Dehumidification – Hot Gas Reheat
Dehumidification control is a mode of Cooling when hot gas reheat is configured with a modulating
reheat valve. Symbio™ 700 supports two configurable dehumidification methods: Dew Point (default) or
Relative Humidity control for occupied and unoccupied operations. All unit types require a valid space
relative humidity, space temperature, discharge air temperature sensor, and Evaporator Entering
Refrigerant Temperature for dehumidification operation. Dew Point control additionally requires outdoor
air temperature and relative humidity.
Dehumidification is allowed when the outdoor air temperature is between 40°F and 100°F, but it is disabled
outside this range. Dehumidification can be enabled and disabled directly via the Occupied
Dehumidification Enable and Unoccupied Dehumidification Enable points (UC700 J20-1 and UC700 J20-2).
Dew Pont- Initiation
Occupied dehumidification Dew Point control initiates when the following conditions exist:
- Space Dew Point is greater than the Space Dew Point Setpoint and
- Outdoor Air Dew Point is greater than the Outdoor Air Dew Point Setpoint
Unoccupied dehumidification Dew Point control initiates when:
- Space Dew Point is greater than the Space Dew Point Unoccupied Setpoint and
- Outdoor Air Dew Point is greater than the Outdoor Air Dew Point Setpoint and
- [Space has been unoccupied for less than Unoccupied Dehumidification Timer and relative humidity is greater than 65% or
- Space has been unoccupied for greater than or equal to Unoccupied Dehumidification Timer]
The Unoccupied Dehumidification Timer defaults to 12 hours. Setting the timer to 0 effectively disables the timer and, unoccupied dehumidification initiates only according to Space and Outdoor Dew Point.
Relative Humidity- Initiation
Occupied dehumidification control initiates when the Space Humidity is greater than the Space Dehumidification Setpoint. Unoccupied dehumidification control initiates when the Space Humidity is greater than the Space Dehumidification Unoccupied Setpoint.
Humidistat — Initiation
The Humidistat binary input on the Customer Options Module is active. If a Space Humidity sensor is installed, the Humidistat input will be ignored.
Operation – CVZT/VVZT
CVZT and VVZT units, the supply fan operates at the minimum speed defined for the compressor stage. Compressors are increasingly staged to 100% capacity. Space temperature is managed by modulating the hot gas reheat valve. The reheat valve will not open until the space temperature is below the space cooling setpoint minus 1.5°F and modulates to the reheat discharge air temperature setpoint calculated by the Symbio™ 700 to maintain space temperature control.
Anytime during occupied modes of operation, if the space temperature falls 3°F below the cooling setpoint and the reheat valve is at the maximum position for 5 minutes, dehumidification control terminates.
During unoccupied dehumidification, the reheat valve modules hold the space temperature at a fixed 73°F. If the space temperature falls below 60°F and the reheat valve is at the maximum position for 5 minutes, dehumidification control terminates.
Operation – VVDA
Variable Volume Discharge Air units continue to modulate the supply fan to maintain duct static pressure control. Compressors will stage to achieve a calculated evaporator dew point temperature. The reheat valve modulates to control discharge air temperature to the Discharge Air Cooling Setpoint Status setpoint.
Unoccupied dehumidification, if the space temperature is greater than the unoccupied cooling setpoint, the control will satisfy the sensible cooling demand first. If the space has a dehumidification demand, compressors will stage to achieve a calculated evaporator dew point temperature, and discharge air temperature will be reheated to a fixed 70°F until the dehumidification demand terminates.
Dew Point - Termination
Occupied dehumidification Dew Point control terminates when:
- Calculated Space Dew Point is less than the Space Dew Point Setpoint minus the Dew Point Setpoint Offset
or
- Outdoor Air Dew Point is less than the Outdoor Air Dew Point Setpoint minus the Dew Point Setpoint Offset
Unoccupied dehumidification Dew Point control terminates when:
- Space Dew Point is less than the Space Dew Point Unoccupied Setpoint – Dew Point Setpoint Offset or
- Outdoor Air Dew Point is less than the Outdoor Air Dew Point Setpoint – Dew Point Setpoint Offset or
- [Space has been unoccupied for less than Unoccupied Dehumidification Timer and Space Relative Humidity is less than 65% minus Space Dehumidification Setpoint Offset]
Relative Humidity - Termination
Occupied modes of operation, Space Dehumidification control terminates when the Space Humidity is less than the Space Dehumidification Setpoint minus the Space Dehumidification Setpoint Offset. In unoccupied mode, Space Unoccupied Dehumidification control terminates when the Space Humidity is less than the Space Dehumidification Unoccupied Setpoint minus the Space Dehumidification Setpoint Offset.
Humidistat — Termination
The Humidistat binary input on the Customer Options Module is inactive.
Dehumidification Purge Cycle
During dehumidification, a 3-minute compressor cooling purge cycle initiates according to the Reheat Purge Interval (120 minutes adjustable). The modulating hot gas reheat valve moves to a fixed position and compressors go to full capacity.
Dehumidification – Humidistat Input
Utilizing a Humidistat, dehumidification becomes active when the Humidistat input is active. Dehumidification terminates when the Humidistat is no longer active.
Dehumidification– Thermostat Control
Dehumidification control can be initiated utilizing a Space Humidity input or a Humidistat input. If Space Humidity input is used for control, dehumidification initiates when Space Humidity exceeds the Space Dehumidification Setpoint. Dehumidification terminates when Space Humidity is less than Space Dehumidification Setpoint minus the Space Dehumidification Setpoint Offset.
Utilizing a Humidistat, dehumidification becomes active when the Humidistat input is active and a Y1 call. All compressor outputs energize and economizer cooling is disabled. The reheat valve will open and modulates to the Discharge Air Reheat Setpoint (default 70°F). Dehumidification terminates when the Humidistat input is no longer active or with any Y2 call or call for heat.
Click the e-library symbol for the latest version
WARNING:
Information in this article is intended for use by individuals possessing adequate backgrounds of electrical and mechanical experience and who comply with all federal, state, and local laws, rules, orders, or regulations related to the installation, service, or repair of a heating or central air conditioning product. Any attempt to install, service, or repair a heating or central air conditioning product may result in personal injury and/or property damage. The manufacturer or seller cannot be responsible for the interpretation of the information contained herein, nor can it assume any liability in connection with its use.