What is a boiler? How does a boiler work?

What is a Boiler? How do Boilers Work?

Before the invention of the steam boiler, humans relied on coal fires and the burning of wood as the sources of lightning and heat. This used to be a major inconvenience, as we’d need to light and tend a fire in every room of the house, not to mention we also have to clean the wood and coal wastes.

Today, thanks to boiler technology, we no longer have to deal with such inconveniences.

Here, we will learn about all you need to know about steam boiler technology, how they work, and other interesting facts you’ll want to know.

Let us begin.

What is a Boiler and What is its Function?

A steam boiler is a type of closed container that acts as a heat exchange system. It mainly works by heating the water to its boiling point with some energy source, typically via combustion or a type of fuel blown through a tube submerged in water.

This process will produce heat, which is then rejected into the system as steam. This steam will flow at low pressure through pipe systems to the equipment being heated.

Thus, a boiler is, in essence, a power generation device. It mainly works by applying the heat energy to water to produce steam (or water vapor). Both water and steam are environmentally-friendly heat carriers and are actually pretty efficient at transferring heat.

The water’s boiling point of 100°C or 212°F, but the boiling system increases the pressure by making the system an airtight closed container, increasing the boiling point. This allows a higher heat than 212°F to be delivered via the boiler system.

The main function of the boiler system is to produce, store, and distribute the steam. We can then use the steam for various heating applications including but not limited to cooking, power generation (boiler-based power plants), water heating, central heating for rooms, and so on.

How Do Boilers Work?

Boilers can be considered a part of a hydronic heating system. Hydronic systems use water to transfer heat to other equipment and/or distribution source. The boiler is a part of the system that heats the water, and it actually consists of many different key components:

  1. Burner: the component that burns the fuel to produce heat that will then heat the water. Various types of fuels can be used, but typically natural gas or oil.
  2. Firebox: the part where the fuel meets the air to create a burning fire (hence the name)
  3. Heat exchanger: the heat exchanger allows the transfer of heat between the burner and the fluid (water). The heat exchanger carries the heat from the burner to the fluid without making any direct contact with the fluid.
  4. Supply lines and return lines: a boiler uses a piping system to distribute the heated water/steam to the distribution points and target equipment. The supply lines are the pipes that distribute the steam (or hot water) to a distribution point, while the return lines are the pipes that bring the water back to the boiler when the water cools for re-heating.
  5. Refractory: refractory materials used for filling any gaps around the firebox to ensure the fire stays in the firebox at all times.
  6. Circulator pumps: the pump (or pumps) pushes the hot steam from the boiler system through the supply lines to the distribution point
  7. Condenser: condenser tanks are only used in steam boilers (and not hot water/hot oil boilers). The condenser ensures all steam is properly condensed before being pumped back to the de-aerator before the steam is then passed to the boiler. In a steam boiler with condenser tanks, new water is typically fed into this condenser.
  8. De-aerator: de-aerator tanks are also only available in steam boilers, and their function is to reduce oxygen and carbon dioxide levels (O2 and CO2) of the water inside the boiler system, mainly to protect the boiler against corrosion. Depending on the de-aerator construction, the O2 and CO2 levels can be reduced to about 0.02 mg/l of O2 and 0 mg/l of CO2.
  9. Superheater: a very important part of any boiler is the superheater, and its main function is to increase the temperature of the saturated steam before entering the circulator pumps without increasing its pressure.
  10. Economizer: an economizer is essentially a heat exchanger that is placed in the boiler’s exhaust to lower energy consumptions.

There are several different ways of how a boiler can work to heat steam/water and distribute the heat, but basically, we can differentiate them into just two categories:

Water-Tube Boilers

A fuel source is burned in the furnace, heating the water tubes inside. The water is then boiled to generate steam, and the steam will be distributed downstream.

A water-tube boiler is typically more energy-efficient, but they are more complex and expensive to manufacture, and is very reliant on water quality. That is, you might need to constantly filter the water feed to ensure the boiler can work at its maximum efficiency.

What is a boiler - Water Tube Boiler
Source: https://commons.wikimedia.org/wiki

Fire-Tube Boilers

A fire-tube boiler has tubes running through the water tank. Fuel is burned in the furnace, while the smokestack vents the gases and heat produced by this heating effect. This is performed to regulate pressure so it doesn’t continue to rise above the intended level.

The tubes then will transfer the heat of the furnace through the water in the tank, and once it’s heated, the steam can move downstream.

Fire-tube boilers are easier to manufacture, so they tend to be more affordable than water-tube boilers. However, they are not as energy-efficient and thermally-efficient (not as good as water-tube boilers in maintaining heat).

What is a boiler - Fire Tube Boiler
Source: https://commons.wikimedia.org/wiki

Are Boilers Dangerous?

Boilers, after all, work by burning fuel, and anything that involves heat and burning fire obviously can be dangerous.

There are indeed accidents related to boilers, whether they are caused by a system error or human error, but they are actually relatively safe due to their pretty good safety controls that are implemented in most boilers that follow the industry standards.

We can pretty confidently say that a home furnace (forced-air) is much more dangerous than a typical boiler.

Nevertheless, according to the National Board of Boiler and Pressure Vessel Inspectors’ Boiler Incident report, there are several common causes of boiler accidents:

  1. Operator error or poor maintenance;
  2. Low-water cutoff;
  3. Unknown;
  4. Limit controls;
  5. Burner failure;
  6. Faulty design or fabrication;
  7. Improper installation;
  8. Safety valve; and
  9. Improper repair.

As you can see, human error and poor maintenance are the most common causes of various boiler incidents, so as long as you maintain the boiler properly, you shouldn’t have much issue regarding safety.

On the other hand, the low-water cutoff is the most common mechanical cause of boiler accidents, so it’s very important to test the low-water cutoff on a regular basis. You can refer to the boiler’s instruction manuals on how to test the low-water cutoff.

Also, you might need to replace the low-water cutoff control if the boiler is older than five years. Some newer boilers actually have a test button for this function.

To summarize, as long as you perform the recommended best practices when operating your boiler, and perform regular maintenance, your boiler should be pretty safe.

Due to the ASME CSD1:2009 code for boiler products, manufacturers are now required to perform systematic and thorough testing before the product can be launched, and this alone has significantly reduced the risks related to boiler accidents.

When Is a Water Heater Considered a Boiler?

It’s important to note that not all water heaters are boilers, while on the other hand, all boilers can be used as water heaters.

With that being said, the main differences between the two are the amount of water needed, the operating temperatures, and the internal pressures of the system.

In general, if the device stores more than 120 gallons of water, operates at 160 psi or higher pressures, and generates a heat output of at least 200,000 BTUs per hour, it can be classified as a boiler.

Boilers can run on various types of fuel from electricity, gas, heating oil, propane, and others. A common misconception is to think that a boiler is limited to certain types of fuel, but that isn’t actually the case.

Otherwise, regular water heaters (that are not a boiler) typically use much smaller tanks, usually around 20 to 50 gallons. In fact, there are also tankless water heaters that do not store water at all.

There are, however, some exceptions, as some water heaters can hold 120 gallons or so. In that case, look at the operational pressure and generated heat to determine whether the unit is a boiler.

Typically regular water heaters have maximum operating temperatures of less than 210°F (below the water’s boiling point), and most water heaters are set between 120°F  and 150°F  to prevent scalding. If the unit has a higher operating temperature, most likely it’s a boiler.

To summarize, check the amount of stored water/water needed, internal pressure of the system, and operating temperatures.


We hope we’ve answered some of your questions with respect to what is a boiler? and how a boiler works?

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