Recovery time for HVAC

how the recovery time (related to HVAC) in cleanrooms can be calculated?

Take the particle count at different location as per protocol then identify the point with highest particle count. Now switch off the HVAC and generate the particle as per SOP (100 times or according to decay method). start the HVAC immediately and take the particle count (at same location where u did find highest particle) continuously until desire level of particle count achieved (Particles less than previous ).
Recovery time should not be more than 20 minutes as per ISO 14644

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What about ISO 8 area ? Guideline doesn’t suggests for iso 8 and iso 9 but if some one wants to recovery study for iso 8 area then what to do ?

for ISO 8 usually time to maintain temp and humidity is noted.
switch off hvac increade humidity and temp of the room.
switch on hvac and note the time to bring temp n humidity within range.