Special Considerations for Large Units (30 Tons and Over) - Verasys - LIT-12012331 - General System Information - Verasys System - 5.0

Verasys System Changeover Bypass Zoning System Design Application Note

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Verasys System
Document type
Application Note
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Due to the large airflow capacities of larger units, careful consideration is essential when designing an effective zoning system. Consider the following factors when designing a COBP Zoning System with a larger RTU:

  • Use constant volume units in your zoning system design.
  • Size the bypass dampers for the remaining airflow volume when all zones are at minimum airflow:

    Bypass Airflow (cfm) = Unit Airflow (cfm) – All Zones Minimum Airflow (cfm)

  • Use rectangular dampers that have a larger air volume capacity than the round dampers. See Table 2 for rectangular damper sizes and cfm ratings.
  • To prevent bypassing large amounts of conditioned air, locate bypass dampers toward the end of the main supply duct run.
  • Always have a minimum of six zones on large units due to the high airflow capacities.
  • Set the zone damper total minimum airflow settings equal to, or preferably greater than, 30% of the unit’s rated cfm to prevent excessive noise in the system.
  • To prevent over-pressurization of the duct work, install a high duct static safety switch such as Johnson Controls® P32 Series Sensitive Pressure Switch or similar.