Boiler Basics 101: Types of Boilers

When we think about boilers, there are a two types that typically come to mind; firetube, or scotch marine, and watertube boilers. These types of boilers can be classified as hot water, steam, high pressure, and low pressure. In today’s blog post we will be answering the question: what are the basic differences between the different types of boilers?

Although their final function is the same, the main difference between a firetube and watertube boiler is the construction and design of each system. In a firetube boiler, water inside a vessel is surrounded by tubes that contain combustion gases. In other words, the ‘fire’ is inside the tubes, making it a ‘firetube’. Watertube boilers are essentially the opposite in design. Combustion gases surround a series of tubes that contain water, coining the name, watertube.

By definition, high pressure boilers are built to a maximum allowable working pressure (MAWP) above 15 psig, while low pressure boilers are designed for operation at 15 psig or below. Low pressure boilers are most commonly utilized in heating applications and require less maintenance than that of a high pressure unit. Furthermore, firetube boilers can be built for both low and high pressure applications, while watertube boilers are typically built for high pressure needs.

Some may think that firetube and watertube boilers are in the same category as hot water and steam boilers. However, steam and hot water boilers are actually a classification, and can be considered a subcategory to firetube & watertube boilers.

Hot water and steam boilers operate in a very similar manner, but hot water boilers don’t actually produce steam. In reality, a hot water boiler is just a fuel fired hot water heater, in which heat is added to increase the temperature to a level below the boiling point. Hot water boilers are not as powerful as steam boilers, which is why they are more commonly used in heating applications providing hot water at 120 – 220F.

Steam boilers heat water to levels that are above boiling point, in order to produce steam. They are much more powerful and are utilized in more industrial and heavy-commercial applications. Steam boilers can be designed to produce either saturated or superheated steam, which we will discuss further on in a future post.  

Be it a firetube, watertube, hot water, or steam boiler, they are all effective and efficient in their own unique ways. To learn in more detail about the differences between boiler types, visit the section on our website, “What Boiler Is Best For You”.