Scheduled Natural Ventilation Data

HVAC tab in model data

See the Scheduled natural ventilation and infiltration tutorial

Summary of Scheduled Natural Ventilation data definition

 

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

 

• Check the On checkbox under the Natural Ventilation header.
• Select the Outside air definition method
• Set the maximum zone natural ventilation rate in air changes per hour (if flow type is 1-By zone), or in m3/s or ft3/min (if the flow type is 3-Design flow rate).
• Select the operation schedule.
• Enter the relevant control parameters on the HVAC tab and on the Activity tab.

Find out more about Natural Ventilation Modelling under Modelling Advice.

Natural Ventilation

Natural ventilation On

Check the On checkbox under the Natural Ventilation header to activate natural ventilation. This causes all of the natural ventilation data on the HVAC tab to be accessible.

Outside air definition method

The Outside air definition method allows you to select the method to be 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 design air flow rate is calculated from the ac/h data using:

V_design = 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 design air flow rate in m3/s is calculated as:

V_design = 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)   

• 3-Design flow rate - where the maximum natural ventilation rate is entered directly in m3/s or ft3/min.

The actual ventilation rate in m3/s is calculated at each time step of the simulation using the equation below:

Ventilation = V_design F_schedule [A + B (T_in - T_out) + C . Wind Speed + D (Wind Speed ²)]

Each of the parameters A-D in the equation above are explained below under Delta T and Wind Speed Coefficients.

F_schedule is described below under Operation > Schedule.

Operation

Schedule

Select the schedule used to modify the maximum design natural volume flow rate (Vdesign) (see Design Flow Rate Calculation Method field and related subsequent fields). This fraction between 0.0 and 1.0 is included as F_schedule in the above equation.

Outdoor Temperature Limits

Outdoor min temperature control

Check this checkbox if you wish to define a restriction on natural ventilation operation when outdoor temperature drops below a setpoint temperature. Use this option to help avoid over-cooling a space, which could result in a heating load.

Min temperature definition

There are 2 ways to define the outdoor min temperature:

 

• 1-By value - use this option when the setpoint does not change with time in which case you can enter a fixed setpoint temperature.
• 2-By schedule - use this option when the setpoint changes with time in which case you can select a schedule of time-varying temperature setpoints.

Outdoor min temperature

This is the fixed outdoor temperature (in °C or °F) below which ventilation is shut off. This control is visible when the 1-By value option is selected for Min temperature definition.

Outdoor min temperature schedule

This is the schedule of outdoor temperatures (in °C) below which ventilation is shut off. This control is visible when the 1-By schedule option is selected for Min temperature definition.

 

See also Defining setpoint temperature schedules.

Outdoor max temperature control

Check this checkbox if you wish to define a restriction on natural ventilation operation when outdoor temperature rises above a setpoint temperature. Use this option to help avoid over-heating a space, which could result in a cooling load.

Max temperature definition

There are 2 ways to define the outdoor max temperature:

 

• 1-By value - use this option when the setpoint does not change with time in which case you can enter a fixed setpoint temperature.
• 2-By schedule - use this option when the setpoint changes with time in which case you can select a schedule of time-varying temperature setpoints.

Outdoor max temperature

This is the fixed outdoor temperature (in °C or °F) above which ventilation is shut off. This control is visible when the 1-By value option is selected for Max temperature definition.

Outdoor max temperature schedule

This is the schedule of outdoor temperatures (in °C) above which ventilation is shut off. This control is visible when the 1-By schedule option is selected for Max temperature definition.

 

See also Defining setpoint temperature schedules.

Delta T Limits

Delta T limit control

Check this option to limit natural ventilation operation based on the temperature difference between the indoor and outdoor air dry-bulb temperature. It allows ventilation to be stopped if the temperature outside is too warm and could potentially heat the space.

Uncheck this option to keep the ventilation on even if the outdoor temperature is greater than the indoor temperature. This is useful for uncontrolled natural ventilation (open windows) or as a way to estimate the effect of required ventilation air for load calculations.

Delta T definition

There are 2 ways to define the Delta T to be used in temperature difference control:

 

• 1-By value - use this option when the Delta T does not change with time in which case you can enter a fixed value.
• 2-By schedule - use this option when the Delta T changes with time in which case you can select a schedule of time-varying Delta T values.

Delta T

This is the temperature difference (in °C or °F) between the indoor and outdoor air dry-bulb temperatures below which ventilation is shut off. This field allows ventilation to be stopped if the temperature outside is too warm and could potentially heat the space. For example, if the user specifies a delta temperature of 2°C, ventilation is assumed to be available if the outside air temperature is at least 2°C cooler than the zone air temperature. If the outside air dry-bulb temperature is less than 2°C cooler than the indoor dry-bulb temperature, then ventilation is automatically turned off. The values for this field can include negative numbers. This allows ventilation to occur even if the outdoor temperature is above the indoor temperature.

 

The Delta Temperature is used in the code in the following way:

 

IF ((IndoorTemp - OutdoorTemp) < DeltaTemperature) Then ventilation is not allowed.

 

For example:

 

Value of Natural ventilation delta T	Effect
0	Natural ventilation only operates when OutdoorTemp < IndoorTemp
-100	Natural ventilation unrestricted by Inside-Outside delta T
2	Natural ventilation only operates when OutdoorTemp + 2 < IndoorTemp

Delta T schedule

This is the schedule of temperature difference values (in °C) between the indoor and outdoor air dry-bulb temperatures below which ventilation is shut off. See above Delta T description for more details.

Delta T and Wind Speed Coefficients

The coefficients below can be used to set up inside to outside Delta T and wind speed dependency of the Scheduled natural ventilation rates using the equation at the end of the Outside air definition method section.

Technical

 

The question of typical values for these coefficients is subject to debate. Ideally, one should do a detailed analysis of the ventilation situation and then determine a custom set of coefficients using methods such as those laid out in Chapter 26 of the ASHRAE Handbook of Fundamentals. The defaults are 1,0,0,0 which give a constant volume flow of ventilation under all conditions.

 

The equation must always yield a non-negative results; negative values are set to 0.0. BLAST (one of the EnergyPlus predecessors) used the following values as defaults: 0.606, 0.03636, 0.1177, 0. These coefficients produce a value of 1.0 at 0C deltaT and 3.35 m/s (7.5mph) wind speed, which corresponds to a typical summer condition. At a winter condition of 40C deltaT and 6 m/s (13.4 mph) wind speed, these coefficients would increase the ventilation rate by a factor of 2.75.

 

In DOE-2 (the other EnergyPlus predecessor), the air change method defaults are (adjusted to SI units) 0, 0, 0.224 (wind speed), 0. With these coefficients, the summer conditions above would give a factor of 0.75, and the winter conditions would give 1.34. A wind speed of 4.47 m/s (10 mph) gives a factor of 1.0.

 

The source of the BLAST defaults is noted in the BLAST documentation as: "Empirical equation and the coefficient default were determined from ASHRAE journal articles and other data on the effects of outdoor weather conditions." The source of the DOE-2 defaults is based on examining the infiltration relationships described in the ASHRAE Handbook of Fundamentals.

 

The local outdoor dry-bulb temperature used in the above basic equation (T_out) is typically a function of the height of the zone centroid above ground. The local outdoor dry-bulb temperature calculation procedure is described in the “Local Outdoor Air Temperature Calculation” section of the Engineering Reference.

 

The local outdoor wind speed used in the above basic equation (Wind Speed) is also a function of the height of the zone centroid above ground. The local outdoor wind speed calculation procedure is described in the “Local Wind Speed Calculation” section of the Engineering Reference.

Constant

This constant term coefficient is the “A” parameter in the above ventilation equation. It is part of the user specified modifying parameters that are a function of environmental factors. This parameter, however, is a constant under all conditions and is not modified by any environmental effect. As a result, it is dimensionless.

Temperature

The temperature term coefficient is the “B” parameter in the above ventilation equation. It is part of the user specified modifying parameters that are a function of environmental factors. This parameter is modified by the temperature difference between the outdoor and indoor air dry-bulb temperatures. The units for this parameter are inverse Celsius.

Velocity

The velocity term coefficient is the “C” parameter in the above ventilation equation. It is part of the user specified modifying parameters that are a function of environmental factors. This parameter is modified by the speed of wind being experienced outside the building. The units for this parameter are s/m.

Velocity squared

The velocity squared term coefficient is the “D” parameter in the above ventilation equation. It is part of the user specified modifying parameters that are a function of environmental factors. This parameter is modified by square of the speed of wind being experienced outside the building. The units for this parameter are s2/m2.

Summary of Scheduled Natural Ventilation Operation Rules

Scheduled natural ventilation is active at any time in the simulation based on the rules in the table below.

Where:

T_in is the inside dry bulb air temperature

T_out is the outside dry bulb air temperature

Note: The Outdoor temperature and Delta T control rules described on this page can be used in combination with any indoor temperature min/max controls that may also have been selected on the activity tab to set up multiple restrictions on natural ventilation operation.

Tip: To ensure that natural ventilation operates regardless of temperature inside and outside you should uncheck all of the checkbox options listed on the "Condition" column in the above table.

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 ZoneMixing object. The way this works can be defined on the Advanced tab of the Model options dialog under the Natural Ventilation > Scheduled headers.

In addition, you can override operation of internal windows, vents and doors to be off as follows: