3.6.5.4 Outdoor Air Ventilation | COMNET

Ventilation Source
Applicability	All HVAC zones
Definition	The source of ventilation for an HVAC system. The choices are: Natural (by operable openings) Mechanical (by fan)
Units	List: natural or mechanical
Input Restrictions	As designed.
Baseline Rules	For residential units, set to natural, for all other occupancies, set equal to mechanical.

Ventilation Control Method
Applicability	All HVAC zones
Definition	The method used to determine outside air ventilation needed for each hour in the simulation. This information is reported to the system serving the zone. The method of controlling outside air at the system level in response to this information is discussed under secondary systems. Options at the zone level are: Demand control ventilation: Occupant sensors: When the space is occupied, the outside air requirement is equal to the design ventilation rate, otherwise, the outside air requirement is the minimum ventilation rate. CO₂ sensors in the space: The outside air is varied to maintain a maximum CO₂ concentration in the space. This shall be approximated by multiplying the ventilation rate per occupant times the number of occupants for that hour. Turnstiles to determine the number of occupants in the space (appropriate for theatres, etc.): The outside air requirement is modulated based on the number of occupants in the space, based on the turnstile counts. Fixed ventilation rate. Outside air is delivered to the zone at a constant rate and is equal to the design ventilation rate (see above).
Units	List (see above)
Input Restrictions	As designed
Baseline Rules	The baseline building shall use the same ventilation control method as the proposed design except for the following: When the proposed design in systems with outdoor air capacity less than or equal to 3000 cfm serving areas with an average design capacity of 100 people per 1000 ft² or less use demand control ventilation, the baseline building shall be modeled as fixed ventilation (G3.1.2.5 exception 1).

Design Ventilation Rate
Applicability	All HVAC zones
Definition	The quantity of ventilation air that is provided to the space for the specified thermal block at maximum occupancy.
Units	cfm or cfm/occupant
Input Restrictions	As designed. Appendix B provides default values.
Baseline Rules	Same as the proposed design except in the following cases: When designing systems in accordance with ASHRAE Standard 62, where the proposed design has a zone air distribution effectiveness greater than 1.0, the baseline ventilation shall be calculated using the proposed design ventilation rate procedure calculation with the zone air distribution effectiveness changed to 1.0 (G3.1.2.5 exception 2). If the proposed design ventilation rate exceeds that required by the rating authority or building official, then the baseline design ventilation rate shall be only what is required by the rating authority or building official, whichever is greater (G3.1.2.5 exception 3). For baseline systems serving only laboratory spaces that are prohibited from recirculating return air by code or accreditation standards, the baseline system shall be modeled as 100% outdoor air (G3.1.2.5 exception 4). For systems serving kitchens, the minimum outside air ventilation rate is the exhaust air ventilation rate minus available transfer air. Refer to building description ‘Kitchen Exhaust’.

Minimum Ventilation Rate
Applicability	All HVAC zones
Definition	The minimum quantity of ventilation air that must provided to the space when the HVAC system is operating. This ventilation rate must be provided whether or not the space is occupied.
Units	cfm or cfm/ft²
Input Restrictions	As designed
Baseline Rules	Same as the proposed design

Exhaust Fan Schedule
Applicability	All HVAC zones
Definition	Demand control ventilation (DCV) is required for spaces larger than 500 ft2 and with a design occupancy for ventilation of greater than 40 people per 1000 ft2 of floor area and served by systems with one or more of the following: a.an air-side economizer, b.automatic modulating control of the outdoor air damper, c.a design outdoor airflow greater than 3000 cfm. Exceptions: a.Systems with the exhaust air energy recovery complying with Standard 90.1 2010 Section 6.5.6.1. b.Multiple-zone systems without DDC of individual zones communicating with a central control panel. c.Systems with a design outdoor airflow less than 1200 cfm. Spaces where the supply airflow rate minus any makeup or outgoing transfer air requirement is less than 1200 cfm.
Units	CFM or CFM/occupant
Input Restrictions	As designed. DCV is a mandatory requirement for spaces larger than 500 sq.ft, with a design occupancy of more than 40 people per 1000 sq.ft unless any of the exceptions specified above apply.
Baseline Rules	Same as proposed for spaces or systems where DCV is required as described above. For other spaces and systems DCV is not modeled.

90.1-2019

Outdoor Air Ventilation Source
Applicability	All HVAC zones
Definition	The source of outdoor air ventilation for an HVAC system. The choices are: Natural (by operable openings) Mechanical (by fan)
Units	List: Natural or mechanical
Input Restrictions	As designed
Baseline Building	Ventilation is set to mechanical for the baseline building

Minimum Outdoor Air Ventilation Rate
Applicability	All HVAC zones
Definition	The minimum quantity of outdoor ventilation air that must be provided to the space when it is occupied
Units	cfm or cfm/ft²
Input Restrictions	As designed
Baseline Building	Minimum ventilation system outdoor air intake flow shall be the same for the proposed and baseline building designs. Except in the following conditions: Demand control ventilation shall not be modeled in zones served by systems with outdoor air less than 3000 cfm and occupant density of 100 people per 1000 ft2 or less. If the minimum outdoor air intake flow in the proposed design is provided in excess of the amount required by the rating authority or building official, then the baseline building design shall be modeled to reflect the greater of that required by the rating authority or building official and will be less than the proposed design. For systems serving kitchens, the minimum outside air ventilation rate is the exhaust air ventilation rate minus available transfer air. Refer to building descriptor Kitchen Exhaust. For systems serving laboratories, refer to building descriptor Laboratory Exhaust, Section 3.6.5.3. When designing systems in accordance to Standard 62.1, Section 6.2, “Ventilation Rate Procedure”. Refer to Section 3.6.5.5 of this document for requirements.
Example	Example 1: Should DCV be required in the following case? Can a savings credit be taken? Proposed Design: Every other floor DOAS air handlers with 8,000cfm of OSA controlled by demand controlled ventilation. Baseline: System 7 VAV, floor by floor with 4,000cfm of OSA Answer: No the design does not meet the exceptions listed above, therefor both designs should be modeled with DCV. If the design airflow of the system in question was less than 3000 CFM then DCV would only be modeled in the proposed case and credit could be taken.

Design Outdoor Air Ventilation Rate and Schedule
Applicability	All HVAC zones
Definition	The quantity of outdoor air ventilation that is provided to the space for the specified thermal zone at maximum occupancy
Units	cfm or cfm/occupant
Input Restrictions	The outdoor air ventilation rate would be as designed. Outdoor air ventilation schedule for proposed building can be either of the two: a. As designed b. The outdoor air ventilation schedule follows the HVAC availability schedule. In this case outdoor air isn’t supplied during unoccupied times or during night cycling operation. Some proposed building designs might bring in ventilation air during unoccupied hours for night flush or economizer operation. Due to these reasons, an option for user input of schedule for outdoor air availability is provided.
Baseline Building	Same as proposed, except: When designing systems in accordance with ASHRAE Standard 62.1, Section 6.2, Ventilation Rate Procedure, where the proposed design has a zone air distribution effectiveness greater than 1.0, the baseline ventilation shall be calculated using the proposed design ventilation rate procedure calculation with the zone air distribution effectiveness changed to 1.0. Refer to Section 3.6.5.5 for details on the ventilation rate procedure. If the proposed design ventilation rate exceeds that required by the rating authority or building official, then the baseline design ventilation rate shall be only what is required by the rating authority or building official, whichever is greater.

Ventilation Control Method
Applicability	All HVAC zones
Definition	The method used to determine outside air ventilation needed for each hour in the simulation. This information is reported to the system serving the zone. The method of controlling outside air at the system level in response to this information is discussed under secondary systems. Options at the zone level are: Occupant sensors: When the space is occupied, the outside air requirement is equal to the design ventilation rate; otherwise, the outside air requirement is the minimum ventilation rate. CO₂ sensors in the space: The outside air is varied to maintain a maximum CO₂ concentration in the space. This shall be approximated by multiplying the ventilation rate per occupant by the number of occupants for that hour. (When turnstile counts are used to automatically adjust ventilation levels based on occupancy, this method may also be used.) Fixed ventilation rate: Outside air is delivered to the zone at a constant rate and is equal to the design ventilation rate (see above).
Units	List (see above)
Input Restrictions	As designed
Baseline Building	Baseline building shall be modeled with a fixed ventilation rate which would be equal to the design ventilation rate (see above). No ventilation control methods shall be modeled for the baseline building design except for the following: Demand control ventilation shall be modeled in zones served by systems with outdoor air greater than or equal to 3000 cfm and occupant density of 100 people per 1000 ft2 or less.

Design Ventilation Rate: Demand Control Ventilation
Applicability	All HVAC zones
Definition	Demand control ventilation is required for spaces larger than 500 ft² and with a design occupancy for ventilation of greater than or equal to 25 people per 1000 ft² of floor area and served by systems with one or more of the following: a. An air-side economizer b. Automatic modulating control of the outdoor air damper c. A design outdoor airflow greater than 3000 cfm Exceptions: a. Systems with the exhaust air energy recovery complying with Standard 90.1-2019, Section 6.5.6.1 b. Multiple-zone systems without dynamic demand control (DDC) of individual zones communicating with a central control panel c. Systems with a design outdoor airflow less than 750 cfm a. Spaces where >75% of the space design outdoor airflow is required for makeup air that is exhausted from the space or transfer air that is required for makeup air that is exhausted from other spaces. b. Spaces with one of the following occupancy categories as defined in ASHRAE Standard 62.1: correctional cells, daycare sickrooms, science labs, barbers, beauty and nail salons, and bowling alley seating.
Units	cfm or cfm/occupant
Input Restrictions	As designed. DCV is a mandatory requirement for spaces larger than 500 ft², with design occupancy of more than 25 people per 1000 ft² unless any of the exceptions specified above apply.
Baseline Building	DCV is not modeled for the baseline building except for zones served by systems with outdoor air greater than or equal to 3000 cfm and occupant density of 100 people per 1000 ft² or less