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Note 17. Energy Analysis Computer Programs

Many of the improvements to system efficiency recommended by the Energy Efficiency Manual require detailed calculations to determine the energy savings they will provide. For HVAC applications, accuracy often requires sophisticated computer energy modeling programs.

This Note explains energy analysis programs, such as DOE2, TRACE, HAP, BLAST, and AXCESS. The major programs have the capability of modeling the energy performance of virtually all conventional building systems, including the thermal properties of the structure, HVAC systems, and lighting. They can calculate the relative efficiency or cost effects of individual energy system components, entire system designs, architectural features, energy prices, and so forth. The same programs can be used to calculate peak loads for equipment sizing.

These calculation tools require a great deal of input effort by the user. You will learn the program structure that is common to most programs, typical program output, how to use the programs, the accuracy limitations of simulations, different ways of dealing with weather data, and tricks for catching errors. You will be able to judge how far you can trust the results you get.

If used properly, energy analysis software provides good accuracy in calculating relative performance for making design and investment decisions. They are much less capable of providing accuracy in absolute terms, primarily because the conditions existing in actual buildings, such as weather, maintenance, and human behavior, cannot be predicted. Unfortunately, computer programs are often used to give a false veneer of authority to bad analysis, for example, erroneously indicating that a building design fulfills the requirements of an energy efficiency codes.