3.6.5 Zone Level Air Flow

Design Airflow

Applicability

Systems 5 through 8 and 11 that vary the volume of air at the zone level

Definition

The air delivery rate to each zone at design conditions

Units

cfm

Input Restrictions

As designed. If the UMLH in the proposed design are greater than 300, the simulation should not proceed and the user may have to modify the design airflow value.

Baseline Building

For systems 5 through 8 and 11, the software shall automatically size the system airflow to meet the baseline building loads based on

• a supply-air-to-room-air temperature difference of 17°F for systems serving laboratory spaces and 20°F for all other spaces
• or the minimum outdoor airflow rate,
• or the airflow rate required to comply with the applicable codes or accreditation standards, whichever is greater.

The baseline system airflow is determined by the load to be met by the airflow and the 20°F (11°C) temperature difference. The loads to be used would be the design load as determined by the sizing runs specified in Section G3.1.2.2, not the cooling or heating capacity of the system as determined using the sizing factors, also specified in G3.1.2.2. Using the system cooling and heating capacity will result in oversized baseline system airflows and energy cost because of the oversizing factors used in G3.1.2.2.

For zones served by baseline systems with multiple zone thermostat setpoints (systems 5 to 8), the design set points used should result in either the lowest supply air cooling setpoint or highest heating supply air heating setpoint.

If the proposed design HVAC system airflow rate based on latent loads greater than the same based on sensible loads, then the same supply-air-to-room humidity ratio difference (gr/lb) used to calculate the proposed design airflow should be used to calculate the design airflow rates for the baseline building.

For baseline systems 9 and 10, the design supply airflow rates shall be based on the temperature difference between a SAT setpoint of 105°F and the design space heating temperature setpoint, the minimum outdoor airflow rate or the airflow rate required to comply with applicable codes, whichever is greater. 

Terminal Minimum Stop

Applicability

Systems that vary the volume of air at the zone level

Definition

The minimum airflow that will be delivered to a zone. For systems with reheat (system 5 to 8) the minimum flow before reheating occurs

Units

Unitless fraction of airflow (cfm) or specific airflow (cfm/ft²)

Input Restrictions

This input must be greater than or equal to the outside air ventilation rate

Baseline Building

For systems 5 to 8, the minimum airflow for the VAV reheat boxes should be set to 30% of the zone peak supply air volume or the outside air ventilation rate, or the airflow rate required to comply with applicable codes or accreditation standards, whichever is larger. Refer to the section Zone Exhaust for requirements related to systems serving laboratory spaces.

For baseline system 11, minimum volume setpoint shall be 50% of the maximum design airflow rate, the minimum ventilation outdoor airflow rate, or the airflow rate required to comply with applicable codes or accreditation standards, whichever is larger. Fan volume shall be reset from 100% airflow at 100% cooling load to minimum airflow at 50% cooling load. In heating mode, supply air temperature shall be modulated to maintain space temperature and fan volume shall be fixed at the minimum airflow.

Systems serving laboratory spaces shall reduce the exhaust and makeup air volume during unoccupied periods to the largest of 50% of zone peak airflow, the minimum outdoor airflow rate, or the airflow rate required to comply with applicable codes or accreditation standards.

Terminal Heating Control Type

Applicability

VAV boxes with reheat

Definition

The control strategy for the heating mode. The nomenclature in simulation programs to simulate this may differ from that provided below.

Single Maximum

In the single maximum control mode, the airflow is set to a minimum constant value in both the deadband and heating mode. This airflow can vary but is typically 30% to 50% of maximum. This control mode typically has a higher minimum airflow than the minimum used in the dual maximum below, resulting in more frequent reheat.

Figure 6. Single Maximum VAV Box Control (Courtesy: Taylor Engineering)

Dual Maximum: raises the SAT as the first stage of heating, and increases the airflow to the zone as the second stage of heating.

1.     The first stage of heating consists of modulating the zone SAT setpoint up to a maximum setpoint no larger than 95ºF while the airflow is maintained at the dead band flow rate.

2.     The second stage of heating consists of modulating the airflow rate from the dead band flow rate up to the heating maximum flow rate (50% of design flow rate) while maintaining the maximum setpoint temperature.

Figure 7. Dual Maximum Control Sequence

Units

List: Single Maximum, Dual Maximum

Input Restrictions

As designed

Baseline Building

Single maximum at 30% of zone peak airflow or the outside air ventilation rate, or the airflow rate required to comply with applicable codes or accreditation standards, whichever is larger. The ventilation outdoor air and exhaust/relief dampers should be shut-off during preoccupancy building warm-up, cool down and setback except for economizer operation.

Refer to the section Zone Exhaust for requirements related to laboratory spaces.

For baseline system 11, fan volume shall be reset from 100% airflow at 100% load to minimum airflow at 50% cooling load. The minimum airflow shall be 50% of the maximum design airflow rate, the minimum ventilation rate or the airflow required to comply with applicable codes or accreditation stations, whichever is larger.

Systems serving laboratory spaces shall reduce the exhaust and makeup air volume during unoccupied periods to the largest of 50% of zone peak airflow, the minimum outdoor airflow rate, or the airflow rate required to comply with applicable codes or accreditation standards.

Fan-Powered Box Type

Applicability

HVAC zones that have fan-powered boxes

Definition

Defines the type of fan-powered induction box. This is either:

• Series, constant volume box fan; or
• Parallel, constant volume box fan

Units

List (see above)

Input Restrictions

As designed

Baseline Building

Applicable to baseline systems 6 and 8 and the fan-powered box type is parallel, with a constant volume fan

Fan Power

Applicability

HVAC zones that have fan-powered boxes

Definition

The rated power input of the fan in a fan-powered box

Units

W or W/cfm

Input Restrictions

As designed

Baseline Building

For baseline building systems 6 and 8, power is prescribed at 0.35 W/cfm

Parallel Fan-Powered Box-Induced Air Zone

Applicability

HVAC zones that have fan-powered boxes

Definition

Zone from which a series or parallel fan-powered box draws its air

Units

List (of zones)

Input Restrictions

As designed

Baseline Building

If the proposed design has a plenum, then induced air will be drawn from that plenum. If the proposed design does not have a plenum, then induced air will be drawn from the space.

Parallel Fan-Powered Box- Induction Ratio

Applicability

HVAC zones that have fan-powered boxes

Definition

The ratio of induction-side airflow of a fan-powered box at design heating conditions to the design primary airflow

Units

Ratio

Input Restrictions

As designed

Baseline Building

50%

Parallel Fan Box Thermostat Setpoint

Applicability

HVAC zones that have parallel fan-powered boxes

Definition

The temperature difference above the heating setpoint at which the parallel fan is turned on

Units

Degrees Fahrenheit (°F)

Input Restrictions

2°F above the heating setpoint schedule

Baseline Building

2°F above the heating setpoint schedule

Fan-Powered Boxes: Terminal Heating Control Type

Applicability

HVAC zones that have parallel fan-powered boxes

Definition

Fan Control: With parallel style fan-powered VAV boxes, the constant volume terminal unit fan is only on when the primary airflow is at design minimum and the zone temperature is less than 2°F above the heating setpoint schedule. When the system is scheduled to operate and the zone terminal fan is running, the box mixes plenum air with primary air.

Heating Operation: During heating mode, the terminal unit discharge air temperature is increased from minimum to the design heating temperature. Throughout occupied heating the cooling, primary airflow is kept at design minimum and the terminal unit fan is running.

Deadband Operation: The cooling primary airflow is kept at minimum airflow and the heating valve is closed. The terminal unit fan will energize as the first stage of heating when the zone temperature drops to 2°F. above heating setpoint.

Cooling Operation: As the space temperature increases, the cooling supply airflow is increased from minimum to design cooling maximum. Throughout cooling the box fan is off. To comply with Standard 90.1-2019 Section 6.5.2.1, Exception (1), the minimum primary airflow for this control logic must be no larger than 30% of the zone design cooling airflow or the minimum airflow for ventilation.

Night Cycle Heating Control: A call for heating during night cycle control shall be met by running the terminal fan and increasing the terminal unit discharge air temperature from minimum to the design heating temperature without the use of primary air.

Single Maximum Parallel Fan Powered VAV Box Control:

Figure 8. Single Maximum Control Sequence for Parallel Fan Powered VAV with Reheat Boxes

Units

List: Single Maximum, Other

Input Restrictions

As designed

Baseline Building

Single maximum with minimum airflow rate set to 30% of peak design primary airflow rate or the outside air ventilation rate, whichever is larger

Kitchen: Exhaust Hood Length

Applicability

Exhaust fans in spaces of type kitchen

Definition

The exhaust hood length

Units

ft

Input Restrictions

As designed

Baseline Building

Same as the proposed design

Kitchen: Exhaust Hood Style

Applicability

Exhaust fans in spaces of type kitchen

Definition

This input defines the style of the kitchen hood

Units

List: Wall-Mounted Canopy, Single Island, Double Island, Eyebrow, Backshelf/Passover

Input Restrictions

As designed

Baseline Building

Same as the proposed design

Kitchen: Exhaust Hood Cooking Duty

Applicability

Exhaust fans in spaces of type kitchen

Definition

The hood cooking duty as defined in Table 6.5.7.2.2 of Standard 90.1-2019

Units

List: Light Duty, Medium Duty, Heavy Duty, Extra Heavy Duty

Input Restrictions

As designed

Baseline Building

Same as the proposed design

Kitchen: Exhaust Airflow Rate

Applicability

Exhaust fans in spaces of type kitchen

Definition

Rate of exhaust from a thermal zone. Standard 90.1-2019, Section 6.5.7.2.2, requires each hood in a kitchen/dining facility with a total kitchen hood exhaust airflow rate greater than 5,000 cfm to have an exhaust rate that complies with Table 39. If a single hood, or hood section, is installed over appliances with different duty ratings, then the maximum allowable flow rate for the hood or hood section shall not exceed the Table 39 values for the highest appliance duty rating under the hood or hood section.

Table 39. Maximum Net Exhaust Flow Rate, cfm per Linear Foot of Hood Length

Units

cfm

Input Restrictions

As designed

Baseline Building

The exhaust air flow rate for the baseline building should be equal to the exhaust flow rate prescribed in Table 39

Kitchen: Demand Ventilation

Applicability

All kitchen HVAC zones

Definition

A demand ventilation system uses a light beam and a photo detector to detect the presence of smoke. It also has a temperature sensor to detect heat. The exhaust fan slows down below design levels and both exhaust and replacement airflow rates are reduced when the system detects no smoke or heat.

Units

Unitless

Input Restrictions

As designed. Exhaust fans controlled with a demand ventilation system with a minimum flow no more than 50% shall be modeled as running at 75% during all exhaust fan operating hours.

Baseline Building

Same as proposed except fan speed would not be reduced due to demand ventilation systems

Laboratory: Exhaust Minimum Airflow Rate

Applicability

All laboratory zones

Definition

Rate of exhaust from a zone

Units

cfm

Input Restrictions

As designed

Baseline Building

Same as proposed except for the unoccupied periods, where systems serving laboratory spaces shall reduce the exhaust and makeup air volume during unoccupied periods to the largest of 50% of zone peak airflow, the minimum outdoor airflow rate, or the airflow rate required to comply with applicable codes or accreditation standards.

Baseline systems serving only laboratory spaces that are prohibited from recirculating return air by code or accretion standard, the baseline system shall be modeled with 100% outdoor air.

Exhaust Fan Name

Applicability

All HVAC zones

Definition

A reference to an exhaust fan system that serves the thermal zone

Units

Text or other unique reference to an exhaust fan system defined in the secondary systems section

Input Restrictions

As designed

Baseline Building

Same as the proposed design

Exhaust Fan Operation Schedule

Applicability

All HVAC zones

Definition

Schedule indicating the pattern of use for exhaust air from the thermal zone. For laboratory exhaust hoods, this input should consider the position of fume hood sash opening. For toilets and other exhaust applications, the schedule may coincide with the operation of the exhaust fan system.

Units

Data structure: schedule, fraction

Input Restrictions

As designed

Baseline Building

Same as proposed except for the unoccupied periods, where systems serving laboratory spaces shall reduce the exhaust and makeup air volume during unoccupied periods to the largest of 50% of zone peak airflow, the minimum outdoor airflow rate, or the airflow rate required to comply with applicable codes or accreditation standards

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:

1. As designed
2. 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:

1. An air-side economizer
2. Automatic modulating control of the outdoor air damper
3. A design outdoor airflow greater than 3000 cfm

Exceptions:

1. Systems with the exhaust air energy recovery complying with Standard 90.1-2019, Section 6.5.6.1
2. Multiple-zone systems without dynamic demand control (DDC) of individual zones communicating with a central control panel
3. Systems with a design outdoor airflow less than 750 cfm

1. 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.
2. 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

Ventilation Standard

Applicability

All projects

Definition

The ventilation standard used for the calculation of minimum ventilation rate required. The ventilation rates are defined based on the ventilation standard selected. This should be specified at the project level.

Units

List. ASHRAE Standard 62.1, IMC-2012, Title-24-2013, ASHRAE 170, Other

Input Restrictions

No restrictions. Any of the defined standards can be selected.

Baseline Building

Same as proposed

The baseline rules for determining the use of DCV are defined according to Standard 90.1-2019, for any specified standard, including ‘Other’.

Ventilation Specification Method

Applicability

All HVAC zones

Definition

The method used to calculate total ventilation rates to a zone. The input must be either “Sum,” “Maximum,” or “No Ventilation.”

• Sum means that the flows calculated from the fields Outdoor Airflow per Person and Outdoor Airflow per Area will be added to obtain the zone outdoor airflow rate.
• Maximum means that the maximum flow derived from Outdoor Airflow per Person, Outdoor Airflow per Area, and Air Changes per Hour (using the associated conversions to cfm for each field) will be used as the zone outdoor airflow rate.
• No ventilation indicates that the zone doesn’t receive any outdoor air.

Units

List: Sum, Maximum, No Ventilation

Input Restrictions

As designed.

For ventilation standard specified as = “62.1” and “IMC-2012,” the only available specification method would be “Sum.” For “T24-2013,” the only available specification method is “Maximum.” For ventilation standard specified as “Other,” the specification method can be specified as either Sum or Maximum, and includes all components.

Baseline Building

Same as proposed

Zone Air Distribution Effectiveness

Applicability

All HVAC zones and spaces

Definition

The zone air distribution effectiveness (Ez) shall be no greater than the default value determined using Table 40.

Table 40. Air Distribution Effectiveness (ASHRAE Standard 62.1-2010)

Units

None

Input Restrictions

As designed

Baseline Building

Same as proposed

Ventilation Multiplier

Applicability

All projects

Definition

The ventilation multiplier is defined at the project level. This multiplier is applied to the user’s design component ventilation rates (Ra and Rp) to uniformly increase the component ventilation rates entered. The principal purpose of the multiplier is to facilitate simulating a uniform increase in proposed design ventilation rates, such as for buildings pursuing the LEED credit for 30% increased ventilation.

Units

No units

Input Restrictions

As designed. The default value is 1.

Baseline Building

The ventilation multiplier is always 1 for the baseline building

Ventilation Rates

Applicability

All HVAC zones

Definition

The method used to calculate total ventilation rates to a zone. The input must be either flow/person, flow/area, flow/zone, air changes/hour, sum, or maximum.

• Flow/Person (Rp) means the program will use the input from the field Outdoor Airflow per Person and the actual zone occupancy to calculate a zone outdoor airflow rate.
• Flow/Area (Ra) means that the program will use the input from the field Outdoor Airflow per Zone Floor Area and the actual zone floor area as the zone outdoor airflow rate. Flow/Zone means that the program will use the input of the field Outdoor Airflow per Zone as the zone outdoor airflow rate.
• Air changes per hour (ACH) means that the program will use the input from the field Air Changes per Hour and the actual zone volume (divided by 3600 seconds per hour) as the zone outdoor airflow rate.
• For non-occupied spaces, the values for Ra, Rp and ACH can be zero for the proposed and baseline building. Software tools can allow a check-box for indicating spaces that would be non-occupied and hence are permitted to have a value for zero for Ra and Rp.

Units

List: Flow/Person, Flow/Area, Air Changes per Hour

Input Restrictions

As designed.
 

If the user specifies ventilation standard as ‘Other’ and DCV is used, a non-zero value for Rp must be specified for the ventilation air flow to vary with occupancy.

Baseline Building

The ventilation rates for the baseline building are determined by the ventilation standard and the space function.

For zones/spaces with ventilation standard specified as ‘Other’

• The code minimum ventilation rates are required to be specified by the user.
• If DCV controls are not specified for the proposed but are required for the baseline, the ventilation rate for the area component is determined based on Standard 62.1 and the balance of the user’s total design air flow is allocated to the Rp component for the baseline.

Total Design Zone Ventilation Airflow

Applicability

All HVAC zones

Definition

The total design outdoor air supplied to a zone (V_z-design).

Minimum outdoor air supplied to a zone (V_bz-min) is calculated based on the user input values of the ventilation multiplier (V_m), Ez, Ra and Rp. The method used to calculate total minimum ventilation airflow to a zone is specified below-

Where,

V_bz-min = Total Breathing Zone Outdoor Airflow (CFM)

V_m = Ventilation Multiplier

R_p = Outdoor Airflow Rate per Person (CFM/person)

R_a = Outdoor Airflow Rate per unit Area (CFM/ft²)

P_z = Zone Population: The number of people in the zone during typical occupancy

A_z = Zone Floor Area (ft²): the net occupiable floor area of the zone.

Units

List: CFM

Input Restrictions

As designed. The total design ventilation airflows should also include the impact of Ez and the ventilation multiplier. The user entered value for space/zone design ventilation airflow is required to be within 3% of the value calculated from the user entered R_a, R_p, E_z and V_m values. The user is required to modify the values for R_a, R_p or E_z such that the total airflow calculated from the components and the user defined airflow is within the tolerance limit. Similarly, if DCV controls are used in the proposed building design, the user entered value for V_z-design should be within 3% tolerance of the value calculated by the design area rate component (Ra) and Ez.

Exhaust airflows are checked at the building story level. If the sum of proposed total design ventilation airflows for all spaces on a building story level (excluding spaces with ventilation standard specified as ‘Other’) is not within 10% of the calculated total code minimum ventilation airflow. To proceed, either the ventilation airflows for spaces that are under-ventilated, need to be increased or alternatively, the ventilation standard should be specified as ‘Other’ for spaces that are under-ventilated. For zones/spaces with DCV controls, the minimum ventilation airflow is the airflow when the space is unoccupied, calculated using the area component and Ez.

Baseline Building

The baseline design ventilation airflow rates for all spaces/thermal zones equal the proposed with the exception of the following:

1. The zone distribution effectiveness for the proposed building (Ez) > 1. In this case, Ez = 1 for the baseline building.
2. The proposed design ventilation airflow, on a building story basis, exceeds the code minimum ventilation airflow for the building story. In this case, the baseline ventilation rates will be calculated by uniformly reducing the proposed rates such that the total ventilation airflow to spaces on the building story equals the code minimum flow. This maintains a proportional distribution of ventilation air to spaces/zones on the building story between the proposed and baseline designs. Spaces with ventilation standard specified as “Other” are excluded from adjustments, and the baseline design ventilation rate equals the proposed.
3. The proposed design ventilation multiplier is greater than 1. In this case, the baseline design ventilation airflow is not increased by the multiplier. Instead, the total baseline design ventilation airflow is adjusted, as described in exception b, to be equal to the code minimum ventilation airflow.
4. The total proposed design ventilation airflow (excluding spaces with ventilation standard specified as “Other”), on a building story basis, is greater than 110% of the total building story exhaust airflow. In this case, the baseline design ventilation airflows will be calculated by uniformly reducing the proposed inputs (excluding spaces with ventilation standard specified as “Other”) such that the total ventilation airflow on the building story equals 110% of the total building story exhaust airflow. This maintains a proportional distribution of ventilation air (at design conditions) to spaces on the building story, while allowing sufficient additional ventilation air for pressurization.

Total System Design Ventilation Airflow

Applicability

All HVAC systems.

Definition

The design outdoor air intake at the system level (V_s-design)

Units

CFM

Input Restrictions

As designed

Software tools are required to verify the input value for V_s-design. For all the zones served by the system, software tools are required to sum the zone level ventilation (V_bz-min) to calculate the minimum ventilation at the system level (V_s-min). If the design outdoor air intake (V_s-design) is less than the minimum ventilation airflow (V_s-min), the input should be flagged and the user should be required to increase the value for V_s-design until it is equal to or greater thanV_s-min.

Baseline Building

The total system ventilation airflow for the baseline building is the sum of the zone level ventilation for the baseline building

System Ventilation Efficiency

Applicability

All HVAC systems

Definition

The efficiency of the ventilation system (Ev). This is the ratio of the design ventilation airflow to the minimum system ventilation airflow.

Units

Ratio (unitless)

Input Restrictions

Calculated by software tool

Software tools are required to calculate and verify the input value for Ev. The maximum value of Ev=1 (for single zone systems). If Ev is greater than 1, an error should be displayed to the user and the user is required to increase the value of V_s-design until the value of Ev is less than or equal to 1.

Baseline Building

Same as proposed for all zones served by the proposed system. In the scenario where the building has a multizone system in the proposed case, but single zone system in the baseline case, all the zones served by the Multizone system in the proposed case have Ev same as the proposed system.

Similarly, if the proposed system is single zone and the baseline building has multizone systems, then the corresponding zones would have Ev specified as 1, same as the proposed building.

This approach ensures that the ventilation rates are the same between the baseline and the proposed building.