Energy Use Calculations

Mechanical engineers perform load calculations to determine the correct size for heating and cooling equipment, airflow rates, and duct and pipe sizes. Architects calculate loads to make sure that the building envelope is adequately insulated, to compare alternative envelope designs, to estimate preliminary mechanical system costs, and to evaluate the potential benefits of solar energy design. Load calculations provide the basis for estimates of annual building energy use. They can become very complicated when used for detailed cost comparisons of alternative systems.

Hourly computer simulations of heating and cooling loads are necessary for accurate calculations for large industrial or commercial buildings. Calculating the hourly heating and cooling energy use for a year requires a substantial computer program. Looking at hourly solar angles and intensities and analyzing shading patterns can determine when the peak load occurs in the course of a year. However, even the best estimates are based on average weather conditions, and can't take into consideration potential problems with construction quality and unusual weather. Any computer program's results are dependent upon the assumptions of the person selecting the input data. Hourly annual calculations are not usually done for simple residences, but may be required to estimate energy use for a passive-solar heated home.

When one design costs less to install but another is more energy efficient, engineers may perform energy design value analyses. For example, an analysis may help with decisions about optimizing the quantity of insulation, choosing between double and triple glazing, selecting types of lighting, deciding on solar energy use, and balancing aesthetic considerations with their cost to the client.

There are four methods of design value analysis. The first looks at the payback period, which is the length of time required for the investment in the building to pay for itself in energy savings. This is a relatively simple method to understand, and is often used to screen out options. A second, lifecycle costing, looks at the total cost of owning, operating, and maintaining a project over its useful life, and considers salvage values and disposal costs. A third method, return on investment (ROI) considers whether the same money could be better invested elsewhere for more profit. The final method, comparative value analysis, also considers issues like improved worker efficiency, reduced air and water pollution, and appearance, and weight factors for relative importance. Then all competing designs are compared in an evaluation matrix, a graphic tool that helps the user decide on which of several complex choices to select.

Energy improvements, such as improving insulation or upgrading windows, can often be planned as part of other building renovations. The payback period for in sulating oil- and gas-heated buildings is around five years, and much faster with electric heat. New energy-efficient construction typically reduces heating bills in cold climates by 75 percent. Building a super-efficient house, with R-30 walls, R-38 ceilings, and R-19 foundations, adds about $5000 to $10,000 to the cost of conventional construction, money that can be recovered in five to ten years in energy savings. Such a house also creates less environmental pollution. Look for contractors with experience in energy-efficient construction for the best ideas and workmanship.

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