Outside Air

HVAC tab in model data under Mechanical Ventilation header - Simple HVAC

You can set the amount, timing and control of outside air delivery through the HVAC air distribution system on the HVAC tab.

Outside air requirement data is used at the zone level for all HVAC configurations and options, though defaults settings can be made at building and block levels.

The Mech Vent + Nat Vent + Infiltration output 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 very useful in checking for occupant discomfort.

Outside air definition method

Defines the method used to set the maximum outside air delivery rate. The actual outside air flow rate during simulation is modified by the Operation schedule described below. The options are:

• 1-By zone - enter the zone outside air delivery rate in air changes per hour in the control directly below. The air flow rate in m3/s is calculated from the ac/h data using:

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.

• 2-Min fresh air (Per person) - maximum mechanical ventilation rate is defined using Minimum fresh air requirements per person as set on the Activity tab. The air flow rate in m3/s is calculated from this as:

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).

• 3-Min fresh air (Per area) - maximum mechanical ventilation rate is defined using Minimum fresh air requirements as set on the Activity tab. The air flow rate in m3/s is calculated as:

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).

• 4-Min fresh air (Sum per person + per area) - maximum mechanical ventilation rate is defined using Minimum fresh air requirements for both as set on the Activity tab. The air flow rate in m3/s is calculated as:

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).

• 5-Min fresh air (Max per person and per area) - maximum mechanical ventilation rate is defined using Minimum fresh air requirements for both as set on the Activity tab. The air flow rate in m3/s is calculated as:

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).

• 6-Design flow rate - where the maximum mechanical ventilation rate is defined directly in m3/s or ft3/min.

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 Operation schedule (below).

Note: For Heating design calculations, the Outside air flow rate value is used unmodified by a schedule when mechanical ventilation is switched on and included in the analysis.

Operation

Schedule

Select the schedule that defines the time-varying operation of the mechanical ventilation system. The fractional values in the schedule are multiplied by the Outside air flow rate (above).