Rulemaking for Alternative Efficiency Determination Methods and Test Procedures for Walk-In 

Coolers and Walk-In Freezers

Data Supporting Evaporator Fan and Electric Defrost Energy Calculations

April 24, 2014

In a Supplemental Notice of Proposed Rulemaking (SNOPR) covering alternative efficiency determination 

methods and test procedures for walk-in coolers and walk-in freezers (WICFs or walk-ins), published 

February 20, 2014, the U.S. Department of Energy (DOE) proposed nominal values to be used when 

rating a condensing unit separately under DOE’s test procedure for WICFs, AHRI 1250-2009, “Standard 

for Performance Rating of Walk-in Coolers and Freezers” (AHRI 1250-2009). These nominal values 

represented the evaporator fan and electric defrost energy contributions to a refrigeration system 

containing that condensing unit. See 79 FR 9818 at 9831. This memo discusses data DOE used to 

support the determination of nominal values for DOE’s forthcoming final rule covering test procedures 

for WICFs.

Unit Cooler On-Cycle Fan Power Factor

DOE presented the data supporting its proposed nominal values for on-cycle fan power in Table III.4 of 

the SNOPR. 79 FR at 9830. However, these data did not cover the full range of capacities that are used in 

WICFs, particularly larger enclosures. For the final rule, DOE surveyed data from manufacturer 

brochures covering capacities up to 250,000 Btu/h. Figure 1 and Figure 2 present the total fan power as 

a function of capacity for a range of systems available from several manufacturers. 

Figure 1: Manufacturer Brochure Data for Fan Power of Medium Temperature Unit Coolers
Figure 2: Manufacturer Brochure Data for Fan Power of Low Temperature Unit Coolers

The results suggest that a nominal value of 0.013 watts per Btu/h of capacity is appropriate for medium 

temperature systems and 0.016 watts per Btu/h of capacity is appropriate for low temperature systems. 

Unit Cooler Electric Defrost Energy Factor

DOE also presented the data supporting its proposed nominal values for electric defrost energy in Table 

III.4 of the SNOPR. 79 FR at 9830. As with the fan power data, these data did not cover the full range of 

capacities used in WICFs. For the final rule, DOE surveyed data from manufacturer brochures covering 

capacities up to 250,000 Btu/h. The available data were in the form of electrical power in watts. Figure 3

presents electric defrost wattage data as a function of capacity for a range of systems available from 

several manufacturers.
Figure 3: Manufacturer Brochure Data for Electric Defrost Power of Low Temperature Unit Coolers

The defrost energy per cycle can be found by multiplying the defrost wattage by the duration of the 

defrost cycle. DOE’s engineering model developed in support of its forthcoming final rule for energy 

conservation standards applied the capacity-dependent defrost durations shown in Table 3.

Table 1: Defrost Duration versus System Capacity

Capacity (Btu/h) Defrost Cycle Duration (hours)

>75,000* 1

50,001-75,000 0.75

0-50,000 0.5

*DOE assumed that defrost durations over 1 hour in length would not be used because they would cause 

excessive heating within the WICF.

Using the manufacturer data for defrost wattage in Figure 3 and the defrost durations in Table 3, DOE 

calculated the defrost energy per cycle for the same range of systems shown in Figure 3. Figure 4

presents the total defrost energy per cycle as a function of capacity. 
Figure 4: Electric Defrost Energy per Cycle for Low Temperature Unit Coolers

DOE observed that the relationship between capacity and defrost energy per cycle approximates a 

power series. Therefore, DOE is expressing the nominal value for electric defrost energy use as a power 

function instead of a linear equation. 

Equation: Electric defrost energy per cycle (Wh/cycle) = 8.5 × 10-3 × (Gross capacity)1.27