York HFC-410A manual Economizer Operation, Dry-BulbEconomizer, Single-EnthalpyEconomizer

FORM 100.50-EG5 (108)

the Return Air Temperature Setpoint, Morning Warm- Up ends when occupancy occurs (BAS, Intelli-Start, or contact closure), or when the Maximum Morning Warm- Up Time has expired.

ECONOMIZER OPERATION

The unit can be equipped with one of three types of optional economizers: dry-bulb, single-enthalpy, or com- parative-enthalpy. When the unit controller determines that Outside Air is suitable for economizing, the unit controller will control the outside air damper(s) open to provide economizer cooling. If economizer cooling alone is insufficient for the cooling load, the unit control- ler shall stage up compressors, one at a time, to meet demand.

The control logic for the three types of economizers is as follows:

Dry-Bulb Economizer

The dry-bulb economizer is the default economizer control scheme. With the dry-bulb economizer, the unit controller monitors the Outside Air temperature only and compares it to a reference temperature setting. Outside Air is deemed suitable for economizing when the Outside Air temperature is determined to be less than the reference temperature setting. This method of economizing is effective, but is prone to some change- over inefficiencies due to the fact that this method is based on sensible temperatures only and does not take Outside Air moisture content into consideration.

Single-Enthalpy Economizer

With the optional, single-enthalpy economizer, the unit controller monitors the Outside Air enthalpy in addition to the Outside Air temperature and compares it to a reference enthalpy setting and a reference temperature setting. Outside Air is deemed suitable for economizing when the Outside Air enthalpy is determined to be less than the reference enthalpy setting and the Outside Air temperature is less than the reference temperature set- ting. This method of economizing allows the reference temperature setting to be set higher than the dry-bulb Economizer and is a more efficient packaged rooftop economizer.

Dual-Enthalpy Economizer

With the optional, dual-enthalpy economizer, the unit controller monitors and compares the Outside Air and Return Air enthalpies, in addition to comparing the Outside Air temperature to the reference temperature setting. Outside Air is deemed suitable for economizing when the Outside Air enthalpy is determined to be less than the Return Air enthalpy and the Outside Air tem-

perature is less than the reference temperature setting. 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 operation, 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 occupied mode, and the economizer is not suitable for cooling.

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 operation, the unit controller shall monitor the speed of the supply fan and open the Outside Air damper to a calculated 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, and the economizer is not suitable for cooling.

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 Economizer and Ventilation control sequences, the pressure inside the building rises. As building static pressure increases to overcome any exhaust duct static pressure, air will be allowed to escape through the barometric relief damper. Because this type of exhaust is not powered, the amount of air exhausted will be limited to the static pressure that will need to be overcome.

Powered, Variable-Volume Exhaust-Discharge Damper Controlled

This optional variable-volume, powered-exhaust system consists of a fixed-speed fan configured with a propor- tionally controlled discharge damper. The Rooftop Unit controller monitors the pressure inside the building and controls the Exhaust Damper and the Exhaust Fan. If the Building Pressure rises, the Exhaust Damper is

JOHNSON CONTROLS 29