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Federal Register / Vol. 86, No. 110 / Thursday, June 10, 2021 / Proposed Rules
analysis because DOE did not have the requisite inputs to develop its LCC
model with a degree of certainty that would meet the statutes clear and convincing evidentiary threshold. DOE
likewise did not expend resources to generate the cost-efficiency curve, as it is unnecessary without an LCC model to feed into.
C. Energy Use Analysis As discussed, UFHWSTs store hot water and do not directly consume fuel or electricity for the purpose of heating water, so any potential amendments to the standard would reduce standby loss of heat from the stored water. Further, DOE currently only prescribes a minimum insulation requirement as opposed to a minimum efficiency requirement for UFHWSTs.
Accordingly, the energy use analysis determines the annual energy consumption of paired water heaters and boilers due to standby loss of the UFHWSTs and assesses the energy savings potential of increasing the stringency of the required insulation for UFHWSTs.
1. Tank Thermal Loss Model
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For this determination, DOE adapted the thermal loss model described in the technical support document TSD for the commercial water heating energy conservation standards ECS NOPR
published in the Federal Register on May 31, 2016 81 FR 34440; May 2016
CWH ECS NOPR, with some modifications to how the tank surface areas are defined.7 These modifications were introduced to capture equipment performance that results from differences in surface insulation thickness over different areas of tank i.e., insulation around fittings and access ports. These differences are described in section IV.C.1.a of this document.

Where:
Qhr, j = The hourly heat loss for the UFHWST
for each efficiency level EL j Btu/hr.
i = The surface area of the cylindrical tank is divided into different zones each indexed i.
Ai, j = The area of each zone i at each EL jft2.
Ti = The constant internal water temperature for each tank zone i F.
7 Available at: https www.regulations.gov/
document?D=EERE-2014-BT-STD-0042-0016, section 5.5.3 Last accessed: April 8, 2020.

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Tamb,z = The ambient air temperature for each climate zone z F.
Ri, j = The net R-value of the insulation for each zone i at each EL j Flft2hr/Btu.

a. Tank Surface Area Ai, j As discussed in section IV.B.2 of this document, DOE used a conservative assumption in its energy use analysis that water temperature would remain uniformly at 140 F and did not consider stratification of water temperatures inside the tank. Therefore, although tanks can be installed horizontally or vertically, there is no difference in thermal losses between these configurations, and DOE only used vertical tanks in its analysis. The UFHWSTs total external surface area was divided into separate zones, where i is the index for each zone. Zones represent the different areas of an UFHWST that would have unique insulative values. These zones are described in more detail in in section IV.B of this document.
ATankTop = When the UFHWST is oriented vertically, this represents the tanks top surface.
AFittings = Is the sum of all uninsulated areas of the tanks surface devoted to fittings.
AFittingInsulation = Is the sum of all insulated areas of the tanks surfacesurrounding the uninsulated fittings.
AAccessPort = Is the sum of all insulated areas of the tanks surface devoted to the tanks cleanout hand hole port or manhole.
ATankWall = When the UFHWST is oriented vertically, this represents the tanks walls.
ATankBottom = When the UFHWST is oriented vertically, this represents the tanks bottom surface.

b. Tank Internal Water Temperature Ti For this analysis, DOE assumed that the water inside the UFHWSTs is at a constant uniform temperature of 140 F, which is the average water temperature required by the current Federal test procedures for storage-type CWH
equipment during standby loss testing.
See generally 10 CFR 431.106; 10 CFR
part 431, subpart G, appendix A, section 6; 10 CFR part 431, subpart G, Appendix B, section 5. Because UFHWSTs serve the same function as storage-type CWH
equipment in standby mode, DOE
expects that similar conditions would be appropriate for UFHWSTs as for storage-type CWH equipment in standby mode. DOE used a conservative assumption that internal water temperatures would remain indefinitely at 140 F. In reality, the rate of heat loss from a UFHWST would decrease slowly as the temperature difference between the internal stored water and the ambient air decreased. However, because this effect would be minimal,
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DOE did not consider stratification of water temperatures inside the tank and assumed that a tank would always be full of hot water. Therefore, DOE held the temperature T constant across all tank zones i.
DOE requests comment on the appropriateness of its assumption regarding the use of a constant internal water temperature of 140 F.
c. Tank Ambient Temperature Tamb, z Based on feedback from manufacturers during interviews conducted under NDA, DOE assumed that 90 percent of UFHWSTs would be installed indoors and that the remaining 10 percent would be installed outdoors.
DOE assumed that all tanks that are installed indoors would have a constant ambient temperature of 75 F, which is the average air temperature required by the current Federal test procedure for storage-type CWH equipment during standby loss testing. See generally 10
CFR 431.106; 10 CFR part 431, subpart G, appendix A, section 6; 10 CFR part 431, subpart G, Appendix B, section 5.
For the fraction of UFHWSTs that are installed in outdoor, or nonconditioned, spaces, DOE defined each climate zone z and calculated the monthly average temperatures from Typical Meteorological Year 3 TMY3 8
data for the Building America climate regions 1A, 2A, and 2B.9 10 The temperatures for each region are represented by the cities in Table IV.4.
The monthly regional averages were then weighted using the regional city populations based on data from 2018
Census.11

8 The TMY data sets hold hourly values of solar radiation and meteorological elements for a 1-year period. Their intended use is for computer simulations of solar energy conversion systems and building systems to facilitate performance comparisons of different system types, configurations, and locations in the United States and its territories. Because they represent typical rather than extreme conditions, they are not suited for designing systems to meet the worst-case conditions occurring at a location.
9 Wilcox, S. and W. Marion, 2008 Users Manual for TMY3 Data Sets, NREL/TP58143156 April 2008 Available at: https www.nrel.gov/docs/
fy08osti/43156.pdf.
10 Building America Best Practices Series, Volume 7.3, Guide to determining climate regions by county 2015 Available at: https
www.energy.gov/sites/prod/files/2015/10/f27/ba_
climate_region_guide_7.3.pdf.
11 U.S. Census Population Estimates by County, as of 2018 Available at: https www.census.gov/data/
tables/time-series/demo/popest/2010s-countiestotal.htmlpar_textimage.

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