Types of Boilers


Great article with photos

This article will discuss all the different types of boilers there are and what not to do. The boiler is almost like a member of the family. Having a good knowledge of all the different boiler types (their technology, how they use fuel, their impact on the environment, their efficiency when it comes to the new regulations, etc) is not only very useful but also informative as it will help you when you need to make a decision on what type of boiler to buy. When the time comes, refer to this article because it will help you to choose the most suitable boiler for your needs.


First things first: What is a boiler?

To gain a better understanding of the different kinds of boilers, we first need to understand what a boiler is. A boiler is an appliance that consumes fuel and heats water to generate steam which can be used for various purposes. The boilers referred to in this article are ones that provide heat.

Heat can be generated in many ways. However, boilers are one of the most commonly used appliances for heating and hot water in the domestic sector. In the past, boilers weren’t as safe, efficient, or elegant as they are now.

Over time, boilers have been improved and they are now wonderfully fuel-efficient, quiet, user-friendly, compactly designed, and have almost zero waste. Their design is elegant and blends in well with the home’s interior design.


How boiler types are classified

Traditionally, there have been many different kinds of boilers, including the famous locomotive boilers. Today, the most commonly-used boilers are energy-generating boilers for thermoelectric plants, fire-tube boilers, commercial kitchen boilers, medical boilers, boilers for heating fluids in the oil sector, and heating boilers and DHW (domestic hot water).

In this section, we will discuss the various ways of classifying heating and DHW boilers, which are boilers that provide hot water to warm your home when it’s cold and provide comfort to the people within the household.


KEY QUESTION 1: What are the different kinds of boilers? Boilers are classified according to various criteria. Depending on how they are used, they can be classified as either domestic, commercial or industrial. Depending on their function, they can be classified as heating-only boilers or mixed boilers (heating and hot water). Depending on where they are located, boilers can be classified into wall-mounted boilers (or wall boilers), floor-standing boilers (or floor boilers), and indoor or outdoor boilers. According to the fuel they use (natural or LPG), they can be diesel, electric, biomass, solar, coal or oil. Finally, depending on their technology, boilers can be classified as either atmospheric or watertight (condensation, low NOx and modulating).

Let’s look more closely at the main classifications for the various boiler types for heating and DHW.

Boiler types according to their scope of use

Depending on the sector or field in which they are used, the different types of boilers are:

  • Domestic boilers: These are used in the household sector and are for individuals and families. Domestic boilers can be individual or communal. They typically work with pressures between 1 and 2 bars, and although they can (in some cases) heat water up to 90 degrees C, it’s recommended that you keep the temperature well below boiling temperature (100 degrees C). The main kinds of domestic boilers are mixed heating and DHW wall boilers that work with gas, diesel, electric or solar. We will, however, take a look at this in more detail later.
  • Commercial boilers: These boilers are neither as small as domestic boilers, nor as large as industrial ones, and have an intermediate performance level. They can be used in the workplace, a public eating space, etc.
  • Industrial boilers: As their name suggests, these boilers are used in the industrial sector and they naturally need to perform at a higher level, have stronger materials, a larger size, and have much more sophisticated safety systems. Industrial boilers can work at pressures between 30 and 350 bars, and they can generate water temperatures of between 300 and 600 degrees.

These categories are sometimes relative. It could be that in the case of a small commercial space, a family-sized domestic boiler can be used. Also, it sometimes happens that a building’s central heating appliance is as big as one that can be found in a large commercial space.

KEY QUESTION 2: What types of industrial boilers are there? Generally speaking, the kinds of industrial boilers that exist can be grouped into two broad categories: fire-tube boilers (up to 30 bars and up to 300 deg C) and water tube boilers (up to 350 bars and up to 600 deg C). Further within these two categories are more industrial boiler types which have different uses, such as steam generating boilers, flash steam boilers, inverted or double hearth boilers, multi-step steam boilers, superheated steam boilers, and more.

Boiler types according to function

Depending on the functions they perform, boilers can be classified according to the following types:

  • Heating only boilers: As its name suggests, these boilers only generate heat. They do this by heating the water and using it as a heat carrier or transmitter, sending it to heating radiators, from which heat from the water dissipates and increases the temperature of the living space. The water then returns to the boiler to be reheated, completing the cycle, and the process is repeated.
  • Mixed boilers: These are also known as combined boilers (or combi boilers). In addition to being able to generate heat, they can provide domestic hot water (DHW). The heating and DHW circuits are normally separate. The heating circuit reaches the radiators, and the DHW circuit reaches the hot water taps in the home. Combined boilers can save on fuel costs and space in the home since they use the same equipment and same fuel for both functions.

KEY QUESTION 3: What is a hot water boiler? These are boilers that, in addition to generating heat, also supply sanitary hot water via the home’s hot water taps. There are no boilers that only provide hot water, but combined or mixed heat and DHW boilers are available. When the unit provides only DHW, it is not called a boiler but a heater. There are various kinds of heaters: tank heaters, pass-through heaters, instant heaters, electric heaters, solar heaters, etc.

Boiler types according to fuel usage

Boilers can run on a variety of fuel types, namely traditional and modern fuels. They are very versatile and are perfectly adapted to use clean and renewable fuels. In terms of the fuel they use, the following are the different kinds of boilers that there are:

  • Gas boilers: These use gas as fuel. They are still one of the most commonly used boilers since they operate silently and do not produce pollutants from the combustion process. Gas boiler types include natural gas (NG) and Liquefied Petroleum Gas (LPG). Natural gas requires a special installation that can be quite expensive, but it’s well worth it. LPG can be either propane or butane. Its flame is bright blue.
  • Diesel boilers: These boilers use diesel. They have the disadvantage of a noisier operation and they also produce pollution resulting from the combustion process. However, they have the advantage of not requiring too much maintenance and you can keep a large reserve of fuel at home without having to rely on a remote fuel supply which can sometimes be unreliable. The types of diesel boilers vary according to their technology. Like boilers that use other kinds of fuel, they can be either atmospheric, sealed or condensing boilers.
  • Electric boilers: These boilers don’t need fuel to run and operate with the use of electrical resistance. This kind of boiler is not recommended for places where there is a high demand for hot water (large families, for example) as they can use a lot of energy when used extensively. It is best suited for small families, couples, single people, etc. Aside from their excessive energy consumption, there are advantages in using these kinds of boilers as they don’t use combustion, produce combustion gases, and all your household energy expenses are conveniently combined.
  • Biomass boilers: Biomass boilers use the so-called biofuels from natural materials (peels, fruit pits, first waste, grains, sawdusts, etc) or transformed natural materials (that have been compressed into pellets). Their burners can produce a flame, generate heat and domestic hot water (DHW) from this fuel type so its design is therefore quite different from that of other boilers. Generally speaking, these boilers are considered environmentally-friendly units.
  • Solar boilers: These are boilers that use various forms of solar radiation to heat water for heating or DHW. They are considered green appliances. They can be used alone or combined with other boiler types (in this case, they are known as hybrid solar boilers). This solves the age-old problem of running out of energy when there is no sun. Thankfully, this problem is becoming something of the past thanks to new technologies that harness the sun’s ultraviolet rays so that the boiler can work even when there is no sun or it is cloudy.
  • Boilers that use other energy sources: Boilers that use other energy sources are less frequent and these include coal and oil boilers.

Boiler types according to technology

Boiler technology has changed a lot over the years. These technological improvements have meant that boilers (which have seen less traditional use) are now once again mainly used for heating and DHW.

The technology types a boiler uses largely depends on factors such as design, fuel efficiency, and how environmentally friendly it is. Some boiler technology has, in fact, become obsolete as it has failed to meet modern high standards.

Depending on the technology used, boiler types can be any of the following:

  • Atmospheric boilers: They have open combustion chambers and take air for combustion from the air around them. These boilers have more traditional technologies, are less fuel efficient, more environmentally polluting, and not as safe inside the home. These boilers have not been allowed to be manufactured since 2013, and their use in new installations has recently been banned. This is according to the new European (ErP – ELD) and Spanish (R.I.T.E) regulations.
  • Watertight boilers: These are also known as forced draft boilers. Their technology is more modern and safer. Currently, if a boiler is considered safe and environmentally friendly, it should at least be a sealed boiler. Sealed boilers have hermetically sealed chambers, meaning that they take in air for combustion through a device that takes the air from the area around the boiler. They also expel residual combustion fumes directly out of this device with the use of auxiliary fans. This is often carried out through coaxial smoke outlets that use the same tube with a double concentric channel to absorb the combustion air and to expel the residual smoke. (More details about coaxial smoke outlets can be found in this article).

Watertight boilers are, in turn, subdivided into other kinds of boilers that, even though they use watertight technology, differ from watertight boilers in many respects, such as fuel efficiency and decreasing polluting emissions.

  • Condensing boilers: A condensing boiler is essentially a sealed boiler that can reuse the latent heat in the residual combustion fumes. Its technology is based on not immediately expelling the combustion fumes, but on condensing them and reabsorbing their heat. In this way, they ensure fuel efficiency above 100% (sometimes up to 100%), resulting in a reduction to a minimum of polluting gases and the temperature of expelled smoke.
  • Low NOx boilers: These are watertight boilers that focus on reducing polluting emissions of nitrogen oxides (NOx), which are one of the most dangerous pollutants for the environment and are the result of fossil fuel burning at high temperatures. This reduction in NOx emissions is due to the burner’s cooling technology. Condensing boilers are also low NOx emitters.
  • Modulating boilers: These are watertight boilers that have high levels of efficiency and precision as they continuously reduce or increase their power and operation depending on the actual needs of the space they are heating and the ambient temperature.

KEY QUESTION 4: In conclusion to this article, this is the final question. What is the best boiler type? Until a few years ago, it was difficult to answer this, and you would have needed to choose a criterion instead and ask: What is the best boiler type from this aspect? However, today with environment and climate issues being of the utmost importance, these aspects have become crucial in determining the best boiler type.


The best boiler type is one that has excellent fuel efficiency, meaning it is able to produce the greatest amount of heat using the least amount of fuel. Similarly, it emits the least amount of polluting gases. From a fuel efficiency perspective, the award for the best boiler goes to the condensing boiler as it is able to achieve extraordinary fuel efficiency as much as 110% due to its recycling technology and smoke condensate.

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