheaters and other accessories that make the boiler or heating system more robust. Some boilers may also have additional accessories or features installed, such as a coil for directly heating potable water in the boiler.

Other Types of Boilers

While firetube boilers are the oldest and perhaps the most common type of boiler, other types of boilers also exist. Listed below are just a couple of the different varieties of boilers you may find.

Watertube Boilers: Watertube boilers are the inverse of firetube boilers. Water runs through tubes that make up the “wall” of the furnace or combustion chamber. Watertube boilers are typically used in heating large building complexes and providing process steam. The main advantage of using a watertube boiler is the ability to produce steam at higher pressures. Watertube boilers are also better at quickly reacting to changes in steam demand. However, watertube boilers are expensive and huge. Some watertube boilers are many stories tall, and are typically housed in their own dedicated building.

A condensing boiler.

Condensing Boilers: A relatively new introduction, condensing boilers are compact boilers which are more efficient than their larger firetube and cast iron counterparts. Condensing boilers are typically at least 90% efficient, meaning 90 percent of the heat is used to heat water or produce steam, and the remainder exits the flu. Some condensing boilers can achieve up to 97% efficiency. Condensing boilers are typically used to heat homes and small buildings, though larger models (or multiple boilers installed in tandem) can even heat larger buildings.

Combination Boilers: Also referred to as combi-boilers, combination boilers are small residential boilers used to heat both water for heating purposes and for tap use. These boilers are typically found in homes and small buildings, especially outside of North America in areas like Europe.

Cast Iron Boilers: Cast iron boilers are boilers which are made mainly of cast iron. These heavy boilers are difficult to change out due to their size and weight. Typically, cast iron boilers must be cut up into pieces to remove from the boiler room, and a new boiler is typically built on site. These boilers are typically found in older homes and buildings.


You may be wondering: “now the boiler has produced the steam or hot water for the load, what happens now?”

For Heating

For heating purposes, the steam or hot water is piped throughout the building to heat exchangers. These heat exchangers “exchange” or transfer the heat into the space they’re heating. 

In some heating systems, the steam never leaves the boiler room. Instead, the steam is piped through a coil submerged in a tank of water. In the tank, the steam in the coil heats the water. This water may be used to heat the space in the same manner the water from a hot water boiler would (or the steam would) or it may be potable water that comes out of a tap for washing your hands, bathing/showering, cooking, doing dishes/laundry, etc.

In the rooms, heat exchangers come in many different shapes and varieties.

Radiators are the most common associated with boilers, and provides radiant heat to heat the space. A simple radiator consists of fins surrounding a pipe. The pipe contains the hot water or steam, and the fins help radiate the heat. An upside to radiators is they are cheap, simple, and easy to repair or replace if needed. Their simple design means they’re very reliable. However, radiators are often heavy (many older radiators are made of cast iron) and can pose a safety hazard since people (especially young children) may touch the hot radiator and get burned. Radiators can also create hot and cold spots in a room.

In-floor loops are radiators that are placed below the floor surface, and have become common in recent years. In-floor loops are better able to evenly heat the room since an in-floor loop consists of a “coil” that covers the entire room. Unlike radiators, in-floor loops don’t consume any space in the room because they are hidden under the flooring surface. However, this poses a problem when an issue arises – the flooring surface must be removed to replace the loop. In-floor loops eliminate the safety hazard associated with traditional radiators. Most in-floor loops used with hydronic heating systems use hot water instead of steam, and some in-floor loops are electric.

Fan coils are forced-air systems used with a hydronic heating system. A fan coil is essentially an air handler (a blower inside of a cabinet connected to return and supply air ductwork) with a heat exchanger installed inside. Air blown across the heat exchanger is heated, and is fed through supply ducts and into the room. Fan coils are often located in a dedicated mechanical room (sometimes in the boiler room itself) and have supply and return air registers located throughout the building. In many cases, a fan coil also has a separate coil used for air conditioning, which is connected to a condenser or chiller system. (In some cases, the second coil can be connected to a heat pump for a “duel fuel” heating system.)

A commercial fan coil or air handler installed in a building. Steam or hot water is piped into coils inside for heating, and chilled water or refrigerant for air conditioning.
A unit heater.

Unit heaters are fan coils sans the ductwork. Typically mounted from a ceiling and used in shop areas, hydronic unit heaters operate similar to a fan coil. A fan located on the back of the heater blows air across the heat exchanger, which heats the room in which the unit heater is installed.

Boilers are typically connected to control systems which control the boiler, fan coil/unit heater (if equipped), and solenoid valves that may be present throughout the system depending on the demands for heat. These control systems may be pneumatic, or electronic. Often times this control system also controls the air conditioning/chiller systems.

For Process Applications

The steam supplied by the boiler may be used for more than heating a space or water. Process steam can be used to turn turbines for generating electrical or mechanical power. Process steam can also be used to operate machinery, the manufacture of alcoholic beverages, laundry/dry cleaning, or numerous other purposes.

For process applications, the pressurized steam (typically high-pressure boilers are used for process applications, which outputs steam at >100 PSI) is piped into machinery or turbines.

Process steam can also be used for air conditioning. Steam absorption chillers allow for space-saving, efficient air conditioning using the steam typically produced by a central boiler that may also be used to heat the building.

Kewanee Boilers were made between 1892 and 2002, with many still in use. While Kewanee’s boilers were made in several different shapes, sizes, and colors over the years – most worked the same way and used the same parts.

Originally written 1-1-2021. Last updated 2-12-2021.