Outside Air

HVAC tab in model data under Mechanical Ventilation header

 

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 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 very useful in checking for occupant discomfort.

Outside air definition method

Defines the method used to set the maximum outside air delivery rate. 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 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).

Outside air flow rate

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: 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.

Minimum AHU outside air requirement schedule

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.

Maximum supply air flow rate

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.

System availability schedule

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.

Outside Air Mixing

This data is defined for CAV/VAV air handling units (AHU) at building level.

Outside air mixing

Select the way outside air is used:

 

Outside air control minimum flow type

This option allows you to specify the way minimum outside air flow rate is controlled for 3-VAV and 5-CAV Compact HVAC types when the Outside air mixing type (above) is 1-Recirculation:

 

 

The minimum outside air flow rate through the AHU outside air inlet is automatically calculated as the sum of the individual zone ventilation requirements. This is the 'design minimum'.  During the simulation, 'minimum' and 'maximum' represent the limits for economizer operation.  If  there is no economizer, then the minimum flow of outside air is delivered.  If 1-Fixed is selected, then this value will remain fixed (unless the total VAV system flow rate falls below this value, in which case, the system will deliver 100% outside air at whatever the system flow rate is).  If, 2-Proportional is specified, then the operating minimum falls as system flow rate falls.  This simulates a typical system with a fixed lower stop on the outside air supply damper.  Delivering 1-Fixed requires enhanced control systems.

 

Note: To ensure minimum outside air is delivered to all zones you should use 1-Fixed.

 

If you select 2-Manual HVAC sizing you can enter the Outside air control minimum flow rate manually at building level.

Outside air sizing

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 as well.