MEMORANDUM

To: Margaret Sheppard, U.S. EPA

CC: Bella Maranion, Melissa Fiffer, and Carolyn Hayek, U.S. EPA

From: Rebecca Ferenchiak, Jenny Tanphanich, Neha Mukhi, and Mark Wagner, ICF International 

Date: August 1, 2011

Re: Additional End-Use Modeling for Household Refrigerators and Freezers

(EPA Contract Number EP-W-10-031 Task Order 005, Task 01)

ICF has reviewed the public comment submitted by Honeywell (EPA-HQ-OAR-2009-0286-0170.1) in 

response to EPA’s proposed SNAP ruling which approves the use of hydrocarbon refrigerants in 

refrigerators ([EPA-HQ-OAR-2009-0286; FRL-9147-9] RIN 2060-AP54). In the comment, Honeywell 

specifically notes that in EPA’s risk screen for isobutane in household refrigerators and freezers, the 

flammability risk for isobutane is underestimated. To support this notion, Honeywell performed leak 

testing of an isobutane refrigerator and provided the test results in their comment. In this memorandum, 

ICF examines the impact of using more conservative assumptions in the flammability assessment for 

isobutane and R-441A to determine whether a flammability risk exists.

Please contact Mark Wagner at 202-862-1155 with any questions or comments.
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Background

In Comment EPA-HQ-OAR-2009-0286-0170.1(FRL-9147-9, RIN 2060-AP54), Honeywell states that 

EPA underestimated the flammability risks associated with the use of isobutane as a refrigerant in 

household refrigerators and freezers. Consequently, ICF, at the request of EPA, has revisited the end-use 

modeling performed for both substitutes proposed for use in household refrigeration, isobutane and R441A, and performed an additional flammability analysis and threshold analysis.

Discussion of End-Use Scenarios Modeled

ICF reviewed the assumptions made in the original risk screens for isobutane and R-441A (formerly 

known as HCR-188C1), dated May 22, 2009 and November 6, 2009 respectively, to determine whether 

they were sufficiently conservative to represent a reasonable worst-case scenario. The assumptions used 

in the original risk screens for worst-case scenario modeling are summarized in Table 1. 

Table 1. Worst-Case Scenario Model Assumptions in Risk Screen

Parameter Risk Screen Assumptions: 

Isobutane

Risk Screen Assumptions:

R-441A

Appliance Household Refrigerator Household Refrigerator

Room Household Kitchen Household Kitchen

Charge size (g) 50 40

Percentage of Charge Leaked 100% 100%

Length of release (minutes) 1 1

Room size (volume - m3) 18 18

Room ventilation (air changes 

per hour) 2.5 2.5

Horizontal stratification 95% in bottom 0.4 m (~16 in) of room 95% in bottom 0.4 m (~16 in) of room

Charge Size

The charge sizes originally modeled for isobutane and R-441A were 50g and 40g, respectively. These 

values were chosen for modeling based on statements in the SNAP submissions for these substitutes that 

indicated the intended charge sizes to be used in appliances. 

There is the potential to be more conservative in the setting of this parameter for additional flammability 

analysis, since EPA’s proposed rule allows a household refrigerator to have a charge size of up to 57g. 

The proposed rule also effectively requires that the charge size be consistent with the standard UL 250 

Supplement SA, which calls for a maximum leak amount of 50 grams for refrigerants with flammability 

limits and a heat of combustion greater than 19,000 kJ/kg, such as isobutane or R-441A. However, the 

test to determine leak amounts is not documented in the UL 250 Standard and could potentially be 

administered incorrectly. As a result, in the additional flammability analysis (described in the following 

section) ICF uses a leak amount of 57g as a more conservative assumption for purposes of additional 

flammability analysis.

Room Size

In the original risk screen analysis, a kitchen with a floor area of 7.4 m

2

and a ceiling height of 2.4 m was 

assumed (EPA, 1994). This is equivalent to a volume of 18 m

3

. Although very limited data are available 

regarding the distribution of kitchen volumes in the United States, an analysis by Murray (1997) which 

aggregated data from over 60 different projects reported the minimum kitchen volume encountered as 31 

m

3

.

1

Further, only one percent of houses sampled had a kitchen smaller than 53 m3. Therefore, ICF does 

 

1 Murray (1997) uses data from the Brookhaven National Laboratory PFT database. Data regarding kitchen volumes 
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not believe there is a need to be more conservative in the setting of this parameter for the purpose of the 

additional flammability analysis.

Room ventilation

The range of potential flow rates for a residential kitchen is believed to be 2.5 to 4.5 air changes per hour 

(ACH) (Sheldon 1989). The more conservative rate of 2.5 ACH was used in the analyses, and ICF does 

not believe there is a need to be more conservative in the setting of this parameter for the purpose of 

additional flammability analysis.

Horizontal Stratification

In the original risk screen, it was assumed that horizontal stratification will occur since both isobutane and 

R-441A are denser than air and as a result, settle in higher concentrations closer to the ground. In order to 

simulate the horizontal concentration gradient that will occur because of the weight differential between 

the refrigerant and air, ICF assumed that 95 percent of the leaked refrigerant mixes evenly into the bottom 

0.4 meters of the room, and the remainder of the refrigerant mixes evenly in the remaining volume 

(Kataoka 1999). 

There is the potential to be more conservative in the setting of this parameter for additional flammability 

analysis, since the release point for a refrigerant may vary with the type of appliance in use. To capture 

the scenario of a bottom-freezer refrigerator, which would likely have the lowest release point, this 

flammability analysis will assume that 95 percent of the leaked refrigerant mixes evenly into the bottom 

0.22 meters (~ 9 inches)2of the room, and the remainder of the refrigerant mixes evenly in the remaining 

volume.

Additional Flammability Analysis and Results

Table 2 details the assumptions that will be used for the additional analyses of flammability risks 

associated with the use of isobutane or R-441A in the household refrigeration end-use. 

Table 2. Assumptions for the Additional Worst-Case Scenario Flammability Analysis

Parameter New Assumption

Refrigeration Unit Household Refrigerator

Room Household Kitchen

Leak Amount (g) 57*

Length of release 

(minutes) 1

Room size (volume - m3) 18

Room ventilation (air 

changes per hour) 2.5

Horizontal stratification 95% in bottom 0.22 m (~9 in) of 

room*

*More conservative than the assumption used in the original risk screens.

Based on the re-analysis, assumptions are the same as those modeled in the original risk screen, with the 

exception of the charge size (increased to 57g) and the stratification level assumptions (95% of the 

 

were only available for the Los Angeles area and the kitchen volumes are for a “kitchen zone” which may include 

associated areas, such as utility rooms, dining rooms, living rooms, and family room. The inclusion of these other 

spaces in the “kitchen zone” volume is realistic, as the presence of adjoining rooms would increase the volume into 

which the refrigerant could leak.

2 This value is consistent with the location for a potential leak from the condenser-compressor department for a 

bottom freezer depicted in comment EPA-HQ-OAR-2009-0286-0170.1.
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refrigerant will settle into the bottom 0.22 m [~9 in.] of the room). The results of this secondary analysis 

for isobutane and R-441A compared to the results of the original analysis are shown in Table 3. 

Table 3. Maximum Concentrations in Lower Compartment of Kitchen

Proposed Substitute Maximim Concentration: 

Original Assumptionsa(ppm)

Maximum Concentration:

Conservative Assumptionsb(ppm)

Isobutanec 6,647 13,778

R-441Ad 6,236 16,153

a

See Table 1

b

See Table 2

c

Isobutane LFL is 18,000 ppm

d R-441A LFL is 16,800 ppm

Although the concentrations in the lower compartment of the kitchen increased for both isobutane and R441A after using more conservative charge size and stratification assumptions, the maximum 

concentrations remained below the LFL for both compounds. The maximum concentration for R-441A is 

within 95% of the LFL (16,800 ppm); however, it is unlikely that concentrations would reach these levels 

using either refrigerant in a real-life leak situation, due to the conservative nature of the assumptions. As 

noted previously, it is unlikely that a kitchen size would be 18 m3and that the entire 57 gram charge 

would leak.

Threshold Analysis and Results

A threshold analysis was also performed to assess the impact of differences in room volume and leak 

amount and to determine the needed parameter values for leaked concentrations to reach the LFL for 

isobutane and R-441A. One of the two assumptions in the revised flammability analysis (i.e., room size 

and leak amount) was changed while the other assumptions remained constant (see Table 2) to determine 

the value at which concentrations in the lower compartment of the kitchen would reach the LFL for 

isobutane and R-441A. The results of the threshold analysis are shown in Table 4.

Table 4. Room volumes and leak amounts necessary to reach LFL 

Refrigerant a,b Room Size (Volume) Leak Amount

Isobutane 18 m3 75 g

Isobutane 13.8 m3 57g

R-441A 18 m3 59 g

R-441A 17.3 m3 57g

a Lower Flammability Limit of isobutane is equal to 18,000 ppm.

b Lower Flammability Limit of R-441A is equal to 16,800 ppm..

Assuming a room volume of 18 m3, the charge sizes required for the concentration in the lower 

compartment of the kitchen to reach the LFL for isobutane and R-441A were 75 grams and 59 grams, 

respectively. Based on the proposed SNAP ruling, the charge sizes required to reach the LFL would not 

be allowed for household refrigerators. Further, although a charge size of 59 grams for R-441A is close to 

the allowable 57 gram charge size, the UL Standard 250 requires that if a charge size of 57 grams is used, 

engineering controls must be in place to ensure that only 50 grams (or approximately 88%) can leak from 

the system. 

If the charge size of 57 grams was held constant, kitchen volumes of 13.8 m3(487 ft3) and 17.3 m3(611 

ft3) would be necessary for isobutane and R-441A concentrations, respectively, in the lower compartment 

of the kitchen to reach the LFL. Both threshold kitchen volumes are lower than the original assumed 

kitchen volume of 18 m3. However, as noted previously, it is highly unlikely that a kitchen would exist at 
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such small volumes—only one percent of houses sampled in more than 60 different projects had a kitchen 

smaller than 53 m3(Murray 1997).

Additional Discussion

Depending on the mixing conditions, it is still possible that in certain locations at floor-level, or in 

restricted areas such as a space between a refrigerator and a wall, the concentrations of isobutane or R441A could reach their LFLs for a few minutes, posing a threat in the presence of a spark. However, the 

annual probability of an external leak occurring over the course of minutes and an ignition source being 

present simultaneously is approximately 5.0 x 10-7(i.e., 0.5 in one million) (A.D. Little 1991). 

To avoid the risk of fire and explosion, it is recommended that refrigerators containing isobutane or R441A not be installed in small, poorly ventilated spaces such as very small enclosed ‘galley’ kitchens3

or 

storage closets (especially as other equipment or appliances in the space would reduce the effective 

volume of the room). For full-sized kitchens and for kitchens with typical ventilation levels, the risk of 

fire and explosion is minimal.

 

3 Enclosed galley kitchens are kitchens that are not open to the rest of the dwelling, generally have two (or 

occasionally three) sides, and have a pass way or doorway entrance, but no door.
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References

A.D. Little 1991. Risk Assessment of Flammable Refrigerants for Use in Home Appliances. Arthur D. 

Little, Inc. for EPA, Climate Change Division. September 10, 1991. Docket item EPA-HQOAR-2009-0286-0023.

EPA 1994. Significant New Alternatives Policy Technical Background Document: Risk Screen on the 

Use of Substitutes for Class I Ozone-depleting Substances: Refrigeration and Air Conditioning. 

Stratospheric Protection Division. March, 1994.

EPA 2009. Risk Screen on Substitutes for CFC-12 in Household Refrigerators and Freezers: Isobutane. 

Stratospheric Protection Division. Prepared by ICF International. March 2009. 

Honeywell 2010. Honeywell International, Inc. Comment on EPA Proposed Rule. Protection of 

Stratospheric Ozone: Listing of Substitutes for Ozone-Depleting Substances-Hydrocarbon 

Refrigerants. EPA-HQ-OAR-2009-0286; FRL-9147-9. July, 2010.

Kataoka. 1999. “Allowable Charge Limit of Flammable Refrigerants and Ventilation Requirements.” 

Draft Proposal. O. Kataoka/Daikin/Japan, June, 1999.

Murray, D.M. 1997. “Residential House and Zone Volumes in the United States: Empirical and 

Estimated Parametric Distributions.” Risk Analysis. 17(4): 439-446. 

Sheldon, L.S., et al. 1989. "An Investigation of Infiltration and Indoor Air Quality." New York State 

Energy Research & Development Authority, Report 90-11.

UL. 2000. UL 250: Standard for Household Refrigerators and Freezers. Underwriters Laboratory.