Single zone VAV - Johnson Controls - LIT-12011950 - Building automation software - Smart Equipment Controller - 5.0

Smart Equipment Controls Sequence of Operation Technical Bulletin
Product
Building Automation Systems > Building Automation Systems > Smart Equipment Controller
Document type
Technical Bulletin
Document number
LIT-12011950
Version
5.0
Revision date
2024-06-06
Product status
Active

This section details the single zone VAV (SZ VAV) setpoints and related data, inputs, outputs, and operation.

Setpoints and related data

The setpoints and related data include the following items:

  • VAV cooling SAT upper setpoint (SATUp-Sp, AV 29604)
  • VAV cooling SAT lower setpoint (SATLo-Sp, AV 29605)
  • VAV operating cooling supply air temp setpoint (OprClgDAT-Sp, AV 30077)
  • VAV operating cooling zone setpoint (OprSZVAVClg-Sp, AV 29927)
  • Fan control type (FanCtl-Type, MV 29555) - Variable Speed
  • SZ VAV enabled (SZVAVEn, MV29908) - Yes
  • SZ VAV occupied cooling setpoint (SZVAVClgOcc-Sp, AV 29925)
  • SZ VAV unoccupied cooling setpoint (SZVAVClgUnocc-Sp, AV 29926)
  • SZ VAV minimum fan speed (SZVAVMinFanSpd, AV 29913)
  • SZ VAV cooling load (SZVAVClgLd, AV 29935)
  • Staged cooling command (StgClgCmd, AV 29922)
  • Operational occupancy (OprOcc, MV 29517)
  • Cooling manual tuning (ClgManualTune, Not BACnet Exposed)

Inputs

The inputs include the following items:

  • Operational space temperature (OprST, AV 29522)
  • Supply air temperature (SAT, AV 29564)
  • Evaporator coil sensors (EC1, EC2, EC3, EC4, AV 29663 through AV 29666)

Outputs

The outputs include the following items:

  • Fan % command (FanVFD, AV 29551)
  • Compressor stage commands (C1, C2, C3, C4, MV 29611 through MV 29614)

Operation

Note: To operate in SZ VAV mode the control must be in an occupied mode.

When there is no demand for cooling and the staged cooling command is 0%, the fan % command is at the SZ VAV minimum fan speed. The SZ VAV operating cooling setpoint is equal to the VAV cooling supply air temperature upper setpoint.

If the operational space temperature is above the SZ VAV occupied cooling setpoint by more than half of the cooling manual tuning amount, the staged cooling command increases. If the operational space temperature is within half of the cooling manual tuning amount of the SZ VAV operating cooling setpoint the staged cooling command does not change. If the operational space temperature is less than half of the cooling manual tuning amount below the SZ VAV operating cooling setpoint, the staged cooling command decreases.

Compressor outputs energize and de-energize based on the staged cooling command according to the following table.

Table 1. SZ VAV PID percent command
No. stages PID percent command
1st stage 2nd stage 3rd stage 4th stage
On Off On Off On Off On Off
1 100% 0%  
2 50% 0% 100% 50%  
3 33% 0% 66% 33% 100% 66%  
4 25% 0% 50% 25% 75% 50% 100% 75%

As the staged cooling command increases the SZ VAV operating cooling setpoint decreases proportionately between the VAV cooling SAT upper setpoint and the VAV cooling SAT lower setpoint. As the staged cooling command decreases the SZ VAV operating cooling setpoint increases proportionately between the VAV cooling SAT upper setpoint and the VAV cooling SAT lower setpoint.

  • If the staged cooling command is >0%, FanVFD output modulates to control the SAT to the SZ VAV operating cooling setpoint +/-0.6°F.
  • If the SAT drops below the SZ VAV operating cooling setpoint by more than 0.60°F, FanVFD increases to raise the SAT within SZ VAV operating cooling setpoint within 0.60°F.
  • If the SAT rises above SZ VAV operating cooling setpoint by more than 0.60°F, FanVFD decreases to lower the SAT to within 0.6°F of the SZ VAV operating cooling setpoint.
Note: The rate of change for the fan to keep the SAT within the control band must be determined.

If any of the evaporator coil sensor values (EC1, EC2, EC3, EC4) go below 42°F, FanVFD increases proportionally to the number of degrees below 42°F according to the following figure.

Figure 1. SZ VAV additional fan speed
Note: The Fan VFD cannot drop below 20Hz or 33% at any time.
Note: In compliance with ASHRAE 90.1-2010 Section 6.4.3.10, the fan % command (FanVFD) cannot operate above 67% when the staged cooling command (StgClgCmd) is at or below 50%. During free cooling operation, the fan cannot operate above 67% if the SZ VAV cooling load is at or below 50%.

Economizer free cooling operation

If free cooling is available and the operating space temperature is greater than the SZ VAV occupied cooling setpoint, FanVFD output changes proportionately between the SZ VAV minimum fan speed and 67% or 100% (maximum fan speed, dependent on the SZ VAV cooling load).

Note: The operational space temperature and the SZ VAV occupied cooling setpoint drive the SZ VAV cooling load when the unit is free cooling. This process controls the FanVFD speed, as the demand increases, the fan increases CFM to match.

With free cooling available and the SZ VAV cooling load above zero, the SZ VAV operating cooling setpoint resets proportionally between the VAV cooling SAT upper and lower sepoints. The economizer analog output (ECON) modulates to control supply air temperature to the SZ VAV operating cooling setpoint +/-1.8°F. If ECON reaches 100% and remains there for 5 consecutive minutes, the control moves to Econ+Mech mode and the staged cooling command begins to accumulate at a rate determined by how much warmer the supply air temperature is from the SZ VAV operating cooling setpoint (starting above 1.8°F setpoint).

When the SZ VAV cooling load reaches 0%, the control ceases free cooling operations and moves to a satisfied condition where it runs the fan at the SZ VAV minimum fan speed.

1 The percent commands are +/-2%
2 The 4th stage applies to cooling operation only.