VAV Units Only
Sequence of Operation
1
Supply Air Pressure Control
Inlet GuideVane Control
Inlet guide vanes are driven by a modulating 2-10 vdc signal from theVAV Module. A pressure transducer measures duct static pressure, and the inlet guide vanes are modulated to maintain the supply air static pressure within an adjustable user-defined range. The range is determined by the supply air pressure setpoint and supply air pressure deadband, which are set through a unit mounted potentiometer.
Inlet guide vane assemblies installed on the supply fan inlets regulate fan capacity and limit horsepower at lower system air requirements. When in any position other than full open, the vanes pre-spin intake air in the same direction as supply fan rotation. As the vanes approach the full-closed position, the amount of “spin” induced by the vanes increases at the same time that intake airflow and fan horsepower diminish. The inlet guide vanes will close when the supply fan is shut down.
Supply Air Static Pressure Limit
The opening of the inlet guide vanes and VAV boxes are coordinated, with respect to time, during unit start up and transition to/from Occupied/Unoccupied modes to prevent overpressurization of the supply air ductwork. However, if for any reason the supply air pressure exceeds the fixed supply air static pressure limit of 3.5” W.C., the supply fan is shut down and the inlet guide vanes are closed.The unit is then allowed to restart four times. If the overpressurization condition occurs on the fifth time, the unit is shut down and a manual reset diagnostic is set and displayed at any of the remote panels with LED status lights or communicated to the Integrated Comfort system.
Variable Frequency Drives (VFD) Control
Variable frequency drives are driven by a modulating 0-10 vdc signal from the VAV module. A pressure transducer measures duct static pressure, and the VFD is modulated to maintain the supply air static pressure within an adjustable user-defined range. The range is determined by the supply air pressure setpoint and supply air pressure deadband, which are set through a unit mounted potentiometer. Variable frequency drives provide supply fan motor speed modulation. The drive will accelerate or decelerate as required to maintain the supply static pressure setpoint. When subjected to high ambient return conditions the VFD shall reduce its output frequency to maintain operation. Bypass control is offered to provide full nominal airflow in the event of drive failure.
2
Supply AirTemperature Controls
Cooling/Economizer
During occupied cooling mode of operation, the economizer (if available) and primary cooling are used to control the supply air temperature.The supply air temperature setpoint is user-defined at the unit mountedVAV Setpoint Panel or at the remote panel. If the enthalpy of the outside air is appropriate to use “free cooling,” the economizer will be used first to attempt to satisfy the supply setpoint.
On units with economizer, a call for cooling will modulate the fresh air dampers open.The rate of economizer modulation is based on deviation of the discharge temperature from setpoint, i.e., the further away from setpoint, the faster the fresh air damper will open. Note that the economizer is only allowed to function freely if ambient conditions are below the enthalpy control setting or below the return air enthalpy if unit has comparative enthalpy installed. If outside air is not suitable for “economizing,” the
fresh air dampers drive to the minimum open position. A field adjustable potentiometer on the Unitary Economizer Module,Tracer®, or a remote potentiometer can provide the input to establish the minimum damper position.
At outdoor air conditions above the enthalpy control setting, primary cooling only is used and the fresh air dampers remain at minimum position.
If the unit does not include an economizer, primary cooling only is used to satisfy cooling requirements.
Supply Air Setpoint Reset
Supply air reset can be used to adjust the supply air temperature setpoint on the basis of a zone temperature, return air temperature, or on outdoor air temperature. Supply air reset adjustment is available on the unit mountedVAV Setpoint Panel for supply air cooling control.
a
Reset Based on Outdoor AirTemperature
Outdoor air cooling reset is sometimes used in applications where the outdoor temperature has a large effect on building load. When the outside air temperature is low and the building cooling load is low, the supply air setpoint can be raised, thereby preventing subcooling of critical zones. This reset can lower usage of primary cooling and result in a reduction in primary cooling energy usage.
There are two user-defined parameters that are adjustable through the VAV Setpoint Panel: reset temperature setpoint and reset amount.The amount of reset applied is dependent upon how far the outdoor air temperature is below the supply air reset setpoint.The amount is zero where they are equal and increases linearly toward the value set at the reset amount input.The maximum value is 20 F. If the outdoor air temperature is more than 20 F below the
