ASHRAE Standard 62.1-2019 Ventilation for Acceptable Indoor Air Quality specifies minimum ventilation and other measures to provide indoor air quality (IAQ) that is acceptable to human occupants and minimizes adverse health effects. The standard defines three unique calculation procedures to determine the ventilation requirements within rooms and at the system. The Ventilation Rate Procedure (VRP), which is the most widely adopted, is built into the platform for easy and automated calculations. Full details of the procedure can be found in Section 6.2 of ASHRAE Standard 62.1 and the accompanying User Manual.
To have a room follow the ASHRAE 62.1 Ventilation Rate Procedure calculation method, the Vent Calculation Procedure of the Room Template must be set to ASHRAE 62.1 Ventilation Rate Procedure. Once selected there are 144 default Occupancy Categories to choose from. These categories provide the default values from ASHRAE Standard 62.1 - 2019. In addition there is a Custom option that can be used to define a project specific category.
There are eight inputs available to define the air flows for the room. In the image above inputs 1, 2, and 3 control the outdoor air requirements for the room. Inputs 4, 5, and 6 control the exhaust requirements. And finally, inputs 7 and 8 define the ventilation efficiency which i used when calculated to ventilation air flow required.
All values are determined based on the occupancy category, however can be modified to meet specific project requirements.
TIP: To set exhaust airflow per room, simply enter the desired value as the Exhaust per Unit and set the Default Number of units to one.
The Ventilation Rate Procedure has different versions depending on the specifics of the system and rooms that make up the calculation. These versions are
Multi-Zone Air Systems
Single Zone Air Systems
100% Outdoor Air (DOAS) Air Systems
The most common, and the most complex, is the Multi-Zone Air System. This calculation is used for systems that serve more than one zone and that incorporate return or mixed air. Typical examples include central VAV systems, RTU, Packaged units, etc. The platform automatically maps rooms to the require calculation based on the following logic.
If a room is assigned to an exhaust only occupancy category it will go into the Exhaust Only table and the Exhaust Only calculation will be performed
If a room is not assigned to a zone it will go into the Unassigned table and the Single Zone calculation will be performed
If a room is assigned to a Local type Air System it will go into the Single Zone table and the Single Zone calculation will be performed
If a room is assigned to a Central type Air System and Loading Type For System Sizing = Ventilation Requirement it will go into a table for that system and the DOAS calculation will be performed
If a room is assigned to a Central type Air System it will go into a table for that system and the Multi-Zone calculation will be performed
Figure 6-B of the ASHRAE 62.1 User Manual provides a flow chart to determine which method and equations shall be used. Note Exhaust only is not present on this chart, however follows the logic described above.
Calculations - Exhaust Only
The calculations preformed for exhaust air flow rates are based on the Minimum Exhaust Rates found in Table 6-2 of ASHRAE Standard 62.1. There are two means of defining the exhaust requirements. First is per the room area which follows the equation:
Room Area * Exhaust Rate per Area = Exhaust Air Flow Required
The second is exhaust per unit. In this context the units are water closets, toilets, urinal, or domestic stove tops. The equation followed is:
Number of Units in Room * Exhaust per Unit = Exhaust Air Flow Required
The total for each room is assigned into the Load Model and a summary is provided to help track the building air balance. In this case the summary is for information only.
Calculations - Breathing Zone Outside Air & Air Distribution Effectiveness
There are several calculation steps in the Ventilation Rate Procedure that must be done in sequence to arrive at the required room and system level air flow requirements. The first step is calculating the Breathing Zone Outdoor Airflow. This is a combination of the ventilation required per person and per floor area. On the platform the calculation is done in three steps:
Breathing Zone OA for People = OA per person (Rp) * People (Pz)
Breathing Zone OA for Area = OA per Area (Ra) * Area (Az)
Breathing Zone OA (Vbz) = Breathing Zone OA for People + Breathing Zone OA for Area
This determines the required out air per Equation 6-1 of ASHRAE Standards 62.1
Note: Area used in the calculation is the net occupiable area defined here.
Next, the Zone Air Distribution Effectiveness must be accounted for. This is done by following ASHRAE Standard 62.1 Equation 6-2:
Zone Outdoor Airflow (Voz) = Breathing Zone OA (Vbz) / Air Distribution Effectiveness (Ez)
Because the Zone Air Distribution Effectiveness is often different when systems are in heating and cooling mode. This is handled by the platform automatically and engineers can assigned a specific effectiveness to each mode.
The calculation of the Zone Outdoor Airflow is core to all the following VRP methods.
Calculations - Single Zone
The calculations for a single zone system begin with the Breathing Zone Outside Air and Air Distribution Effectiveness. Once complete, for Single Zone Air Systems there is one ventilation zone to serve. As the zone outdoor air flow will equal the system level. Equation 6-3 of ASHRAE Standard 62.1 defines this as
Outdoor Air Intake (Vot) = Zone Outdoor Airflow (Voz)
Once calculated the values will be sent to the Load Model. In addition a summary is provided, in this case it is for information only and reports the number of systems and summed values of all the included systems.
Calculations - DOAS
The calculations for a 100% Outside Air Systems also begin with the Breathing Zone Outside Air and Air Distribution Effectiveness. Once complete, the values are summed per systems as multiple zones are being served by the same system. Luckily, since the system is 100% outside air there are no further calculations! Equation 6-4 of ASHRAE Standard 62.1 defines this as
Outdoor Air Intake (Vot) = ∑Zone Outdoor Airflow (Voz)
Once calculated the values will be sent to the Load Model. In addition a summary is provided, here the maximum of the Heating and Cooling Voz is used.
Calculations - Multi-Zone
The Multi-Zone calculation is the most complex because not all of the air delivered to each room will be outside air. In order to determine that ventilation requirements are meant the system efficiency must be considered. The calculations begin in the same way as other versions with the Breathing Zone Outside Air and Air Distribution Effectiveness.
Next, the Uncorrected Outdoor Air Intake is calculated according to Equation 6-5 of ASHRAE Standard 621.
Uncorrected Outdoor Air Intake (Vou) = Diversity (D) * ∑(Rp * Pz) + ∑(Ra * Az)
The Diversity (D) used is determined by Equation 6-6. On the platform the Diversity is assumed to be 1 by default, however engineers can adjust the expected system populate (Ps) to account for diversity at the system.
Diversity (D) = system populate (Ps) / ∑Pz
Next, the procedure provides two options for calculating the System Ventilation Efficiency which will impact the zone and system results. The Simplified Procedure defined in section 188.8.131.52 of ASHRAE 62.1 and Alternative Procedure defined in Appendix A of ASHRAE 62.1.
The System Ventilation Efficiency (Ev) is calculated depending on on the Diversity (D) value by either equation 6-7 or 6-8 of ASHRAE Standard 62.1 - 2019.
for D < 0.60 | Ev = 0.88 * D + 0.22
for D ≥ 0.60 | Ev = 0.75
Once the System Ventilation Efficiency (Ev) is known, the Design Outdoor Air Intake (Vot) can be calculated with equation 6-10.
Design Outdoor Air Intake (Vot) = Uncorrected Outdoor Air Intake (Vou) / Ev
On the platform the system level calculations can be see on the Calculation Summary panel. System People is an editable input that defines the Ps value used in equation 6-6. The resulting values are also sent to the Load Model for inclusion in the system sizes.
The System Ventilation Efficiency (Ev) is calculated depending on the lowest calculated zone ventilation efficiency (Evz). Typically this zone is called the Critical Zone. The calculation requires several additional inputs and calculations for each zone. However they begin with the Average Outdoor Air Fraction (Xs), calculated by equation A-1 of ASHRAE Standard 62.1.
Xs = uncorrected outdoor air intake (Vou) / system primary airflow (Vps)
Next a series of calculations are preformed to determine the zone ventilation efficiency (Evz). These calculations are handled by the platform in tabular format so that each columns represents a step of the process and each row represents an individual zone. The Critical Zone is automatically highlighted blue for easy identification. The additional inputs available are
Zone Discharge SA (Vdz) - This value will be automatically filled in after the simulation is run with Room Sizing ON, or can be manually edited
Zone Primary Air Fraction (Ep) - This value defaults to 1 and can be edited if there is local recirculation.
Zone 2nd Recirc. Fraction (Er) - This value defaults to 0 and can be edited to reflect the system configuration
Refer to ASHRAE Standard 62.1 Section A1.2.2 for a full description of the equations and a system diagram noting location of each. Once the simulation is run all the zone ventilation efficiencies will be known and the Critical Zone revealed. Common design practice is to adjust specifics of the critical zone to further reduce the outdoor air intake required at the system level. On the platform this can be done live within the calculation table, always referring to the calculation summary to verify the system level results.
Tip: use the toggles to decide between the simplified and alternative calculation of outdoor air intake.
Each version of the VRP calculation includes a unique pdf and Spreadsheet Report. Each room of the calculation will be included along with the system summary. Here is an example of the Multi-Zone Report
net occupiable area: the floor area of an occupiable space defined by the inside surfaces of its walls but excluding shafts, column enclosures, and other permanently enclosed, inaccessible,and unoccupiable areas. Obstructions in the space, such as furnishings, display or storage racks, and other obstructions, whether temporary or permanent, are considered to be part of the net occupiable area.
occupiable space: an enclosed space intended for human activities excluding spaces that are intended to be occupied occasionally and for short periods of time, such as storage rooms, equipment rooms, and emergency exitways.
ASHRAE Standard 62.1 Section A1.2.2 defines the following options for Zone Secondary Recirculation Fraction (Er)
Plenum return systems with secondary recirculation (for example fan-powered VAV with plenum return | ER < 1
Ducted return systems with secondary recirculation (for example fan-powered VAV with ducted return) | ER = 0
Systems with system level recirculation (for example dual-fan, dual-duct systems with ducted return) | ER = 1
Other system types | ER = 0.75