Assuming that the maximum demand is sustained throughout the day, the difference between the maximum demand and recovery rate will be equal to the change in available hot water each hour. Time-based evaluation describes the process of calculating the remaining available hot water at the end of each hour during a typical day. If these constraints cannot be eliminated or they are deemed too costly to eliminate, time-based evaluation can be used to ensure the system’s needs are still being met. Restrictions could include insufficient space, lack of the desired fuel source, budget limitations or even preference from facility maintenance personnel. ![]() However, the engineer may encounter different restrictions requiring a water heater with a smaller tank size or lower recovery rate. The heater should also be selected at a recovery rate just above the maximum demand. If possible, a storage tank size should be selected just larger than the suggested storage capacity. Once the maximum demand and suggested storage capacity have both been identified, the water heater can be selected. Suggested storage capacity = maximum demand x storage capacity factorĬourtesy: RMF Engineering Time-based evaluation for water heaters Maximum demand = possible demand x demand factor Possible demand = sum of fixture subtotals Table 1: For this table, use the following formulas:įixture subtotal = fixture demand x number of fixtures These decisions and any assumptions should be documented in writing for future reference. It may be necessary to request feedback from the architect, owner or users to confirm which building type is most appropriate for each plumbing fixture. The fixture demand selected should always be the one corresponding to the building type that most closely matches the actual use of the plumbing fixture. Table 10 categorizes fixture demand by building type, often resulting in difficulty determining which values are appropriate for more unique buildings. Therefore, the values can be very conservative for most new designs. It is important to realize that these values were informed by various studies between the 1930s and 1960s before the widespread installation of modern, low-flow plumbing fixtures. ![]() The majority of the table presents empirical data on hot water demand, in gallons per hour, for common fixture types during a peak hour. The 2019 ASHRAE Handbook - HVAC Applications provides the relevant parameters for sizing by fixture count in Table 10 of Chapter 51. However, because of the high variability in occupancy and usage, sizing based on fixture count is more common in most commercial buildings. These profiles can be used to select both a recovery rate and corresponding storage tank size based on a per person, per apartment unit or per meal basis. Figures 16 – 23 in Chapter 51 of the 2019 ASHRAE Handbook - HVAC Applications illustrate suggested capacity profiles for office buildings, apartment buildings and restaurants, among others. The per capita approach is often used for smaller, light commercial facilities. Regardless of fuel, storage-type water heaters are typically sized by one of two methods: water demand per capita or water demand per fixture.
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