National Oilheat Research Alliance 39 The heating capacity of an indirect heater depends on the size of the tank, the size of the heat exchanger, boiler water temperature supplied to the heat exchanger, and the flow rate. The efficiency of indirect water heating is directly related to the efficiency of the boiler. Sizing Water Heaters Unlike space heating, the need for hot water will vary widely between houses and families, and oversizing may not have a significant efficiency penalty. Most manufacturers provide recommendations regarding water heater sizing and installation practices. For example, see: http://accusize.bockwaterheaters.com/IntelliSizing/ NewProject.aspx This web page is shown as Figure 8. If manufacturer supplied information is not available, NORA reccomends using the AHRI first-hour sizing method. In addition to the size of the heater in gallons, there are a number of considerations which must be taken into account when determining which water heaters best fit the needs of each individual application. For example, on a spec sheet, there are a number of capacities listed that must be considered before a model is selected. First hour rating is the amount of hot water that a fully heated water heater can deliver in the first hour. This is normally shown as gallons per hour at a 100°F temprerature rise, unless stated otherwise. Figure 6 Figure 7 Figure 8
NORA Advanced Hydronics
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