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Why Pre-Purge & Post-Purge Sequences Matter in HVAC Control

September 1, 2025

When it comes to indoor air quality and energy-efficient HVAC operations, transition modes like Pre-Purge and Post-Purge play a vital role—especially in large buildings with scheduled occupancy.

 

Pre-Purge (Unoccupied ➝ Occupied)
Before occupants enter, the system runs the supply fan at 100% airflow and opens the outdoor air damper fully for 30 minutes.

This flushes out any stale or potentially contaminated air—setting the stage for a healthy, fresh-air environment.

Post-Purge (Occupied ➝ Unoccupied)
Before switching to unoccupied mode, the unit continues operating at full airflow with maximum outdoor ventilation, purging built-up CO₂ and pollutants.

The system stays in this mode until CO₂ levels drop to the minimum threshold—ensuring safe air quality even after people leave.

 These sequences aren't just compliance measures (like ASHRAE 62.1)—they’re smart strategies for healthier buildings, better IAQ, and optimized control logic.

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