Natural Ventilation

HVAC tab in model data

You can control the operation of natural ventilation using the data under the Natural Ventilation header on the HVAC tab. When using Calculated natural ventilation model option there is additional data on the Openings tab to control the timing and extent of operation of openings.

Note: the Natural ventilation checkbox must be ticked here on the HVAC tab to allow window, vent and door operation to be defined on the Openings tab.

The data shown under Natural ventilation on the HVAC tab depends on whether you are using the Scheduled or Calculated Natural ventilation setting in Model options.

Scheduled

When the Natural ventilation model option is set to Scheduled,and for heating and cooling design calculations, natural ventilation is defined simply as follows:

Check the 'On' checkbox under the Natural Ventilation header.
Select the flow type
Set the maximum zone natural ventilation rate in air changes per hour (if flow type is 1-By zone).
Enter the operation information.

Find out more about Natural Ventilation Modelling under Modelling Issues.

Flow type

The Flow type defines the method used to set the maximum outside air natural ventilation rate. The options are:

1-By zone - enter the zone natural ventilation rate in air changes per hour in the control directly below. The air flow rate 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 options for optionally 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 as set on the Activity tab. The air flow rate in m3/s is calculated as:

m3s = 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)

Scheduled natural ventilation is active at any time in the simulation when:

The air temperature in the zone is higher than the Ventilation cooling set-point temperature (as set on the Activity tab under the Environmental Comfort header) AND ...
The difference between inside and outside air temperatures (Tin - Tout) is less than 'Max ventilation in-out delta T' (below the Ventilation cooling setpoint data on the Activity tab) AND ...
The operation schedule is on at that time in the simulation.

Note: to avoid temperature control of natural ventilation set a very low Natural ventilation cooling setpoint temperature and to avoid temperature difference controlof natural ventilation set a high negative value of Max ventilation in-out delta T.

The actual natural ventilation rate at any one time in the simulation is calculated by multiplying the maximum natural ventilation rate (ac/h) by the value of the operation schedule.

When using the 'Typical workday' Timing option, natural ventilation can be either on or off. When it is on, the ventilation rate is the 'maximum natural ventilation rate'.

When using the 7/12 Timing option, it is possible to vary the natural ventilation rate between 0 and the maximum value by using daily profiles which have values between 0 and 100%.

Internal Windows, Doors and Vents

Airflow through external windows, vents, doors and holes are not modelled with Scheduled natural ventilation because the natural ventilation airflow from outside is explicitly defined (above). Internal windows, vents, doors and holes cause air to be exchanged between the two adjacent zones using the EnergyPlus 'Mixing' object. You can override operation of internal windows, vents, doors be off as follows:

Window, Vent, Door position

Override Off Action

Internal adjacent to unconditioned semi-exterior

Switch off the Natural ventilation 'On' checkbox in the occupied zone

Other internal

Unaffected by the Natural ventilation 'On' checkbox. Use the operation schedules on the Openings tab to switch off

Calculated

When the Natural ventilation model option is set to Calculated, natural ventilation is defined as follows:

Check the 'On' checkbox under the Natural Ventilation header.
On the Openings tab by setting window, door and vent opening sizes and operation times.

You can override operation of windows, vents, doors to be off as follows:

Window, Vent, Door position	Override Off Action
External	Switch off the Natural ventilation 'On' checkbox
Internal adjacent to unconditioned semi-exterior	Switch off the Natural ventilation 'On' checkbox in the occupied zone
Other internal	Unaffected by the Natural ventilation 'On' checkbox. Use the operation schedules on the Openings tab to switch off

Note that even when operation is switched off as described above (or because operation schedules on the Openings tab are off) windows, vents and doors still provide a small flow path through the crack between the opening and the surrounding surface if the Model infiltration data is switched on.

Important Notes

For cooling design calculations, natural ventilation is always handled as Scheduled even when the Natural ventilation model option is set to Calculated. In this case you can still enter natural ventilation air change rates and operation schedule data on the HVAC tab but it is used only for design simulations.
With both the Scheduled and Calculated options, natural ventilation can be overr Related Topicsidden to be off in Heating and Cooling design calculations. In Heating design calculations natural ventilation is switched off by default because it is considered to be a cooling mechanism - windows and vents will normally be closed during the coldest winter periods.

Mixed mode

You can also enter Mixed mode and data on opening Modulation under the Natural Ventilation header on the HVAC tab when using Calculated natural ventilation.

Control mode

For calculated natural ventilation you can define the control mode . The data can be entered at building, block or zone level. In the descriptions that follow:

Tout is the outdoor air temperature,

Tzone is the previous time step’s zone air temperature,

Tset is the Vent Temperature Schedule value,

Hzone is the specific enthalpy of zone air from the previous time step, and

Hout is the specific enthalpy of outdoor air.

The four allowed choices for natural ventilation Control mode are:

1-NoVent - all of the zone’s openable windows and doors are closed at all times independent of indoor or outdoor conditions. The operation schedule is ignored in this case.
2-Temperature - all of the zone’s openable windows and doors are opened if Tzone > Tout and Tzone > Tset and operation schedule allows venting.
3-Enthalpy - All of the zone’s openable windows and doors are opened if Hzone > Hout and Tzone > Tset and operation schedule allows venting.
4-Constant - Whenever an opening's operation schedule allows venting, all of the zone’s openable windows and doors are open, independent of indoor or outdoor conditions. Note that “Constant” here means that the size of each opening is fixed while venting; the air flow through each opening can, of course, vary from time step to time step. This option allows modelling of a window that is opened for fresh air regardless of inside/outside temperature/enthalpy.