Boiler Classifications

boiler classificationThere you are, standing in the boiler room, or even worse, in a boiler room you’re not familiar with. The supervisor walks up and goes “Johnson, what type of boiler is this?” And instantly you’re on the spot. Do you know? Can you look at the boiler and tell? Don’t think I’d be able to without doing some leg work and poking around. Take a look in the peephole, see what’s going on inside.

It happens in the military and out in buildings and facilities with decent stationary engineer training programs, a little shaming. Some slight embarrassment at your expense if you will. That fear of being embarrassed should make you want to learn and have some answers beforehand yeah?

Well, we’re here to help out those seeking some quick answers on boiler classifications. We are going to briefly start at the top and then work our way into specific classes such as firetube or watertube boilers. For now, this will be more of an overview so everyone at least has some idea of what they’re looking at.

Boiler Classifications

We already mentioned the first main type of classification the boiler goes through. Firetube or watertube. And that is going to drive me bananas for the entire post as it shows up on my end as being spelled wrong. Fun times.

Fire or Water?

firetube boiler

The flame.

First thing you should do when checking out a boiler (besides looking at the water level and flame condition lol) is determine whether it is a firetube or watertube boiler. Is the water inside the tubes or does it surround the tubes?

Firetube boilers have you guessed it, fire in the tubes. Well not really fire in the tubes but in the combustion chamber. This heat and the gases of combustion then flow through the tubes which are surrounded by water. The heat gets transferred to the water through the thin tube walls.

A watertube boiler is just the opposite arrangement.

Water is inside the tubes and the heat and gases of combustion surround them.

Personally I have never even been in the vicinity of a watertube class boiler. Have you? Leave a comment and tell us about it please.

Boiler Operating Pressure

There are several official types of boilers but mainly it can be divided into working pressures.

For instance, and this is always a question on the boiler operator or steam engineer exam, a low pressure boiler is a boiler that operates at 15 psi or below.

A high pressure boiler is one that runs above 15 psi and over 6 boiler horsepower.

Other types of boilers that can fall into this classification are:

  • low pressure hot water heating boiler (<160 psi & 250 F.)
  • small power boiler (>15 psi but <100 psi and <440,000 btu/hr heat input.)
  • power steam boiler (>15 psi.)
  • hot water supply boiler (>120 gallon volume, or heat input >200,000 btu/hr, or >200 F.)
  • power hot water boiler (>160 psi or >250 F.)

Again the main thing to remember for the exam is <15 psi – low pressure boiler.

Fuel Type

boiler classifications

The gun you use to feed fuel oil to a boiler.

The boilers you see in the pictures around the site can use natural gas or fuel oil. We use natural gas in them unless some emergency occurs.

A boiler that primarily uses natural gas needs a specific type of burner and delivery method.

Coal is another source of fuel and requires different equipment also.

Natural gas, fuel oil, and coal are just a couple. So we have a gas fired boiler. You may have a coal fired boiler.


Just wanted to put out a brief post on types of boiler classifications. Now you can quit being in the dark or afraid to speak up when looking at the latest and greatest boiler.

When you look inside, what do you see? Is it a fire or watertube?

What type of fuel does it use? Natural gas typically has piping, fuel oil is more like a gun or wand that you plug in, and coal usually has an elaborate delivery system.

If you glance at some of the gauges, what are the operating pressures and temperatures?

These three things alone will give you a leg up in identifying exactly what class of boiler you are looking at.

Comment? Question? Tirade? Suggestion? Let’s hear them!

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