DOE prototype Midrise Apartment Water heater size

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Table 2.2 of Enhancements to ASHRAE Standard 90.1 Prototype Building Models (PNNL 2014) describes the behavior, it represents 23 separate 50 gallon hot water heaters. So while both Hi-Rise and Mid-Rise are modeled as a single water heater, the Mid-Rise water unit is representing a non-centralized system, and as a result has a higher combined storage capacity.

This is also addressed in Appendix A.1.1 (Water Heater Type) and Table A.4. (Total Pipe Thermal Loss For Prototype Buildings) in the same document.