Cooling - Verasys - LIT-12012331 - General System Information - Verasys System - 5.0

Verasys System Changeover Bypass Zoning System Design Application Note

Product name
Verasys System
Document type
Application Note
Document number
Revision date

Consider the following factors when you apply a COBP Zoning System for hot weather operation:

  • Low Ambient Temperature Lockout - During cold weather months, mechanical systems may have low temperature lockouts that protect equipment from damage. When outside temperatures are below the safe operating limits for your equipment, the RTU services the interior zones with thermal loads that require cooling. Use an economizer or low ambient mechanical cooling provisions to prevent damage to the equipment. Economizers lower utility costs and provide comfort under conditions when it is not possible to operate the mechanical cooling system. The Verasys Control System integrates these safeguards into the zoning system operation.
  • Low Supply Air Temperatures (SAT) - Under lightly loaded conditions, you can bypass much of the supply air back to the return air side of the RTU. Bypassing the supply air lowers the SAT, and increases the risk of reaching the low temperature safety limit. To protect itself from damage, the control system cuts off the mechanical cooling when you exceed the low SAT safety limit. Excessive cycling of the mechanical system can occur if this condition persists. Comfort can also suffer if the system cannot run long enough to satisfy cooling demands. To avoid excessive cycling of the mechanical system, implement the following operational and design changes:

Operational changes:

  • Increase the cooling minimum airflow or damper position settings to allow more airflow during cooling operation. Avoid minimum settings that are too high and cause overcooling.
  • Increase the static pressure setpoint to reduce the amount of air being bypassed. Use of excessive static pressures can increase noise levels and operating costs. Increasing the static pressure can also cause overcooling of the spaces due to increased airflow at minimum positions.

Design changes:

  • To avoid oversizing the unit, calculate loads carefully. Oversizing the unit is a common cause of excessively low SAT cycling.
  • When cooling loads are minimal, use an economizer to improve operation and ventilation during cool weather.
  • Bypass the air into the ceiling plenum instead of into the return air intake.
Note: Be careful when you bypass the air into the ceiling plenum as you may get dumping of cold air from the return air grills. This method works best with plenum returns, but do not use with ducted returns.