Natural ventilation may be modeled for a thermal block in the proposed design when the following conditions are met:
- The thermal block does not have an air conditioning system.
- The temperature of the thermal block does not exceed the cooling setpoint temperature for more than 300 hours for the year of the simulation with just the natural ventilation system operating.
Under these circumstances, the thermal block in the proposed design is modeled with no air conditioning when the natural ventilation system maintains temperature. For periods when the space temperature is greater than the cooling setpoint, an air conditioner like the one for the baseline building is assumed to operate to maintain temperature. The fans in this simulated system cycle with loads. The corresponding thermal block in the baseline building is modeled with air conditioning and the fans operate continuously.
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Natural Ventilation Method |
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|---|---|
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Applicability |
All thermal blocks with natural ventilation |
|
Definition |
The method used to model natural ventilation. The choices will depend to some extent on the capabilities of the energy simulation program. One procedure that could be used with most energy simulation programs would be to approximate the effect of natural ventilation by scheduling a high rate of infiltration when conditions are right. The schedule would typically be developed through computational fluid dynamic software or with other software that is capable of estimating the cooling benefit of natural ventilation and relating it to climate so that the schedule can be developed. |
|
Units |
List: choices depend on the capabilities of the energy simulation program. |
|
Input Restrictions |
None. If natural ventilation is modeled as an increased infiltration rate, two inputs will be specified: 'Infiltration through building envelope' and 'Infiltration as a result of Automatically Controlled Natural Ventilation System'. |
|
Baseline Rules |
Baseline building is not modeled with natural ventilation. If natural ventilation is modeled as an increased infiltration rate in the proposed design, the component ‘Infiltration as a result of Automatically Controlled Natural Ventilation' is not modeled for the baseline building. |
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Air Flow Rate |
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|---|---|
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Applicability |
All projects with natural ventilation that use a method that require the specification of an air flow rate |
|
Definition |
The rate of air flow through the thermal block when the natural ventilation system is operating |
|
Units |
Air changes per hour or cfm |
|
Input Restrictions |
The air flow rate for the proposed design shall be determined using sound engineering methods and supporting documentation shall be provided. If not modeled directly by the simulation program, an exceptional calculation method shall be used, with proper documentation submitted. |
|
Baseline Rules |
Baseline building is not modeled with natural ventilation. |
|
Natural Ventilation Schedule |
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|---|---|
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Applicability |
All projects with natural ventilation that use a method that requires a schedule |
|
Definition |
A schedule that modifies the airflow rate through the thermal block dictates when windows can be opened to provide natural ventilation. |
|
Units |
Data structure: schedule, fractional |
|
Input Restrictions |
The schedule for the proposed design shall be determined using sound engineering methods and keyed to outdoor temperature and perhaps other conditions on the weather file used for the simulation. |
|
Baseline Rules |
The baseline building is not modeled with natural ventilation. |
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Minimum Indoor Temperature |
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|---|---|
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Applicability |
All projects with natural ventilation or mixed mode ventilation with automatic controls |
|
Definition |
The minimum indoor temperature below which natural ventilation is disabled |
|
Units |
Degrees, F |
|
Input Restrictions |
As designed |
|
Baseline Rules |
Not applicable |
|
Maximum Indoor Temperature |
|
|---|---|
|
Applicability |
All projects with natural ventilation or mixed mode ventilation with automatic controls |
|
Definition |
The maximum indoor temperature above which natural ventilation is disabled |
|
Units |
Degrees, F |
|
Input Restrictions |
As designed |
|
Baseline Rules |
Not applicable |
|
Minimum Outdoor Temperature |
|
|---|---|
|
Applicability |
All projects with natural ventilation or mixed mode ventilation with automatic controls |
|
Definition |
The minimum outdoor temperature below which natural ventilation is disabled |
|
Units |
Degrees, F |
|
Input Restrictions |
As designed |
|
Baseline Rules |
Not applicable |
|
Maximum Indoor Temperature |
|
|---|---|
|
Applicability |
All projects with natural ventilation or mixed mode ventilation with automatic controls |
|
Definition |
The maximum outdoor temperature above which natural ventilation is disabled |
|
Units |
Degrees, F |
|
Input Restrictions |
As designed |
|
Baseline Rules |
Not applicable |
Natural ventilation may be modeled for a thermal zone in the proposed design when the following conditions are met:
a. Outside air intake from natural ventilation systems shall not be less than the minimum required outdoor air ventilation rates during occupied times in the proposed building model.
b. Controls for cooling system operation and availability of natural ventilation are automatic.
c. Rating authority approves of the proposed procedure.
• In the case when the thermal zone does not have a cooling system: The thermal zone in the proposed design is modeled with no cooling when the natural ventilation system maintains temperature. For periods when the space temperature is greater than the cooling setpoint, a cooling system like the one for the baseline building is assumed to operate to maintain temperature. The fans in this simulated system cycle with loads. The corresponding thermal zone in the baseline building is not modeled with natural ventilation. The baseline building HVAC system, as defined in Standard 90.1-2019, Section G3.1.1 (Table 3 and Table 4 of this document), provides cooling to maintain thermostat setpoint.
• In the case where the thermal zone has a cooling system: In the case where the thermal zone has an installed cooling system in the proposed design, the fans are required to operate when natural ventilation is insufficient to meet cooling or outdoor air ventilation loads. The cooling system comes on when natural ventilation is insufficient to maintain the temperature setpoint.
The corresponding thermal zone in the baseline building is not modeled with natural ventilation. In the baseline building, the fans are required to run constantly during occupied periods, and cycle with 3.37 loads during unoccupied periods, same as the proposed building. The baseline building HVAC system, as defined in Standard 90.1-2019, Section G3.1.1 (Table 3 and Table 4 of this document), provides cooling to maintain thermostat setpoint.
The infiltration rate for the proposed building will be as designed. If natural ventilation is modeled as an increased infiltration rate, it will be specified through two inputs: “Infiltration through building envelope” and “Infiltration as a result of automatically controlled natural ventilation system”. Infiltration as a result of automatically controlled natural ventilation is not modeled for the baseline building.
Outputs for the proposed building with natural ventilation need to demonstrate that minimum outdoor air requirements are being met during all occupied hours.
|
Natural Ventilation Method |
|
|---|---|
|
Applicability |
All thermal blocks with natural ventilation |
|
Definition |
The method used to model natural ventilation. The choices will depend to some extent on the capabilities of the energy simulation program. One procedure that could be used with most energy simulation programs would be to approximate the effect of natural ventilation by scheduling a high rate of infiltration when conditions are right. The schedule would typically be developed through computational fluid dynamic software or with other software that can estimate the cooling benefit of natural ventilation and relate it to climate so that the schedule can be developed |
|
Units |
List: choices depend on the capabilities of the energy simulation program. |
|
Input Restrictions |
As designed. If natural ventilation is modeled as an increased infiltration rate, it shall be specified through two inputs: “Infiltration through building envelope” and “Infiltration as a result of Automatically Controlled Natural Ventilation System.” |
|
Baseline Building |
The baseline building is not modeled with natural ventilation. If natural ventilation is modeled as an increased infiltration rate in the proposed design, the component “Infiltration as a result of Automatically Controlled Natural Ventilation” is not modeled for the baseline building. NOTE: If the effect of natural ventilation is approximated by scheduling a high rate of infiltration, additional documentation needs to be provided, supporting the calculation process. |
|
Air Flow Rate |
|
|---|---|
|
Applicability |
All projects with natural ventilation that use a method that require the specification of an air flow rate |
|
Definition |
The rate of air flow through the thermal block when the natural ventilation system is operating |
|
Units |
ACH or cfm |
|
Input Restrictions |
The airflow rate for the proposed design shall be determined using sound engineering methods, and supporting documentation shall be provided. If not modeled directly by the simulation program, the exceptional calculation method shall be used, with proper documentation submitted. |
|
Baseline Rules |
Baseline building is not modeled with natural ventilation. |
|
Natural Ventilation Schedule |
|
|---|---|
|
Applicability |
All projects with natural ventilation that use a method that requires a schedule |
|
Definition |
A schedule that modifies the airflow rate through the thermal zone dictates when natural ventilation is enabled |
|
Units |
Data structure: schedule, fractional |
|
Input Restrictions |
The schedule for the proposed design shall be determined using sound engineering methods and keyed to outdoor temperature and perhaps other conditions on the weather file used for the simulation |
|
Baseline Rules |
The baseline building is not modeled with natural ventilation. |
|
Minimum Indoor Temperature |
|
|---|---|
|
Applicability |
All projects with natural ventilation or mixed mode ventilation with automatic controls |
|
Definition |
The minimum indoor temperature below which natural ventilation is disabled |
|
Units |
Degrees, F |
|
Input Restrictions |
As designed |
|
Baseline Rules |
Not applicable |
|
Maximum Indoor Temperature |
|
|---|---|
|
Applicability |
All projects with natural ventilation or mixed mode ventilation with automatic controls |
|
Definition |
The maximum indoor temperature above which natural ventilation is disabled |
|
Units |
Degrees, F |
|
Input Restrictions |
As designed |
|
Baseline Rules |
Not applicable |
|
Minimum Outdoor Temperature |
|
|---|---|
|
Applicability |
All projects with natural ventilation or mixed mode ventilation with automatic controls |
|
Definition |
The minimum outdoor temperature below which natural ventilation is disabled |
|
Units |
Degrees, F |
|
Input Restrictions |
As designed |
|
Baseline Rules |
Not applicable |
|
Maximum Indoor Temperature |
|
|---|---|
|
Applicability |
All projects with natural ventilation or mixed mode ventilation with automatic controls |
|
Definition |
The maximum outdoor temperature above which natural ventilation is disabled |
|
Units |
Degrees, F |
|
Input Restrictions |
As designed |
|
Baseline Rules |
Not applicable |