A design tip exists that suggests using the Minimum Design Concentration (MDC) plus 15% extra clean agent combined with continuous air mixing ensures a proper hold time as required by NFPA 2001, 2025 edition. (All references to NFPA 2001 in this document refer to the 2025 edition unless otherwise noted.) An FSSA task group was assigned to verify whether this 15% additional clean agent provides a 10-minute hold time prediction for continuous mixing situations.

The task group used the simulation method given in NFPA 2001 to estimate the hold time for an enclosure with continuous air movement versus the predicted hold time in the same enclosure with air movement shut down at the time of discharge. This is the same method commonly used to fulfill NFPA 2001 system acceptance criteria.

The estimates indicate that when a room is sealed to provide a 10-minute predicted hold time with air movement shut down, i.e. descending interface situations, the need to add agent to maintain that 10-minute predicted hold time when there is continuous mixing depends on the height of protected combustibles versus the flooded height and the type of agent. In the cases examined, FK-5-1-12 was most likely to need additional agent or more sealing against leakage while inert gas IG-541 was unlikely to need additional agent or sealing to maintain a 10-minute hold time with continual mixing compared to the descending interface scenario.

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