High efficiency boilers are tanks or vessels that transfer thermal energy from a heat source to water or steam and then recover heat that would typically escape through the flue. Condensing boilers are the most efficient boilers, with efficiencies above 90%.
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The more efficient a boiler, the less fuel it requires, which is beneficial for both the environment and the checkbook, making high efficiency boilers a good investment. Even though they initially cost more, the savings in fuel more than make up for the purchase price. Boilers vary in size and shape depending on the desired output and application. Industrial boilers are often quite large due to the size of the space or the necessity of having hot water for use in manufacturing processes. Boilers are constructed of cast iron, aluminum, steel or stainless steel. They are given ratings from A to G based on their efficiency; "A" boilers are condensing boilers, while conventional boilers more than 15 years old are usually given a "G" rating. High efficiency boilers are used wherever hot water or heat is beneficial or required such as in schools, offices, factories, production plants and other buildings. Mobile boilers provide hot water or steam power in emergency situations or in temporary locations.
The first few steps of the process for high efficiency boilers are identical to those of conventional boilers: heat is generated through the burning of fuels such as wood, coal or natural gas. Hot gases are produced and contained in metal rods or tubes called heat exchangers that are immersed in water or that contain water themselves. The heat is transferred to the water, which then increases in temperature (hence the name "boiler"), and water vapor or steam rises. In conventional boilers the steam would be released through the flue; however, high efficiency boilers retain it and condense it back into water. The latent heat that would have gone to waste is captured and used, thus making the boiler highly efficient. These kinds of boilers usually have dual heat exchangers and sealed combustion whereas older boilers with lower efficiencies have a pilot light and heavy heat exchangers that waste and lose a lot of thermal energy. Boilers are rated by their annual fuel utilization efficiency or AFUE, a measure of how the appliance uses fuel over the course of a typical year. It compares the ratio of heat output to the total energy consumed by the boiler; a rating of 93% means that 93% of the fuel or energy used becomes heat while 7% escapes.
