Boiler efficiency measures the relationship between the energy input and energy output of a boiler system. This ratio is expressed as a percentage and is used to assess the performance of the boiler and the burner as a means to determine the fuel costs of your boiler equipment investment. In fact, the ongoing fuel usage costs are the most significant portion of your investment, even more than your initial capital expense, so it is important to understand boiler efficiency as you make strategic business decisions about your equipment. Even a few percentage points can make a big difference!

The Importance of Boiler Efficiency

Understanding boiler efficiency, also known as fuel-to-steam efficiency or fuel-to-water efficiency, can have an economic impact on your business’ operations. To put it simply, the more efficient that your boiler is, the less you will spend on fuel. By calculating boiler efficiency, you are able to forecast the fuel costs, in which you can, ultimately, determine the full life cycle cost of the boiler. Based on this information, you want to choose a boiler that is sustainable and has a long life. Having the boiler efficiency can allow you to make strategic decisions in purchasing a new or used boiler or help you incorporate certain maintenance practices that aim to improve your boiler efficiency score as a possible solution to reduce fuel and maintenance costs.

How to calculate boiler efficiency fuel costs

We want to show you the financial impact that boiler efficiency has on your business. Even a percentage point may save you thousands of dollars annually! Don’t believe us? Check it out with our cost calculator below.

All you need to know to be able to use this calculator is the boiler horsepower, fuel type, boiler efficiency and yearly operation to determine your annual fuel costs. Knowing your current boiler efficiency is a great place to start, and then you can start moving the dial to understand the cost-benefit of increasing the boiler efficiency for your business.

Typical Measures of Boiler Efficiency

There are several calculations that you can conduct to better understand the overall boiler efficiency of your equipment. In other words, each of the assessments listed below contributes to the overall boiler efficiency. If you notice that one of these ratios deviates from the norm, then you may want to further investigate the factors that influence that particular calculation. In order to determine a course of action to correct the issue, you can work with our service technicians, who will give you a fair assessment of your boiler’s health and help solve all of your industrial boiler repair needs.

Combustion Efficiency

Combustion efficiency helps us to understand the burner’s ability to burn fuel. By analyzing the amount of unburned fuel and excess air in the exhaust, we can determine how effectively the heat content of a fuel is transferred into usable heat. If both the unburned fuel and excess air operate at low levels, then your burner system is considered efficient. The desired range for combustion efficiency is 75 - 89%.

Flue gas oxygen and stack temperature are the primary factors that indicate combustion efficiency. If you fall outside the desired range, then we suggest looking at these two indicators to make adjustments. Even more so, we recommend that you periodically monitor flue gas composition in order to maintain excess air at optimum levels. While excess air is partially responsible for the heat loss, it is necessary to completely burn the fuel. There are ways to signal that you do not meet the desired levels of excess air. For instance, unburned combustibles like fuel, soot and smoke mean that you do not have enough excess air while an increased flue gas flow indicates that you have too much.

Seasonal Efficiency

When the seasons change, they bring in ambient air conditions such as air temperature, humidity, density, etc. These conditions result in significant changes in the operation of a burner. Burners are specifically tuned to the current air properties. It is important to tune a boiler during times of significant seasonal differences. We recommend at least twice a year in the spring and fall.

Seasonal efficiency is how well the boiler uses fuel over the entire heat season. To determine the seasonal efficiency, you compare the total seasonal heat output used by your facility to the total seasonal fuel input. Having this information will let you know how much you will need to pay for fuel over the heating season.

Different seasons bring varying amounts of boiler operations that you will need. The steady-state efficiency of your boiler system along with the downtime and cycling losses affect the overall seasonal efficiency. Ultimately, the longer a boiler operates, the higher the seasonal efficiency, so when you turn the boiler on and off, you create downtime and cycling losses resulting in a decreased seasonal efficiency. As you determine your needs in the heating season, make sure to think about these effects in your decisions.

Thermal Efficiency

Thermal efficiency assesses the effectiveness of the heat exchanger by analyzing its ability to transfer heat energy to the water or steam within the boiler. However, this calculation does not account for external factors such as ON-OFF losses, radiation losses, convection losses and more, so it is not the best indication of the boilers' fuel usage.

There are many factors that can negatively affect thermal efficiency, particularly water scaling and soot build up on the boiler tubes. If the heat exchanging surfaces are in poor condition, then less heat is transferred from the combustion gases into the water. As a result, the boiler operates less efficiently.

Fuel-to-Steam Efficiency

Fuel-to-Steam efficiency is the overall efficiency of the boiler system, including all other factors discussed above. While many times, thermal efficiency is used as synonymous of steam-to-fuel efficiency, they are not identical. This metric is the single most important characteristic of a boiler required to make the best comparison of equipment and the wisest economic decision for an upcoming purchase.

The average values for Fuel-to-Steam efficiency are typically between 80-85%. To achieve the upper end of this metric, manufacturers have incorporated advanced computational fluid dynamics (CFD) modeling to optimize the design of components such as boiler tubes, furnace shape, and more.

What Affects Boiler Efficiency?

Understanding the different types of boiler efficiencies can help you begin to piece together the factors that are positively or negatively affecting the overall performance of your boiler system. We have listed the major components with a brief explanation of their effect for your reference.

Flue Gas Temperature

The flue gas temperature is the temperature of the combustion gases as they exit the boiler. If the flue gas temperature is high, then it is often associated with heat being lost. Flue gas economizers can be installed to recover heat, and enhance the overall boiler efficiency.

Radiation and Convection Losses

Radiation and convection losses are inevitable. However, there are varying amounts of these losses based on factors like boiler type, size and operating pressure as well as velocity across the boiler. Each of these components can have a large impact on the radiation and convection losses, but, in the end, all boilers will have some type of heat loss radiating from the boiler vessel. Insulation is the best solution to minimize losses.

Ambient Air Temperature

The temperature of the combustion air entering the boiler is called ambient temperature, which can have a relatively noticeable effect on boiler efficiency. A variation of 40 degrees Farenheit can affect boiler efficiency by 1% or more, so this can drastically affect your operations.

Fuel Specification

Fuel specification plays an important part in boiler efficiency. In the case with gaseous fuels, the higher the hydrogen content, the more water vapor is formed during combustion, resulting in high water vapor losses and a lower efficiency. This is why oil fires operate at higher efficiency levels than natural gas.


Turndown is the ability of the boiler to achieve a wide range (from low to high) of output. If a boiler has a high turndown ratio, then it is able to supply steam at lower rates without shutting down and restarting. This means that higher turndowns can offer efficiency improvements when demand varies.

While some would argue that the higher the turndown, the greater the efficiency there is evidence to suggest that capability of greater than 4:1 turndown has negligible benefit. If your modulating burner has at least a 4:1 turndown, this is much more efficient than a non-modulating burner. Most manufacturers are producing modulating burners with ranges between 4:1 and 10:1 turndown.

How to Increase Boiler Efficiency

Upon calculating the efficiency of your boiler, you can then determine what needs to be done to increase it a few percentage points. There are several ways that you can improve boiler efficiency, ranging from no-cost options to large capital investments. Remember, even though you might hesitate to spend money to increase boiler efficiency by even one percentage point, you can potentially save thousands of dollars in annual fuel costs. Here’s our list of the top ten ways to improve boiler efficiency:

  1. Incorporate Preventative Maintenance Practices
  2. Install a Flue Gas Economizer
  3. Clean Boiler Internals and Tune Burner
  4. Upgrade Boiler Burners (Retrofit Components or Replace)
  5. Improve Boiler Insulation
  6. Reduce Excess Air with O2 Trim System
  7. Recover Heat from Boiler Blowdown
  8. Install Condensate Return System
  9. Rent a Boiler as a Short-Term Solution
  10. Talk to our Engineered Services for a Permanent Solution

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