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Federal Register / Vol. 86, No. 110 / Thursday, June 10, 2021 / Proposed Rules
options that have been identified in the technology assessment. DOE may also rely on a combination of these two approaches. In this rulemaking, DOE is adopting a design-option approach because there are very few models of UFHWSTs currently on the market that are marketed with higher insulation levels than the current baseline requirement of R12.5.
Based on its review of publiclyavailable equipment information and feedback from manufacturers, DOE had tentatively determined that 2 inches of polyurethane foam insulation is needed to meet the current insulation requirement, and DOE, therefore, considered this insulation thickness as the baseline. As discussed in section IV.A.3 of this document, increased polyurethane foam insulation thickness was the only technology option that was not screened-out for this analysis, and thus, DOE considered more-stringent efficiency levels i.e., increased R-value based on varying levels of increased polyurethane foam thickness.
In response to the August 2019 RFI, AHRI commented that there is a diminishing return from increasing insulation thickness due to the increasing heat transfer rate and surface area as the insulation thickness increases. AHRI, No. 6 at pp. 12 This comment was supported by individual manufacturers during interviews with DOE. Manufacturers stated that surface tension decreases as the foam thickness increases, which results in the foam becoming less stable. To counter this, less blowing agent is used and the foam becomes denser, thereby reducing the added insulating benefit per inch of applied insulation at thicknesses above 3 inches if foam is applied by being poured into a form, which is the typical application method for polyurethane foam on jacketed UFHWSTs.
Manufacturers stated that due to the changing foam density as the insulation thickness increases, the R-value per inch is expected to diminish as insulation thickness is increased, especially as thickness increases beyond 3 inches. As a result, when more than 3 inches of insulation thickness is applied, it is unclear how much additional R-value could be achieved by continuing to increase the thickness of the foam of jacketed UFHWSTs.
Unjacketed tanks, which are intended for outdoor installation and may not have the same space constraints as indoor units, do not have an outer metal jacket enclosing and protecting the foam. As a result, unjacketed tanks can be spray-foamed in layers, which reduces the compression of the foam and mitigates the potential for changes
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in foam density at thicknesses above 3
inches. However, all UFHWSTs were considered in a single equipment class as discussed in section IV.A.1 of this document, so the max-tech level for jacketed UFHWSTs was applied for all UFHWSTs in this analysis.
Furthermore, feedback from manufacturers and DOEs previous knowledge of the UFHWST market indicated that at least 90 percent of UFHWSTs are jacketed and intended for indoor installation.
Therefore, DOE expects uncertainty related to the effective R-value of insulation for insulation thicknesses above 3 inches. Because thicknesses above 3 inches are not typically used on jacketed UFHWSTs, the improvement in R-value as insulation thickness increases beyond 3 inches for jacketed tanks is unclear at this time. Therefore, due to the high level of uncertainty regarding the R-value of foam insulation with thickness greater than 3 inches, DOE has limited its analysis to considering only up to 1 additional inch of insulation thickness above the baseline insulation level of 2 inches, so 3 inches of foam insulation was considered the max-tech efficiency level for UFHWSTs in this analysis.
DOE requests data and information related to achievable R-values of polyurethane foam insulation on jacketed UFHWSTs at thicknesses above 3 inches. DOE also seeks comment on its understanding of the difficulties associated with applying more than 3
inches of foam to jacketed UFHWSTs.
DOE also included one intermediate level of added insulation in its analysis, with 0.5 inch of added insulation above the 2-inch baseline that results in R
12.5. DOE has assumed for its analysis that polyurethane foam has an R-value per inch of 6.25 up to a maximum thickness of 3 inches. The selected ELs used in the analyses for this NOPD are shown in Table IV.1.

TABLE IV.1EFFICIENCY LEVELS FOR
REPRESENTATIVE UFHWSTS BASED
ON INCREASED INSULATION
Efficiency levels Baseline EL0.
EL1
EL2

Insulation thickness polyurethane foam
R-value of insulation
2 inches

R12.5.

2.5 inches
3 inches

R15.625.
R18.75.

DOE seeks comment on the considered efficiency levels analyzed for UFHWSTs. Additionally, DOE seeks comment on its assumption that polyurethane foam has an R-value per
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inch of 6.25, up to a maximum thickness of 3 inches.
2. Representative Equipment for Analysis For the engineering analysis, DOE
analyzed the publicly-available details, including storage volumes and other critical features, of UFHWST models available on the market and conducted interviews with manufacturers under NDAs to determine appropriate representative equipment to analyze. In response to the August 2019 RFI, several commenters highlighted the customized and variable nature of the UFHWST
market. BWC, No. 5 at pp. 12; AHRI, No. 6 at p. 2; A.O. Smith, No. 8 at p.
1 BWC stated that it does not believe it is possible to have one representative volume of UFHWSTs or more in a reasonable quantity. BWC also commented that it would be difficult to have a representative application with associated R-value, ambient conditions, tank setpoint, and draw patterns for UFHWSTs and suggested that DOEs analysis should not be overly simplified if it is acknowledged that tank orientation can affect heat losses. BWC, No. 5 at pp. 23 A.O. Smith recommended that DOE conduct its analysis using various standard models, but the agency should keep in mind the customized nature of the UFHWST
market. A.O. Smith, No. 8 at p. 1
To account for the wide range of UFHWSTs on the market, DOE chose several representative baseline units for analysis. As discussed in section IV.C.1.c of this document, DOE also included several ambient temperature conditions in its energy use analysis to reflect typical installation locations i.e., indoors in mechanical rooms or outdoors in Very Hot and Hot regions. Although UFHWSTs can be installed horizontally or vertically, DOE
used a conservative assumption in its energy use analysis that water temperature would remain uniformly at 140 F as discussed in section IV.C.1.b of this document, 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 determined that installation orientation would not have a significant impact on its energy use analysis results, so the Department calculated estimated standby losses based on all tanks being vertical, because vertical installations are the most common. The characteristics of these representative units are listed in Table IV.2.

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