Refrigerator Test Procedure 

Adjustments to Second Part of Test 

12/9/11 

Background: 

The Samsung waiver to adopt a test procedure for multiple defrost cycle types, and the related 

discussion, have led to a reevaluation of the appropriate test period for the second part of a test 

for variable or long-time defrost products. See 76 FR 16760 (March 25, 2011) (request for 

comment and grant of interim test procedure waiver). This part of the test is used to determine 

the energy use associated with defrost, and all events associated with defrost, including 

precooling and temperature recovery. 

The current (Appendix A1) test period starts when the compressor stops, prior to or coincident 

with the defrost heater being energized, and finishes at the second turn-on of the compressor 

after defrost. 

The interim final Appendix A test period refines this approach to require that the test period start 

prior to any precooling cycles. It adopts temperature-based requirements to confirm that the start 

has been selected to precede precooling, and that the stop has been selected to follow all 

temperature recovery. However, the procedure still ends the test period for the second part of the 

test when the compressor starts. 

Whirlpool commented on the Samsung waiver, suggesting that the test period should stop when 

the compressor stops, rather than when it starts. This suggestion was first made in the test 

procedure NOPR public meeting presentation on June 22 (see page 53 of the presentation). Both 

the Whirlpool comment and the NOPR meeting presentation highlight the unusual result that 

energy use contribution associated with defrost sometimes is negative when determined using the 

Appendix A1 test procedure. This result occurs for some defrosts of an evaporator (i.e. a heat 

exchanger that cools the refrigerator’s air with refrigerant passing through its tubes) serving the 

fresh food compartment of specific refrigerator models. The Whirlpool comments indicate that 

the Appendix A1 test procedure generally underestimates the energy use associated with defrost. 

This memo presents a theoretical discussion demonstrating the merits of adopting Whirlpool’s 

suggestion to modify the test period of the second part of the test so that it ends when the 

compressor stops. 

This issue applies only to products with cycling compressors and long-time or variable defrost. 

Other products are not affected. For example, products with conventional automatic defrost are 

tested using a single test period that captures all product energy use, including the energy used 

for defrost, thus specifications for the second part of the test do not apply. As another example, 

products with non-cycling compressors and long-time or variable defrost are not tested using test 

periods defined by compressor cycles. 

The discussion presented in this memorandum assumes that the test procedure for conventional 

automatic defrost as specified in Appendices A1 and A, section 4.2 accurately measures 

refrigerator energy use, properly capturing the energy use associated with defrost. This procedure 
2

for automatic defrost products has been in effect as part of the DOE test procedures since at least 

1982 and its accuracy has not been questioned. 

Potential questions about the accuracy of the two-part test primarily (Appendix A1 or A, section 

4.2.1) are associated with whether the second part of the test accurately captures the contribution 

of defrost to overall energy use. The concept of the two-part test is that the first part of the test 

measures energy use during steady state operation not including defrost events—this is the 

energy use to maintain particular temperature levels in the compartments, i.e. the average 

temperature levels measured for the first part of the test. Because the compartment temperatures 

fluctuate in products with cycling compressors, increasing when the compressor is not operating 

and decreasing when the compressor is operating, measured temperatures for the first part of the 

test are averaged over a whole number of compressor cycles. 

The defrost cycle, including all events associated with it, represent a departure from steady state 

operation that is generally accepted to increase average energy use (i.e. energy consumption per 

elapsed time). This is because the defrost heater itself expends energy and because the 

refrigeration system must remove from the compartment(s) the heat added by the defrost heater. 

Compartment temperatures generally rise during defrost, hence the thermal load (i.e. heat that 

transfers into the refrigerator from outside) should decrease during the defrost cycle. However, 

the reduction in energy use associated with this thermal load reduction is small, if not negligible, 

when compared with the added energy use of the defrost heater and the refrigeration system 

operation to remove the associated heat addition. 

The energy use contribution associated with defrost is determined from the test measurements by 

subtracting from the energy use of the second part of the test the energy use that would have 

been expended during the same period of time if the product had instead continued to operate in 

steady state mode as it does during the first part of the test. The energy use equation (e.g. 

Appendix A1 section 5.1.2.3) adjusts this energy use per defrost based on defrost frequency per 

day. Conceptually, to assure that energy use associated with defrost is not “lost from” (or “added 

to”) the measurements of the second part of the test, the second part of the test should start and 

end with the product operating in its steady-state mode with the compressor cycling as it does in 

the first part of the test. Further, the start and end should ideally be at times when the product is 

in the same time during a compressor cycle: just as achieving proper temperature and energy use 

averages for the first part of the test requires that the test period for the first part of the test 

includes a whole number of compressor cycles, the second part of the test should represent a 

period of time for which, had there been no defrost cycle, there would have been a whole number 

of compressor cycles. This means that the test period should start and end at the same time 

during a compressor cycle, as suggested by Whirlpool. 

The discussion that follows is presented first for products that operate such that there is a 

complete regular compressor cycle prior to the defrost cycle, since this scenario more readily fits 

the presented theoretical analysis. The memorandum ends with a discussion of how the analysis 

would be affected by situations in which there is a truncated compressor cycle prior to the defrost 

cycle. This latter scenario generally occurs for timer-based defrost initiation. 
3

Operation in Which There is a Full Regular Compressor Cycle Before Defrost 

This section shows that, for those cases where the compressor “on” cycle prior to the defrost is 

not truncated (e.g., through the use of timer-based defrost initiation), the test period should end 

when the compressor stops in order to more accurately capture the energy consumption of 

refrigeration products with cycling compressors and long-time or variable defrost. 

When testing products with long-time or variable defrost, the test is split into steady-state and 

defrost portions (first part and second part) to reduce test burden. For long-time automatic 

defrost, the energy equation (section 5.2.1.2 of both Appendices A1 and A) is, 

ܧܶ ൌ 1440 ൈ

ܲ1ܧ

ܶ1 ൅ ൬ܧܲ2 െ ܧܲ1 ൈ

ܶ2

ܶ1൰ ൬12

൰ܶܥ

where ET is the total energy in kWh/day; EP1 and EP2 are the energies of the first and second 

parts of the test, respectively (in kWh); T1 and T2 are the lengths of time of the two parts (in 

minutes); and CT is the compressor run time between defrosts (in hours). The first term on the 

right side of the equation represents the energy use during steady operation without defrost. The 

second term represents the energy use associated with defrost. 

The energy equation for variable defrost, in section 5.2.1.3, is identical to the above equation 

except that in that case CT is a representative compressor run time between defrost cycles, 

calculated as prescribed in the test procedure, as a weighted average of the minimum and 

maximum values that the control algorithm can select for it. 

In this analysis, the test period provided in Appendix A1, section 4.2 is assumed to result in an 

accurate measure of the average energy use for a time period from one defrost to a second: 

4.2 Automatic Defrost. If the model being tested has an automatic defrost system, the 

test time period shall start after steady-state conditions have been achieved and be from 

one point during a defrost period to the same point during the next defrost period. 

Figure 1 shows a representative power input and freezer temperature history of a generic variable 

defrost refrigerator. 
4

Figure 1: Representative Power and Temperature Histories for a Generic Cycling 

Compression Scenario 

The section 4.2 test period is shown as Tfull. The two test periods associated with long-time or 

variable defrost products, T1 and T2, are labeled so that they correspond to the allocation of time 

that Whirlpool suggested should apply. Specifically, the end of the T2 period is shown as 

occurring at the end of the compressor “on” cycle after the recovery compressor cycle, rather 

than at beginning of that “on” cycle (as is shown in Figure 1 of Appendix A1 or A). 

For convenience, T1 is set to be the entire steady-state period rather than a short portion of that 

period. Note that T2, the test period of the second part (defrost portion) of the test, is divided 

into separate periods TpcB (precooling B) and TdrB (defrost/recovery B). The energy use 

associated with each of these periods is represented by the terms EPfull, EP1, EP2, EPpcB, and 

EPdrB; for example, EPfull is the energy use expended during time period Tfull, 

Note that the figure shows two separate time periods for defrost and recovery, TdrA and TdrB. 

While products may not always operate in a repeatable fashion, the fact that a laboratory test that 

measures only one (or two) defrost cycles is considered to be representative of field use 

presupposes a reasonable degree of repeatability in product operation. Therefore, TdrB should be 

considered to be equal to TdrA. Similarly, the energy use measured during these time periods 

should also be considered equal, EPdrB equal to EPdrA. DEFROST A RECOVERY

PRECOOL

POWER TEMPERATURE

DEFROST B

RECOVERY

Tfull

TdrA TdrB T1

(steady)

TpcB

T2

Same Temperature Before and After T2
5

Considering the Tfull time period, the total energy use for the period starting when the defrost 

heater is energized for the first time (i.e., for defrost A) and ending the second time it is 

energized (for defrost B) is: 

஻௖௣ܲܧ ൅ ܲ1ܧ	 ൅ ஺௥ௗܲܧ 	 ൌ௟௟ ௨௙ܲܧ	

Alternatively, the total energy use determined by the two-part method is: 

஻௖௣ܲܧ	൅ ܲ1ܧ ൅ ஻௥ௗܲܧ ൌ ܲ2ܧ	 ൅ ܲ1ܧ	

Because EPdrB and EPdrA are equal, EPfull is expected to be exactly equal to EP1 + EP2, when T1 

is defined as indicated in the figure to include the entire steady-state period. 

The following discussion will show that the two-part energy equation (section 5.2.1.3 of 

Appendix A1 or A) calculates energy use in a manner consistent with the single-test-period 

equation of section 4.2 only if the test period of the second part of the test ends at the end of a 

compressor “on” cycle. The meaning of the “12” in the two-part energy equation is that 

compressor run time is half of the clock time from one defrost initiation to the next. In other 

words, the equation defines CT to be equal to half of Tfull/60. 

The daily energy use calculated based on EPfull is determined by multiplying average energy use 

per hour for the test period by 24 hours. 

 ܧܶ ൌ 	24 ൬ ா௉೑ೠ೗೗

்೑ೠ೗೗/଺଴൰ ൌ ቀ ଶସ

ଶ஼்ቁ ܧܲ௙௨௟௟ ൌ	ቀ ଶସଶ஼்ቁ ሺܧܲ1 ൅ ܧܲ2ሻ

 ൌ ቀ ଶସ

ଶ஼்ቁ ሾܧܲ1 	൅ ሺܧܲ2 െ ܧܲ1 ൈ ܶ2/ܶ1ሻ ൅ 	ܧܲ1 ൈ ܶ2/ܶ1ሿ

 ൌ ቀ ଶସ

ଶ஼்ቁ ሾሺ1 ൅ ܶ2/ܶ1ሻܧܲ1 	 ൅ ሺܧܲ2 െ ܧܲ1 ൈ ܶ2/ܶ1ሻሿ

 ൌ 24 ቀ்ଵା்ଶ

ଶ஼்	 ቁ ܧܲ1/ܶ1 ൅ ቀଵଶ஼்ቁ ሺܧܲ2 െ ܧܲ1 ൈ ܶ2/ܶ1ሻ

Because TdrA = TdrB and CT = 0.5 × Tfull/60, it follows that 

 2ܥܶ ൌ ்೑ೠ೗೗

଺଴ ൌ ቀ்೏ೝಲା்ଵା்೛೎ಳ଺଴ ቁ 

 ൌ ቀ்೏ೝಳା்ଵା்೛೎ಳ

଺଴ ቁൌቀ்ଵା்ଶ

଺଴ ቁ

Plugging this into the first term of the right side of the above equation for ET gives 
6

 ܧܶ ൌ 24 ቀ ்ଵା்ଶ

ሺ்ଵା்ଶሻ/଺଴ቁ ܧܲ1/ܶ1 ൅ ቀଵଶ

஼்ቁ ሺܧܲ2 െ ܧܲ1 ൈ ܶ2/ܶ1ሻ

 

 ൌ 1440 ൈ ܧܲ1/ܶ1 ൅ ቀܧܲ2 െ ܧܲ1 ൈ ்ଶ

்ଵቁ ቀଵଶ஼்ቁ

Hence, for the time periods as defined in Figure 1, ET calculated using the full-period calculation 

is identically equal to ET calculated using the two-part calculation. 

In contrast, suppose that T2 ends at the start of the compressor “on” cycle that follows the 

recovery compressor cycle, as in Figure 1 of the the current Appendix A, instead of at the end of 

that “on” cycle. In that scenario, TdrB is now unequal to TdrA, and, significantly, the energy 

consumption during TdrB is now less than that of TdrA by the amount of a full compressor “on” 

cycle. Since TdrB ≠ TdrA, we no longer have 2CT = (T1 + T2)/60 and the section 5.2.1.3 equation 

for ET is no longer valid. Defining both TdrA and TdrB to end at the second compressor start after 

the defrost, and defining T1 to start at the end of TdrA would resolve this inequality. However, in 

this case, T1 does not include a whole number of compressor cycles, as required by the test 

procedure. This situation illustrates that establishing equality of the full-cycle energy use 

calculation and the two-part energy use calculation requires that T2 end at the end of a 

compressor “on” cycle. 

The temperature history may be further examined to support ending the second part of the test 

when the compressor stops. The defrost part of the test is intended to capture the energy use 

associated with temperature recovery from defrost. However, at the time of the second 

compressor start after defrost B, the temperature has recovered only to the maximum level it 

normally reaches during the steady state portion of the test (i.e. the upper horizontal line in the 

temperature plot of Figure 1) – but the test period starts when the compartment temperature is at 

the minimum level normally reached during steady state (i.e. the lower horizontal line in the 

temperature plot). The test, as currently written, would be missing both the energy use and time 

associated with additional recovery of the compartment temperature to its level at the start of the 

second part of the test. By extending the test through this additional compressor “on” cycle, the 

product is brought back to the thermal state it was in at the start of the second part of the test, 

thus assuring full temperature recovery. 

Operation in Which There is Not a Full Regular Compressor Cycle Before Defrost 

In some circumstances the compressor “on” cycle before the defrost cycle may be truncated. 

This may occur in products with timer-based long-time defrost and in tests involving initiation of 

instant-defrost. Instant-defrost testing is used by manufacturers in which they can force a defrost 

to occur, rather than waiting for the variable defrost algorithm to initiate the next defrost. This 

approach is used to reduce test time. 

In the extreme case where the compressor stops immediately after it started prior to defrost, the 

steady portion of the test between defrosts could be selected to include a whole number of 

complete compressor cycles from a compressor start to a compressor start. The second part of the 

test could be selected under Appendix A1 section 4.2.1 at the instant the compressor turns on and 

back off and the defrost heater turns on (with all three events considered for the purposes of the 

discussion to be occurring within the same instant). In this case, the logic of the previous 
7

discussion is reversed: in order to avoid improper inclusion of the energy use and time 

associated with a compressor “on” cycle, the end time for the second part of the test would be 

selected to occur at the start rather than at the end of a compressor “on” cycle. This approach 

also holds true when considering the temperature-based view of the test period: the compartment 

temperature would have been at its steady-state maximum level at the start of the second part of 

the test, since the compressor was just restarting. Hence, temperature recovery would involve 

returning to this steady-state maximum, which is coincident with a compressor start, for 

example, the second compressor start after defrost. 

In practice, the timer-based defrost control would stop the compressor at a random time during 

the compressor “on” cycle. Hence, achieving strict equality of Efull and E2part would require 

using an approach that measures the energy consumption of the tested unit by ending the second 

part of the test at a time during a regular compressor “on” cycle after defrost when the 

compartment temperature matches its level at the start of the test, which would be at some point 

between the steady-state maximum and minimum temperatures. Extending the test period 

through to the end of a compressor “on” cycle would add to the test the energy use and time 

associated with the additional compressor operation to bring the compartment temperature to its 

steady-state minimum. 

However, by modifying Appendix A’s test period of the second part of the test so that it starts at 

the “termination of the last regular compressor “on” cycle,” it causes a truncated compressor 

“on” cycle to be considered part of the defrost cycle. Accordingly, by modifying the procedure 

in this manner, the procedure addresses any issues associated with the random nature of timerbased initiation of defrost when using the two-part test.