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Proposal for Dealing with Low Emissions Stack Systems that Will be Excluded 

from Testing Under the Stack Test Option.

If a fab chooses to use the stack test option, it will test stacks that handle exhausts from fab 

manufacturing equipment (tools) using F-gases. Stack testing must account for a minimum of 85% of 

annual emissions determined in multiple ways including options such as:

Use the EPA update of Tier 2a method and the most recent year of F-gas usage to calculate 

overall estimated F-gas emissions. Allocate F-gas usage to individual stack systems that are 

candidates for exclusion using the method described below and estimate emissions from those 

low stacks to demonstrate emissions are less than 15% of the total, or 

Compare the estimated F-gas emissions (using Tier 2a method) of the stack system not being 

tested to the actual total fab F-gas emissions from the prior year reported under Subpart I.

Proposal for Determining Gas Use Associated with “Low Emissions Stack Systems” That Are Not Being 

Tested

A stack system is any set of stacks on a common header. If there is an interest in excluding from testing 

one or more low F-gas emission stack systems, the fab will estimate emissions from each such stack 

system by identifying the number of F-gas using manufacturing tools tied into each stack system. F-gas 

usage for a given stack system will be allocated by multiplying the total fab usage of a particular F-gas 

by the ratio of the number of tools using that F-gas on the stack system to the total number of tools 

using the F-gas in the fab. This will be done for each F-gas to assign a portion of the F-gas use to the 

stack system that might be excluded from testing. 

If a stack is excluded from testing, its F-gas emissions will be determined using the updated Tier 2a 

factors in conjunction with POU abatement DRE factors. The F-gas use assigned to any untested stacks 

will be deducted from the total fab F-gas usage that will be used to determine the emissions factors for 

the stacks that are tested.

Proposal for Addressing Low Use Gases and Non-Detect Emissions Under the Stack Test Option.

Determining F-Gas Use For Low Use F-Gases (for use in determining emissions factors)

Gas usage is typically tracked using weigh scales or pressure transducers. If the usage of an F-gas is low, 

relative to the size of the supply cylinder or cylinder systems and it is not possible to determine usage 

during the test period with confidence from cylinder usage tracking, a number of approaches can be used

including:

- Use a longer period of time (> than duration of stack test) to determine chemical usage. A 

period representative of the operation at the time of the test, and including the test period, 

will be used to estimate usage during the test period. This determination will be fully 

documented in the record of the determination of emissions factors; or
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- If a fab has the ability and chooses to, the fab may track F-gas usage using a flowmeter 

during the test period; or

- For certain systems where multiple cylinders are ganged together to supply tools, it may be 

possible to supply the gas from just a single cylinder for the duration of the test. However, 

this alternative will only work in certain situations.

Determining Emission Factors for Gases that are not Detected or They are Detected Infrequently during 

the Stack Test

The following protocol will apply:

- If an F-gas is used but emissions are not detected, calculate the emission factor by assuming 

emissions at 1⁄2 Field Detection Limit (FDL) of the specific gas and dividing by total usage of 

that gas. 

- If an F-gas is used and detected (for some portion of the test period), calculate emissions 

using detected quantity for the time when it is detected and 1⁄2 FDL when not detected and 

dividing by total use of that F-gas. If calculated EF is greater than 1.0, assume EF=1.0. 

- If an F-gas is not used and no emissions are detected, assume emissions are zero.

- If an F-gas is not used but it is detected (for some portion of the test period), calculate 

emissions, as a by-product, using detected quantity and 1⁄2 FDL when not detected and 

dividing by total F-gas usage. 

- If an F-gas is not used during the test period but is used during the reporting period, 

emissions will be estimated using the Tier 2a method.