Single Zone
Single Zone CV
The design is peak air and peak cooling capacity.
A separate single zone unit, including air handler and cooling and heating coils, is assumed for each zone. Each unit is located on the ROA deck and draws return air from a common return air plenum, i.e., the return air of the interior and perimeter rooms associated with a series of single zone units is uniformly mixed in the common return air plenum. The design cooling and heating supply air temperatures are determined from either the Design Phase or user input and cannot be overridden by outside air reset schedules. The fans are constant volume while the coils are cycled to meet the load.
Each zone assigned to this system will be given its own set of coils. For example, if 10 zones are assigned to an individual Single Zone CV system, 10 systems will be created, one per zone.
System Simulation
When the room drift temperature rises above the room cooling thermostat, the cooling coil is energized (at a constant cooling supply air temperature) for a percentage of the hour that it takes to bring the room temperature down to the cooling thermostat temperature. For the portion of the hour that the cooling coil is de-energized, the supply air will remain at the return/outside air dry bulb temperature (plus supply fan heat).
When the room drift temperature drops below the room heating thermostat the heating coil is energized (at a constant heating supply air temperature) for a percentage of the hour that it takes to bring the room temperature up to the heating thermostat temperature. For the portion of the hour that the heating coil is de-energized, the supply air will remain at the return/outside air dry bulb temperature (plus supply fan heat).
When the room drift temperature lies within the deadband, the supply air will initially remain at the return/outside air dry bulb temperature (plus supply fan heat). If necessary, the return/outside air mixture will be heated or cooled to prevent the room temperature from going out of the deadband because of the introduction of significant quantities of cold or hot ventilation air.
System Options
1. The value of minimum ventilation airflow will be proportional to the percentage defined by the outside air schedule. The value of outside airflow may be greater than the nominal value if an economizer or nighttime purge is activated.
The value of outside airflow may be less than the nominal ventilation value if the supply fan (entered as the main cooling fan) has been scheduled at less than 100%; the outside air percentage for a particular hour will be multiplied by the main cooling fan utilization percent for that hour. If the supply fan has been scheduled off for a particular hour, no outside air can be delivered to the rooms.
2. The supply fan is controlled in time clock fashion, i.e., if the schedule reads 80% for a particular hour, the fan will operate at full rpm for 80% of the hour and remain off the rest of the hour. Since the supply fan delivers conditioned air to the space, reducing the percent it can operate during a given hour will cause the cooling coil to cycle on more often during the percent of the hour the fan is available. If the fan is not allowed to operate long enough, the room temperature will be allowed to drift, the amount dependent on the magnitude of the unmet space load.
3. Neither the cooling or heating supply air temperature may be reset using supply air reset controls since the cooling supply air temperature is cycled at a constant temperature and the heating supply air temperature responds to the room thermostat.
Application Notes
● Humidity compensated controls can be implemented by using the humidity controller on the controls tab of the system properties to modify the default settings. By using the option supply air reset per maximum, for example, whenever a room relative humidity is greater than the design room relative humidity, the room's VAV dampers will continue to open until the room relative humidity is lowered to the design room relative humidity.
● Alternatively, by specifying the option supply air reset per multizone maximum avg, whenever the return air relative humidity is greater than the design system relative humidity (which is the average of the design room relative humidities for the zones served by the system), supply air reset controls are deactivated and the main cooling coil leaving air temperature is depressed to its design value for that hour.