Application Note: One AHU Serving Multiple Floors of VAVs

Current Standard Solution:

75F offers a VAV sequence in which the zones and the systems work together to achieve a perfect balance of comfort and energy.  This works by all zones will reporting data back to a CCU, where Trim and Respond algorithms operate to for premium operations.

Problem:

This setup works amazingly when all VAVs are dedicated to one CCU and one AHU.  However, if the AHU serves multiple floors where multiple CCUs need to be installed, there are two options:

1. Use one CCU to operate the AHU.
   • Limitations
     • The AHU is driven by only one area in the building, which may not account for variances in solar load or occupancy.
     • If the CCU is installed on the AHU, the distance from the CCU to the zones may be too far and cause an unstable mesh network.
     • If the CCU is installed in the space, there may need to be long cable runs back to the AHU.
     • The CCU is limited in what it can control on an advanced AHU.
2. Operate the VAVs as an independent system from the AHU
   • Limitations
     • The AHU's schedule may not match the floor schedules.  Any special or vacation mode schedules, setback conditions, and preconditioning will not be communicated to the AHU.  The best option may be to run the AHU 24/7 in this scenario.
     • 75F optimization leveraging ASHRAE Guideline 36 will not apply to the AHU.

Solution:

Use a custom AHU package combined with our API to optimize the AHU considering all floors and zones.  Add a CCU dedicated to the AHU.

The base program for the AHU based on ASHRAE Guideline 36 sequences will be created by 75F, including the following points:

1. Outdoor Air Temperature
   • If a hardwired OAT sensor is installed, use this point as your higher priority.
2. Outdoor Air Relative Humidity
   • If a hardwired RH sensor is installed, use this point as your higher priority.
3. Supply Air Temperature Setpoints
   • Create a unique point for each CCU and set an average value sequence.
4. Duct Static Setpoint
   • Create a unique point for each CCU and set an average value sequence.
5. Occupancy
   • Create a unique “Occupancy Mode” point for each CCU.  Set three modes:
     • Unoccupied
     • Preconditioning
       1. Note: OA and EA dampers fully closed and MA damper fully open
     • Occupied
6. Conditioning Mode
   • Create a unique “Conditioning Mode” point for each CCU.
     • If all CCUs that have zones that serve the AHU are in heating mode, set the supply air temperature setpoint to 95 degrees.  When one CCU returns to cooling mode, return the AHU SAT Setpoint logic to the average of the CCU SAT Setpoints.
7. CCU Heartbeat
   • Create a unique “CCU Heartbeat” point for each CCU.
     • If a CCU is not communicating via API, make sure the points from that CCU do not impact averages or min/max sequences.
     • If all CCUs fail, be sure to program an onboard, standalone sequence.
8. Stage Down Timer
9. Stage Up Timer
10. Supply Air Temp Setpoint Reset Max
11. Supply Air Temp Setpoint Reset Min
12. Duct Static Pressure Setpoint Reset Max
13. Duct Static Pressure Setpoint Reset Min
14. Outside Air Temp Cooling Lockout
15. Outside Air Temp Heating Lockout

The following points will be discovered and linked to the points above:

1. From each floor level CCU:
   • staticPressureTRSp (Duct Static Pressure Setpoint)
   • satTRSp (Supply Air Temp Setpoint)
   • occupancy (Occupancy Mode)
   • conditioningMode (Conditioning Mode)
2. From the AHU dedicated CCU:
   • outsideTempCoolingLockout
   • outsideTempHeatingLockout
   • stageDownTimerCounter (staged heating/cooling only)
   • stageUpTimerCounter (staged heating/cooling only)

Result:

• Limitations mentioned for the two problem solutions will be resolved:
  • The floor level data will be averaged to best satisfy all floors and optimize AHU operations.
  • The AHU operations can be adjusted using the standard 75F tuners on the AHU CCU.
  • Setbacks, vacations, special schedules, and preconditioning will be in sync with the AHU.
  • No concerns about distance and count of SmartNodes from the CCUs, creating a stable system.
  • Long cable runs are eliminated.
  • Programming of large, complex AHUs is achievable with an end-to-end 75F solution.