Consider the following factors when install a COBP Zoning System for cold weather operation:
- High SAT under lightly loaded conditions - you can bypass large volumes of the supply air back into the return air side of the RTU. Bypassing the supply air raises the SAT and increases the risk of reaching the high temperature safety limit. If the supply air high temperature safety limit is exceeded, the control system cuts off the mechanical heating to protect it from damage. Excessive cycling of the mechanical system can occur if this condition persists. If the system cannot run for long enough to satisfy heating demands, occupant comfort is affected. To avoid excessive cycling of the mechanical system, implement the following operational and design changes:
Operational changes:
- Increase the heating minimum airflow or the damper position settings to allow more airflow during heating operation. Avoid minimum settings that are too high and cause overheating.
- Increase the static pressure setpoint to reduce the amount of air being bypassed. Use of excessive static pressures may increase noise levels and operating costs. This technique may 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 excessive low SAT cycling.
- 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 hot air from the
return air grills. This method works best with
plenum returns, but do not use with ducted
returns.