Double Duct VAV
Double Duct VAV
The design is block air and block cooling capacity.
System Description
This system is composed of a separate hot and cold deck that supplies air to each room's variable air volume mixing box. The single, variable air volume supply fan is located on the ROA deck and will modulate in proportion to the sum of the room hot and cold deck airflows. Return air is drawn from a common return air plenum; the return air from the associated interior and perimeter rooms is assumed to be uniformly mixed before being routed to the ROA deck.
Nonmixing Event
For nonmixing situations, the sequence of events is as follows. When the room drift temperature has risen above the room cooling thermostat the cold deck side of the room mixing box is opened. The resulting room airflow is proportional to the room's design cooling load. When the room drift temperature drops below the room heating thermostat, the hot deck side of the room mixing box opens. The resulting room heating airflow is assumed to be proportional to the room's design heating load. When the room drift temperature lies within the dead band region both the hot and the cold sides of the room mixing box are closed and no supply air is admitted to the space. It is possible for some rooms to be in the heating mode (using hot deck air only) and some rooms in the cooling mode (using cold deck air only) simultaneously since the hot and cold decks are separate.
Mixing Event
Mixing situations can occur in a room mixing box only when a reheat minimum airflow has been specified. A minimum stop defines the minimum amount of cold and hot deck airflow that must be delivered to each space. When the drift temperature rises above the room cooling thermostat, mixing can occur when the cooling airflow required to meet this hour's cooling load is less than the minimum stop airflow; the proportions of cold and hot deck airflow are then adjusted to control the room temperature to the cooling thermostat temperature. Similarly, when the drift temperature drops below the room heating thermostat, mixing can occur when the heating airflow required to meet this hour's heating load is less than the minimum stop; the proportions of cold and hot deck airflow are then adjusted to control the room temperature to the heating thermostat temperature. When the drift temperature lies within the dead band, the proportions of hot and cold deck airflow are adjusted to control the space at the beginning hour's room temperature.
Note that shutting off the cooling coil availability does not shut off the central air handler. The supply fan will modulate accordingly, supplying untreated return/outside air to the room mixing boxes that are open in response to the thermostat settings. However, if the supply fan is scheduled off for a particular hour, no mechanical cooling or heating is possible.
System Options
1. The value of minimum outside air nominally follows the outside air schedule. The amount of outside air introduced into the ROA deck may be greater than the ventilation minimum if an outside air economizer or nighttime purge is activated this hour.
In addition, the amount of outside air brought into a particular room is a function of how much supply air is needed by the room. If, for example, the drift temperature is in the dead band both the cold and hot decks are closed and no outside air can be admitted (unless a reheat minimum has been specified). When the drift temperature is above the cooling thermostat or below the heating thermostat, a proportional amount of outside air is admitted into the space.
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 available whenever the schedule reads 1% or greater for a particular hour. Its energy consumption will be proportional to the amount of supply airflow required each hour.
3. Input fan cycling on the availability manager tab of the systems properties will allow the supply fan to cycle with the heating load for rooms which have minimum reheat airflow scheduled during unoccupied hours. Fan cycling can only apply to rooms that have a design reheat minimum airflow > 0 and their reheat minimum schedule reading > 0% for the hour(s) that the people schedule reads 5% or less.
4. Supply air temperature reset control can be used to reset the cold deck temperature upward based on the worst case cooling room and reset the hot deck temperature downward based on the worst case heating room.
5. Reheat Minimum. A reheat minimum will determine the minimum airflow into the space each hour.
Application Notes
● The return/outside air mixture, which is the air routed through the ROA deck, is used on both decks. Consequently, when an economizer is specified, greatly increased heating loads can result since the ROA deck temperature will be controlled to the required cooling supply air temperature regardless of the cooling load. (Only one room out of ten may need cooling, for example.) Input of a supply air reset control will minimize the excess heating during unoccupied hours but will have minimal effects during occupied hours.
● 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.
Two-Fan Double Duct VAV
The design is block air and block cooling capacity.
System Description
This system is composed of a separate cold deck and a separate heating runaround deck that supply air to each room's mixing box. The cooling and heating supply fans are both variable air volume and will modulate in proportion to the sum of their respective room cooling and heating airflows.
Nonmixing Event
For nonmixing situations, the sequence of events is as follows. When the room drift temperature has risen above the room cooling thermostat the cold deck side of the room mixing box is opened. The resulting room airflow is proportional to the room's design cooling load. When the room drift temperature drops below the room heating thermostat, the runaround deck side of the room mixing box opens. The resulting room heating airflow is assumed to be proportional to the room's design heating load. When the room drift temperature lies within the deadband region both the supply and the runaround sides of the room mixing box are closed and no conditioned air is admitted to the space.
Mixing Event
Mixing situations can occur in a room mixing box only when a reheat minimum airflow has been specified. A minimum stop defines the minimum amount of cold deck airflow that must be delivered to each space. As long as the room drift temperature stays above the room heating thermostat no mixing takes place and the mixing box supplies air only from the cold deck. If, however, the room drift temperature falls below the heating thermostat settings, the hot deck also opens such that the mixture of the hot and cold deck airflow controls the room temperature to the heating thermostat. Thus, as long as the room drift temperature remains in the dead band region, the cold deck air quantity is equal to the room reheat minimum airflow.
Note that shutting off mechanical cooling does not shut off the cooling supply fan. If the cooling fan schedule reads 1% or greater the cold deck fan will modulate accordingly, supplying untreated return/outside air to the room mixing boxes that are open. However, if the main cooling fan has been scheduled off for a particular hour, no mechanical cooling is possible.
Similarly, shutting off the main heating coil does not shut off the heating supply fan. If the main heating fan schedule reads 1% or greater the heating supply fan will modulate accordingly, supplying untreated runaround air to the room mixing boxes that are open. However, if the main heating fan has been scheduled off for a particular hour, no mechanical heating is possible.
System Options
1. The value of minimum outside air nominally follows the outside air schedule. The amount of outside air introduced into the cold deck may be greater than the ventilation minimum if an outside air economizer or nighttime purge is activated this hour.
In addition, the amount of outside air brought into a particular room is a function of how much cold deck air is needed by the room. If, for example, the drift temperature is below the cooling thermostat, the cold deck is closed and no outside air can be admitted (unless a reheat minimum has been specified). When the drift temperature is above the cooling thermostat, a proportional amount of outside air is admitted into the space.
Note that scheduling the cooling supply fan off means that no outside air can be delivered to the rooms.
2. The cooling supply fan is available whenever the main cooling fan schedule reads 1% or greater for a particular hour. Its energy consumption will be proportional to the amount of cold deck airflow required each hour.
The heating supply fan is available whenever the main heating fan schedule reads 1% or greater for a particular hour. Its energy consumption will be proportional to the amount of runaround (heating) airflow required each hour.
3. Supply air reset control can be used to reset the cooling supply air temperature upward based on the worst case cooling room. The heating supply air temperature will be reset downward based on the worst case heating room.
4. Reheat Minimum. A reheat minimum will determine the minimum cold deck airflow into the space each hour.
Application Notes
● While supply air reset control will save cooling energy, the room relative humidity will be higher and the supply fans will consume more energy.
● 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.