Boiler Heating Surface
The boiler heating surface (expressed in square feet) is that portion of the surface of the heat transfer apparatus in contact with the fluid being heated on one side and the gas or refractory being cooled on the other side. The direct or radiant heating surface is the surface against which the fire strikes. The surface that comes in contact with the hot gases is called the indirect or convection surface.
The heating capacity of any boiler is influenced by the amount and arrangement of the heating surface and the temperatures on either side. The arrangement of the heating surface refers to the ratio of the diameter of each passage to its length, as well as its con- tour (straight or curved), cross-sectional shape, number of passes, and other design variables.
Boiler Efficiency
The boiler efficiency is the ratio of the heat output to the caloric value of the fuel. Boiler efficiency is determined by various factors including the type of fuel used, the method of firing, and the control settings. For example, oil- and gas-fired boilers have boiler efficiencies ranging from 70 to 80 percent. A hand-fired boiler in which anthracite coal is used will have a boiler efficiency of 60 to 75 percent.
Boiler Energy Efficiency
Two government programs have been created within the last 20 years to rate the energy efficiency of different heating appliances such as furnaces, boilers, water heaters, and heat pumps. These two programs are (1) the annual fuel utilization capacity (AFUE) program and (2) the Energy Star Certification program.
• Annual Fuel Utilization Capacity (AFUE). The energy efficiency of an oil-, gas-, or coal-fired boiler is measured by its annual fuel utilization capacity (AFUE). The AFUE ratings for boilers manufactured today are listed in the boiler manufacturer’s literature. Look for the Ener Guide emblem for the efficiency rating of that particular model. The higher the rating, the more efficient the boiler. The government has established a minimum rating for boilers of 78 percent. Mid-efficiency boilers have AFUE ratings ranging from 78 to 82 percent. High-efficiency (condensing) boilers have AFUE ratings ranging from 88 to 97 percent. Conventional (noncondensing) steam and hot-water space heating boilers have AFUE ratings of approximately 60 to 65 percent.
• Energy Star Certification. Energy Star is an energy performance rating system created in 1992 by the U.S. Environmental Protection Agency (EPA) to identify and certify certain energy-efficient appliances. The goal is to give special recognition to companies who manufacture products that help reduce greenhouse gas emissions. This voluntary labeling program was expanded by 1995 to include furnaces, boilers, heat pumps, and other HVAC equipment. Both the Energy Star label and an AFUE rating are used to identify an energy-efficient appliance.