The efficiency of a steam boiler can be calculated by dividing the energy going into the boiler by the resulting energy output of the boiler. To directly measure the efficiency of a steam boiler, measuring the energy input to the boiler and its subsequent energy output is necessary.
The measurements will vary depending on the type of fuel used and the boiler being correctly calibrated. The overall efficiency of a boiler depends upon the efficiency of its combustion process, the boiler's thermal efficiency and its fuel-to-fluid efficiency. All three of these aspects of a boiler's functionality must be considered when assessing the overall efficiency of a steam boiler. That the boiler maintains an accurate control of the internal air supply is necessary for achieving greatest possible efficiency. If too much air is allowed inside the boiler, it will have a cooling effect as it carries away some of the heat produced, and if there is too little air, the combustion process will not be complete, and smoke will be produced from any resulting unburned fuel. The effectiveness of the transition of heat from the point of combustion to steam is also key to a boiler's efficiency, and any heat that escapes the boiler in the transition process will add to the inefficiency of the overall process.