PWR boiler repair certified boiler professionals category

The number 1 Industrial Boiler Sizing Guide

Contact our Boiler Experts ➜ 1-800-377-8326

When you start the process to replace your current boiler system, one of the first questions you’ll need to answer is “What size boiler do I need?” You shouldn’t just replace your current boiler system with the latest model. Instead, you need to do your homework first. After all, the goal of upgrading your boiler system is to improve the efficiency of your operation.

Using our industrial boiler sizing guide, you’ll be able to understand:

  • Problems with undersized and oversized boilers
  • The difference between sizing a hot water boiler and a steam boiler
  • How to size an industrial boiler with our easy-to-use calculators
  • The inputs that affect the size of your boiler

Problems with undersized and oversized boilers

Taking the time to properly size your boiler helps to make sure you have a reliable and efficient operation. More often than not, businesses operate with oversized boilers, which causes issues with boiler efficiency. While not as common, undersized boilers can also lead to production issues. Here’s what we mean:

Hot Water Versus Steam Boiler Sizing Requirements

Based on your production needs, you will select either hot water or steam boiler for your operation. A steam boiler is used to meet your high or low-pressure steam system requirements while a hot water boiler is typically used to provide hydronic heat. Since these types of boilers have different usages, there are certain inputs that matter more when estimating the size of your boiler.

Note: Some facilities require 100% redundancy while others operate on an N+1 scheme. This just means that the total load is split between N number of boilers and an additional boiler sits idle for availability if the main boiler was to go out of service (due to failure, safety device tripping, or maintenance).

Steam Boiler Sizing Inputs

  • Required operating pressure
  • Load profile in pounds per hour of steam
    • Maximum load
    • Average load
    • Minimum load
  • Hours of operation
  • Fuel input
  • Redundancy required

Hot Water Boiler Sizing Inputs:

  • Circulation medium such as hydronic hot water, domestic hot water or glycol mixture
  • Required operating pressure
  • Load Profile in “btu/hr: or “MMbtu/hr”
    • Max, Min, and Avg Flow (GPM)
    • Max, Min, and Avg Supply Temp (°F)
    • Max, Min, and Avg Return Temp (°F)
  • Hours of operation
  • Fuel Input
  • Redundancy required

How to size an industrial boiler with our custom calculators?

We’ve created three custom calculators to help you size an industrial boiler, including

  1. Boiler Horsepower Calculator
  2. BTU/HR to Boiler Horsepower Calculator
  3. BTU given Flow and △T Calculator

Boiler Horsepower Calculator

Boiler horsepower is used for sizing steam boilers. This calculation translates the steam load (in pounds per hour) to boiler horsepower. Each boiler is given a boiler horsepower (BHP) rating based on the steam output capacity at 212°F and 0 psig.

For this calculation, you will enter the steam load in pounds per hour, which will be divided by a constant of 34.5. This number represents the thermal energy rate required to turn 34.5 lbs of water at 212 °F to steam in one hour.

BTU/HR to Boiler Horsepower Calculator

The BTU/hour to Boiler Horsepower calculator is used to help size both steam and hot water boilers. This calculation translates the heat transfer input requirement to BHP. To do so, we divide the BTU/hour by a constant of 33,475. This constant represents water being converted to steam, which is 34.5 pounds of water multiplied by 970.3 BTU/lb, or latent energy.

For this calculation, you will enter the BTU/hr of your operation along with the efficiency of your boiler. This will then tell you the boiler horsepower.

Note: To estimate the expected boiler horsepower output from the total heat input from the fuel, the overall boiler efficiency (fuel-to-steam efficiency) is required. Since a boiler horsepower is a measurement of the energy required to convert the water to steam, we need to account for the difference between the heat input of the fuel and the efficiency of the boiler to transfer that energy to the water in order to produce steam. Typically, in industrial boiler applications, the fuel-to-steam efficiency is between 80-85%.

If the BTU/hr requirement for the desired output is known, boiler efficiency is not required for the calculation. This is because neither the output requirement nor boiler horsepower takes the boiler efficiency into account - they are both a measurement of boiler output capacity only.

Boiler BTU Calculator given Flow and △T

Hot water boilers do not experience any phase change in the heating process so the calculation is different than for steam boilers. Since there are no phase changes, the latent energy is not utilized in hot water boilers. Instead, the flow of water and the required temperature change, which directly corresponds to the difference in sensible heat, are the main drivers for the BTU/hr load of the system. Meanwhile, Specific Heat (SH), Specific Gravity (SG) and Density of the medium being circulated are important properties to include to ensure proper sizing.

For this calculation, you will enter Delta T and Gallons Per Minute (GPM). With the Specific Heat, Specific Gravity and Density values representing the properties of water, the output of this calculation will tell you the BTU.

Note: While the SH, SG and Density values are somewhat temperature-dependent, the given values are good for estimation purposes when sizing a hot water boiler that uses water only as the heating medium. The SG, SH, and Density values would change if a different medium was used (a glycol mixture, oil, etc.)

Boiler sizing definitions

Boiler Horsepower

Boiler Horsepower is defined as the thermal energy rate required to turn 34.5 lbs of water at 212 °F to steam in one hour.


A BTU, or a British Thermal Unit, is a unit of heat defined as the amount of heat required to raise the temperature of a single pound of water by 1 °F. BTU/hr is a measurement of heat energy transferred per unit time, in this case 1 hour.

Specific Heat

Specific Heat is the intensive property of a material's heat capacity, or the amount of heat required to result in a unit change in it’s temperature. The specific is identified by dividing the heat capacity by the mass of sample material.

Specific Gravity

Specific Gravity, also known as relative density, is the ratio of the density of a substance to the density of a reference. The specific gravity of liquids is typically measured in reference to water at 40 °F).


The density of a substance refers to its mass per unit volume. When comparing materials of the same size (volume) those with a higher density will have a larger mass (be heavier) than those with a lower density.

Delta T or ΔT

Delta is a Greek letter that is used in Mathematics and Engineering to denote a change in a value. Delta T (ΔT) identifies the change in temperature between two points. Typically, in boilers, this is referring to the difference between the supply and return temperatures.

Gallons Per Minute

GPM is a measurement of volumetric flow measuring the number of gallons to flow past a point of reference every minute.

How to size an industrial boiler with powerhouse

We’ve given you the frameworks to help estimate the size of your boiler. If you’re looking for someone to double-check and triple-check the math, our team at Powerhouse is happy to help.

Contact us and we’ll walk you through the following questions so we can make sure you find the right boiler for your operation.


Related Boiler Education Pages