4–91CDS-PRM001-EN • TRACE 700 User’s Manual System Design OptionsASHRAE Standard 62.1

ASHRAE Standard 62.1

ASHRAE Standard 62.1 prescribes minimum breathing zone ventilation rates and a calculation procedure to find the minimum intake airflow needed for different ventilation systems. These rates and procedures must be used to find the design outdoor air intake flow, which establishes the required capacity of the mechanical system equipment.

The first step is to determine the minimum ventilation that is required for each different space type. This is also known as the zone level procedure. The ventilation requirements for individual space types can be found in Table 6-1 of Standard 62.1.

Once the minimum ventilation requirement for each space type has been determined, equation 6-1 of the standard is used to calculate the required amount of airflow for the breathing zone. The effectiveness of the air mixture within the space, called the Zone Effectiveness, is determined based on the position of the supply diffusers and return air grilles in each zone. This factor also accounts for any ventilation that would not be used within the breathing zone that could be recirculated into the space.

The zone level procedure concludes with the solving of equation 6-2 to determine the required outdoor air for each zone. This is determined by dividing the Breathing Zone Outdoor Airflow by the Zone Effectiveness. This process must be completed for each zone that will comply with Standard 62.1.

Now that each zone level ventilation requirement has been determined, the system level ventilation can be calculated, which is also known as the Outdoor Air Intake for the system. This procedure varies for different system types. For single zone systems, equation 6-3 indicates that the ventilation requirement will be identical for both the zone and the system because they are one and the same. For 100% Outdoor Air systems (also known as Dedicated Outdoor Air systems within TRACE), equation 6-4 is used, which sums the zone level outdoor airflow to derive the system level. For multiple zone, recirculating systems (i.e., variable-air-volume reheat systems), sections

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6.2.5.1 through 6.2.5.4 are used to determine the required system level ventilation. Once the system level ventilation has been determined, the procedure is complete.

Application considerations■ ASHRAE Standard 62.1 may not be as stringent as local codes.

You can create new airflow requirements using the Internal

Loads and Airflows library.

■ The minimum discharge airflow for each space is critical in determining the system level ventilation requirements when using ASHRAE Standard 62.1 calculation methods. The ASHRAE Standard 62.1 calculations in TRACE assume that each VAV box is at the minimum reheat setting when determining the critical fraction of outside air for the system. This can be conservative if all VAV reheat minimums are set to the same value or when utilizing the default reheat minimum that comes with each TRACE system selection.

In reality, the expected minimum airflow at the worst case outside air condition analyzed will likely be greater than the reheat minimum for most VAV boxes when in cooling mode. Adjusting the reheat minimums in each VAV box to the expected minimum values can reduce required outside air intake; however, in most cases, this will increase reheat energy when performing an energy simulation.

■ Designers should account for occupant diversity in cases where the design population is significantly larger than what the actual population will be in the building.

■ To ensure that each space receives its specified ventilation airflow by automatically increasing VAV reheat minimum settings when they are set lower than the ventilation requirement, check the Force VAV minimum always >= nominal ventilation

during design check box on the Change Load Parameters screen.

4–93CDS-PRM001-EN • TRACE 700 User’s Manual System Design OptionsASHRAE Standard 62.1

Related reading■ “Addendum 62N Breathes New Life into ASHRAE 62,” Engineers

Newsletter (volume 33, number 1)

■ “CO2-Based Demand-Controlled Ventilation with ASHRAE 62.1-2004,” Engineers Newsletter (volume 34, number 5)

■ “Potential ASHRAE Standard Conflicts—Indoor Air Quality and Energy Standards,” Engineers Newsletter (volume 37, number 4)

Sample scenarioASHRAE Standard 62.1 ventilation requirements will be applied to an elementary school to ensure proper space ventilation for the occupants.

Note: Ventilation requirements can be applied either in the Airflows tab of Create Rooms or in the Ventilation templates. For a discussion of templates, see “Using templates” on page 6–108.

1 In the Apply ASHRAE

Std 62.1-2004/2007 field, select Yes to set the zone-level outside air requirement per the standard.

2 Select the appropriate space type. These values are from Table 6.1 of Standard 62.1.

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3 Select the appropriate cooling and heating effectiveness numbers. Be sure to specify a recirculation rate.

4 Click Advanced for each system that will have outside air calculations based on Standard 62.1.

5 Select the appropriate Standard 62.1 calculation.

Note: The ventilation reset option allows system outdoor intake dampers to dynamically reset ventilation airflow for hourly changing system loads and airflows.

Note: If the recirculation effectiveness is unknown, select Default based on system type. TRACE has a default value for each available airside system.

4–95CDS-PRM001-EN • TRACE 700 User’s Manual System Design OptionsASHRAE Standard 62.1

Additional items

1 The ASHRAE Std62 Max Vent (Z) Ratio Allowed field can be used to optimize the trade-off between reheat and outside air intake. Spaces that have lesser loads and airflows may have outside air requirements that force them to become a critical factor in the Standard 62.1 ventilation calculations. By examining the Z fractions in the ASHRAE Std 62.1 report, users can set the maximum outside air fraction for the system such that the program automatically adjusts the reheat minimum for those spaces to match the maximum outside air fraction (Z). In doing so, zones that have their reheat minimum adjusted based on this constraint will see larger minimum airflows and an increase in reheat energy necessary to maintain space conditions. However, the resultant system outside air intake can be reduced and therefore may result in overall energy savings. An iterative process may be required to determine the optimal ratio.

2 There is an additional option that is available when ASHRAE Std

62.1-2004/2007 w/ Vent Reset is selected. The CO2-based

Demand Controlled Ventilation (DCV) is enabled and can be set to None, Proportional Control, or Single Setpoint.

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Proportional Control is detailed in Appendix A of Standard 62.1-2004.

a When Proportional Control is selected, the outdoor air controller will adjust the outdoor air intake flow proportionally between the minimum ventilation flow and the design ventilation flow.

b When Single Setpoint is selected, the intake airflow is adjusted to maintain the indoor CO2 concentration at the minimum CO2 concentration in the space for any population. If the OA damper reaches the minimum outdoor air intake flow and the population in the zone continues to drop, the OA damper remains at the minimum outdoor air intake flow as specified in the DCV Minimum OA Intake field on the Create

Rooms – Airflows screen.

The Outdoor Carbon Dioxide Level, which defaults to 400 ppm for each weather location, is located on the Weather Overrides screen. This value can be changed if desired.

CO2 sensor locations must be specified within Create Rooms or the Rooms template.