Determining Applicability of Front-End Modular Boilers, Modeling Retrofit Performance Assessing Project Management

A spreadsheet model is used to estimate the performance of front-end boiler retrofits in eight multifamily buildings. A bin analysis is used to estimate the fraction of annual space heating load that occurs above a given outside air temperature. Front-end boiler performance is then modeled by considering the boiler as an internal gain that provides the estimated space heating fraction above each outside air, or balance point, temperature. Below the given balance point temperature, space heat is assumed to be provided by both the front-end boiler and the existing boiler in the building. The life-cycle cost and simple payback are calculated for each bin, or boiler size, to find the optimum boiler size. The model includes boiler temperature reset and cutout controls. Three separate domestic hot water (DHW) systems are modeled: 1) existing boiler heating DHW year round, 2) existing boiler heating DHW during heating season only, and 3) a separate DHW system.

Input parameters for the model consist of the input rate, steady state efficiency, and jacket and stack loss rates of the existing boiler; the boiler operating temperature; the existing DHW system efficiency and loss rate; and the efficiency of the front-end boiler. Building input parameters consist of the annual space heat and DHW energy uses. Boiler operation parameters include the system cutout temperature, the reset setpoint, the reset ratio, and the high temperature limit. Economic parameters include fuel cost, the fixed and incremental boiler cost, the investment term, and the annual discount rate.
The eight multifamily buildings selected ranged in size from 31 to 110 apartment units. The selected buildings were all high space heat energy users, and, except for one, the domestic hot water was heated by the space heat boiler. A parametric analysis was applied to four of the eight buildings analyzed. Long and short payback cases yielded average values of 9.3 and 2.0 years, respectively. The average value for all cases using a best estimate for all parameters was 3.3 years. For these cases the optimum size front-end boiler provided from 50 to 75 percent of the space heat demand. Shortest payback periods were for those buildings that had the largest boilers relative to the building heating demand. The average pre-retrofit efficiencies for the three DHW system options examined were nearly equal.

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Determining Applicability of Front-End Modular Boilers, Modeling Retrofit Performance Assessing Project Management