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.

Outside air requirement data is set at the zone level on the HVAC tab for all HVAC configurations and options.
Outside air flow control through the AHU (when using 2-Unitary multizone, 3-VAV and 5-CAV Compact HVAC) is set at building level on the HVAC tab.
Outside air flow control through zone units (1-Unitary single zone and 4-Fan coil Compact HVAC) is set at zone level on the HVAC tab with the rest of the zone unit data.

The Mech Vent + Nat Vent + Infiltrationdata 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:

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 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 natural 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 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 natural 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 natural 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 natural 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).

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:

1-Recirculation - the outside airflow supply to the AHU is mixed with recirculated return air from the zones. This is done to save energy often in conjunction with an economiser to ensure that optimal use is made of outside air. When heat recovery is used with the 1-Recirculation outside air mixing option, the economiser must be switched on selected in the interface for the heat recovery to be by-passed.
2-Full fresh air - the airflow into the AHU is 100% outside air, without any recirculation. This is modelled in EnergyPlus by setting the supply airflow rate to the outside air flow rate. Note that when using this option, fixing the supply air flow rate to the outside air rate will in many cases mean that the system will not be capable of meeting the design cooling load. To increase the maximum cooling capacity of a Full fresh air VAV/CAV system you can increase the outside air flow rate (such systems could be very inefficient in energy use without heat recovery). When heat recovery is used with the 2-Full fresh air outside air mixing option, the heat recovery is by-passed when this is advantageous in reducing AHU cooling coil loads. The heat recovery also operates when the outside air temperature is less than 12°C.

Important note for Full fresh air systems

In order to ensure that there is no recirculation you must set the HVAC sizing factor to 1 (the default is 1.2)

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:

1-Fixed - the minimum outside air flow rate is fixed as set above no matter what the actual system flow rate is. In practice even with 'Fixed' outside air control minimum it is possible for the outside air flow rate into individual zones to be slightly different from the requested value. This is because the minimum outside air is controlled at the AHU level to ensure that the total fresh air entering the AHU is the total required for the building (the sum of the individual zone outside air requirements). But depending on the cooling loads in each zone, some zones will require more supply air to meet the cooling load than others. So the total fresh air entering the building will be correct but the distribution around the building may not be perfectly maintained.
2-Proportional - the minimum outside air flow rate varies in proportion to the total system air flow rate.

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.