Weekend/weekday multipliers for daily hot water use were derived from data collected in the 1200 house Aquacraft study (AWWA 1999). In addition, three vacation periods with no hot water use were designated: May 26–28, August 12–18, and December 22–25. The multipliers that adjust for these effects in single-family homes or multi-family homes with in-unit clothes washers are summarized in Table 11. This is an optional level of detail for DHW analysis, and is not required if the simulation tool being used by the analyst does not allow variable daily hot water use.
Table 11. Hot Water Use Multipliers for Specific Day Types
Weekend Weekday Vacation (May 26–28, August 12–18, December 22–25) Not Vacation
Certain advanced hot water measures may require the use of detailed hot water events with realistic frequency, flow rates, durations, fixture assignment, and clustering. Such measures include solar hot water systems with demand-side heat exchangers, tankless water heaters, distribution system improvements, and recirculation loops. NREL has developed an interactive spreadsheet tool (DHW Event Generation Tool) that generates an annual set of event schedules automatically based on a series of user inputs, such as TMY3 location and number of bedrooms. This tool will soon be available for download from the BA performance analysis Web site, along with standard Benchmark event schedules for two-, three-, and four-bedroom houses (http://www1.eere.energy.gov/buildings/building_america/perf_analysis.html). The events generated by the spreadsheet are consistent with the average daily volumes calculated based on Table 10 and Table 11. Additional characteristics of the Benchmark hot water events for a three- bedroom house are summarized in Table 12.
T T ∆Tamb,max
= = =
0.986 day# offset ratio lag
= = = = =
The mains water temperature for a typical house varies significantly depending on the location and time of year. Equation 5, based on TMY3 data for the location of the Prototype, shall be used to determine the daily mains water temperature for both the Benchmark and the Prototype:
mains (supply) temperature to DHW tank (°F) annual average ambient air temperature (°F) maximum difference between monthly average ambient
/ 2) × sin (0.986 × (day#
Julian day of the year (1–365)
6°F 0.4 + 0.01 (T 35 – 1.0 (T – 44) – 44). amb,avg amb,avg
offset) + ratio × (∆Tamb,max
temperatures (e.g., Tamb,avg,july degrees/day (360/365)
15 - lag) - 90)