Outdoor Air Ventilation

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.

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).

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’.

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

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.