HVAC tab in model data under Mechanical Ventilation header - Compact HVAC
Check the mechanical ventilation checkbox to indicate that outside air and/or re-circulated air is delivered to the zone. You can use this setting to define mechanical ventilation and air delivered through centrally ducted air conditioning systems.
Note that Mechanical Ventilation data refers to the fan-powered air delivery through the main HVAC system and not a separate mechanical ventilation system.
You can set the amount, timing and control of outside air delivery through the HVAC air distribution system on the HVAC tab.
The Mech Vent + Nat Vent + Infiltration data can be displayed on the Heating design, Cooling design and Simulation screens when you select 1-All or 3-Comfort data. This is the sum of fresh air delivered through the HVAC system, infiltration and natural ventilation outside airflow in air changes per hour (ac/h) and when used together with the other environmental output it can be useful in checking for occupant discomfort.
Defines the method used to set the maximum outside air delivery rate. The actual outside air flow rate during simulation is modified by the Minimum AHU outside air requirement schedule described below. The options are:
m3/s = ac/h x ZoneVolume / 3600
where ZoneVolume is the actual air volume of the space calculated according to Model options for excluding floor/ceiling constructions.
Note: this zone volume may be slightly different from the value used for the purposes of calculating air flow rates for infiltration which uses the total zone volume.
m3/s = MinFreshAir x NumberPeople / 1000
NumberPeople = Occupancy density (people / m2) * Zone floor area (m2).
MinFreshAir is the minimum fresh air rate per person (l/s/person or CFM).
m3/s = l/s-m2 x ZoneFloorArea / 1000
where ZoneFloorArea is the actual floor area of the space.
MinFreshAir is the minimum fresh air rate per floor area (l/s-m2 or CFM/ft2).
m3/s = (l/s-m2 x ZoneFloorArea + MinFreshAir x NumberPeople)/ 1000
where ZoneFloorArea is the actual floor area of the space.
MinFreshAir is the minimum fresh air rate per floor area (l/s-m2 or CFM/ft2).
NumberPeople = Occupancy density (people / m2) * Zone floor area (m2).
MinFreshAir is the minimum fresh air rate per person (l/s/person or CFM).
m3/s = Maximum of (l/s-m2 x ZoneFloorArea / 1000) and (MinFreshAir x NumberPeople/ 1000)
where ZoneFloorArea is the actual floor area of the space.
MinFreshAir is the minimum fresh air rate per floor area (l/s-m2 or CFM/ft2).
NumberPeople = Occupancy density (people / m2) * Zone floor area (m2).
MinFreshAir is the minimum fresh air rate per person (l/s/person or CFM).
Enter the minimum zone outside air delivery rate in air changes per hour when Outside air definition method (above) is 1-By zone. The actual minimum amount of outside air entering the AHU can be modified by the Minimum AHU outside air requirement schedule (below).
Note however that this value is used unmodified by any schedules for Heating design calculations when mechanical ventilation is switched on and included in the analysis.
Note: in Compact HVAC systems, the total mixed air flow rate into the zone is calculated by EnergyPlus and will often be higher than the outside air flow rate due to the mixing of recirculated air to provide enough airflow to delivery any additional heating/cooling.
You can modify the total outside airflow using the Minimum AHU outside air requirement schedule. Whatever the outside air flow would have been without the schedule is multiplied by the current schedule value. So, for example, if you select a schedule which is always 0.5, then the system will supply half the outside air flow at all times vs. the same run without the schedule. You can also use this schedule to switch off outside air during unoccupied periods.
The maximum supply air flow rate is the total flow rate of air entering the zone (not necessarily all fresh air). It can be defined at the zone level when using Compact HVAC or it can be left in the default state (Auto), in which case the maximum supply air flow rate is calculated based only on the maximum heating and cooling capacities.
This data is available at the zone level when using Unitary single zone, Unitary multizone or VAV Compact HVAC and the plant sizing options Autosize when not set or Manual.
The System availability schedule defines the operation of the air system fans. For Compact Unitary systems the Fan operation mode (below) can be set to 2-Cycling in which case air distribution only occurs when there is heating or cooling demand.
When the Outside air definition method is 2-Min fresh air per person, sizing for minimum Outside air (OA) flow rate is done using the 'Number of People' value (see above) with no regard for the schedules. The total OA requirement for each zone on the air handler is summed and used to set the minimum OA flow rate for the air handler. The OA controller sets an OA flow rate for the air handler each time step. After that, each zone receives a pro-rated share of the total OA flow proportioned by the current supply flow rate to each zone. There is no further zone-by-zone allocation of OA quantities, which is what happens in a real system.