All elements of the existing AHU should be upgraded when doing a refurbishment, including Coils, Filter banks (typically 400FPM Max with 2” thich MERV 8 filters), Fan systems (clean / degrease / lubricate / repair damage fan housing/supports) and drive, Dampers (OA, EA,RA, bypass, deck zone dampers, air tunnel reheat box dampers, etc) and Actuators and controls. Typically multi-zone and air tunnel reheat box zone dampers are straightened, cleaned and lubricated, not replaced and new actuators are provided. Not all systems will require replacement, but all should be evaluated for proper performance and may require some level of repairs or modifications for the new system or controls. If new system elements are needed, refer to Volume 3, Section 6.3.3 for acceptable fan element types and requirements.
Coils shall only be replaced if there is a need based on condition or a change in coil medium (change from Steam to Hot Water). Preheat coils are to be eliminated if possible. If the new coil is to be dual temperature, it shall be sized for cooling capacity and airflow. Both chilled water performance and hot water performance shall be scheduled for the dual temperature coil. Provide a coil circulation pump on all dual temp coils. The circ pump does not need to be sized for the chilled water flow as heating capacity required is significantly less and coils are selected for the higher cooling load. Often the pump is sized for minimum turbulent water flow through the coil (this often turns out to be approximately half the required chilled water coil flow requirement). Circulation pump is operational only during heating. Provide a bypass around the pump with a check valve to allow flow to bypass the circ pump during cooling. See CPS details. Typically these coils require straight fins and air velocities of 300 FPM or less to work with the existing to remain low static supply air fans. Coil selection can be a very iterative process to accomplish both heating and cooling requirements. Often a bypass damper is required around the dual temp coil to reheat the air after cooling/dehumidifying above the building dewpoint (typ around 60F drybulb) as much of the existing supply ductwork to be reused is inaccessible and condensation in the duct is to be avoided. Provide a separate heating and cooling control valve for all dual temp coils. Identify each control valve GPM on drawing. The heating control valve max GPM is often much lower than the heating flow through the coil as the circ pump flow is higher and blends in return water. The valve GPM value must be identified in order for temperature controls to select the correct valve. This is commonly missed resulting in a grossly oversized control valve.
In most cases, the fan wheel shall remain unless there is evidence of failure or ongoing issues. All drives and motors to be changed to VFD capability.
Typically, if fan refurbishments are included in the scope, there will also be an upgrade to the existing building controls to a BAS. All new AHU controls, including dampers, actuators, sensors and sequences, shall follow the CPS Master Specifications and Details.