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There are three basic configurations of residential units at McLean Gardens: bi-level units on the first two floors, middle-level units, and upper-level units that include loft vinyle and replacement space under the slope of the roof (see Figure 2). For our energy simulations, we selected six actual floor plans, windows two each of lower, middle, and upper units, with one of vinyle each type having a relatively small window area and one a large area. The idea was to bracket the energy use of most units with high and low estimated energy use and energy savings.[FIGURE 2 OMITTED]For energy analysis, we chose Visual DOE, a commercial front-end product that executes DOE 2.1E in performing hourly simulations of energy use in the building. Visual DOE doesn''t perform air leakage calculations, however, which we needed to get a good comparison between existing and replacement options. To do this, we selected CONTAM, a program developed by the National Institute for Standards and Technology and available flee through its Web site (see information at the replacement end of this article). CONTAM calculates instantaneous values of air flow through openings in buildings, using the flow characteristics windows of each opening, orientation, height, and other factors, based on a weather file that provides temperature and wind information. We set up our CONTAM runs to calculate air flow for each hour of the typical year, and then summarized vinyle and replacement the results by month.To complete the energy windows simulation inputs, we gathered information on building envelope construction and estimated appliance loads, hot water usage, and occupancy for each unit type. Water-to-air heat pumps installed vinyle in each unit provide heating and cooling at McLean Gardens. Six central plants equipped with boilers and cooling towers provide conditioned water at roughly 70[degrees]F to the heat pumps as a source or sink for energy. We took advantage of Visual DOE''s ability to model this system by grouping an upper, middle, and lower unit together with a central plant for each analysis. Projected Cost SavingsWe calculated cost savings for the residential units in a spreadsheet, using monthly energy consumption results from Visual DOE and the applicable utility rate, which includes seasonal differences replacement and a block rate that makes energy more expensive after the first 400 kwh. (Visual DOE can do cost analysis, but not using the combination of rates that we needed.) Similarly, we estimated central plant operating costs by applying commercial electric and gas rates to the estimated monthly energy consumption and demand (see Table 2).The value of energy savings attributable windows to window replacement fell between $.072 and $.092 per kWh of electric energy saved. Estimated total savings across vinyle all 720 replacement residential units and the central plants range from about $35,000 per year for storm window replacement to $91,000 per year for the most efficient windows. These savings represent about 5%-12% of the total gas and electric bills for McLean Gardens residents and plants. The simple payback based on energy savings is estimated at around 65 years for storm window replacement, and over 80 years for full window replacement windows.
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