FORM
less than the Return Air enthalpy and the Outside Air temperature is less than the reference temperature set- ting. This method of economizing is the most accurate and provides the highest degree of energy efficiency for a packaged rooftop economizer.
VENTILATION CONTROL SEQUENCES
Minimum OA Damper Position (CV Units)
When the unit goes into the Occupied mode of opera- tion, the unit controller shall open the Outside Air Damper to a fixed minimum position. The damper shall remain at this position as long as the unit is in the occu- pied mode. The minimum position may be overridden more open by the unit controller when Outside Air con- ditions are suitable for economizing.
Minimum OA Damper Position (VAV Units)
With Variable Air Volume units, there are two Minimum OA Damper Positions, one when the unit is at full speed and the second when the unit is at approximately half speed. These two points allow the control to linearly reset the position of the OA damper in response to fan speed.
When the unit goes into the Occupied mode of opera- tion, the unit controller shall monitor the speed of the supply fan and open the Outside Air damper to a calcu- lated minimum position based on the fan speed. This minimum position shall vary as the speed of the fan changes. The damper shall remain at this calculated position as long as the unit is in the occupied mode. The minimum position may be overridden more open by the unit controller when Outside Air conditions are suitable for economizing.
Air Measurement Stations
When the unit is equipped with an air measurement station, the unit controller shall control the Outside Air damper to a measured flow rate through the Air Mea- surement Station.
When the unit goes into the Occupied mode of opera- tion, the unit controller shall control the Outside Air damper to maintain the Minimum AirFlow Setpoint through the Air Measurement Station. The unit control- ler shall control the Outside Air damper to this flow rate as long as the unit is in the Occupied mode. The Out- side Air damper may be overridden more open by the unit controller when Outside Air conditions are suitable for economizing.
Demand Ventilation
If an optional CO2 sensor is connected to the unit, the unit controller can reset the minimum OA damper position(s) or minimum flow rate based on demand.
The unit controller shall monitor the CO2 level within the building. If the CO2 level rises above the CO2 setpoint, the controller will temporarily increase the Minimum OA Damper Position or Minimum OA flow rate to increase ventilation. If the CO2 level drops below the CO2 set- point, the controller will decrease the Minimum OA Damper Position or Minimum OA flow rate to decrease ventilation.
Demand Ventilation shall remain active as long as the unit is in the Occupied mode of operation.
EXHAUST CONTROL SEQUENCES
Barometric
The optional barometric exhaust system consists of a lightweight barometric relief damper installed on the end of the unit in the Return Air section. As more outside air is introduced into the controlled zone due to Econo- mizer and Ventilation control sequences, the pressure inside the building rises. This increase in building pres- sure forces the barometric relief damper open to allow exhaust air to escape. Because this type of exhaust system is not powered, it is limited to small amounts of exhaust.
Powered Fixed Volume Exhaust Based on Outside Air Damper Position
This optional fixed volume powered exhaust system consists of a fixed speed fan that is controlled ON and OFF based on the position of the Outside Air Damper. During operation, when the Outside Air Damper opens to a selected
Powered Fixed Volume Exhaust Based on Building Pressure
This optional fixed volume powered exhaust system consists of a fixed speed fan that is controlled ON and OFF based on the pressure inside the building. During
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