When to use: Use to size minimum outdoor air for a zone with the ASHRAE 62.1 Ventilation Rate Procedure: Vbz = Rp·Pz + Ra·Az, combining a per-person rate (Rp) and a per-area rate (Ra) from the space type. For demand-controlled ventilation (DCV), the steady-state indoor CO₂ at the design per-person airflow is Css = Cout + N·10⁶ / Vp, where N is the per-person CO₂ generation (≈0.0084 cfm sedentary) and Vp is the per-person breathing-zone airflow (Rp). Keep the equilibrium CO₂ near ~1000 ppm (≈ Cout + 700) so DCV ramps outdoor air before occupants notice stuffiness.
This tool calculates the minimum outdoor air required for a zone using the ASHRAE 62.1 Ventilation Rate Procedure and determines the steady-state CO2 setpoint for demand-controlled ventilation (DCV) systems. Engineers use it to size OA dampers, program DCV CO2 setpoints, and verify compliance with ventilation codes.
The ASHRAE 62.1 Ventilation Rate Procedure calculates the breathing-zone outdoor airflow as: Vbz = Rp × Pz + Ra × Az, where Rp is the per-person outdoor air rate (cfm/person), Pz is the design occupant count, Ra is the per-area outdoor air rate (cfm/ft²), and Az is the zone floor area in square feet. For an office, Rp = 5 cfm/person and Ra = 0.06 cfm/ft² per ASHRAE 62.1 Table 6.2.2.1.
For demand-controlled ventilation, the steady-state indoor CO2 concentration at design per-person ventilation is: Css = Cout + (N × 10⁶) / Vp, where N is the per-person CO2 generation rate (approximately 0.0084 cfm/person for sedentary activity), Vp is the per-person breathing-zone airflow (Rp), and Cout is the outdoor CO2 concentration (approximately 400 ppm ambient). The DCV controller modulates the OA damper to maintain CO2 near the Css setpoint, typically targeting 1000 ppm (outdoors + 600 ppm above ambient).
ASHRAE 62.1 is the primary ventilation standard for commercial buildings, mandatory in most US jurisdictions. Section 6.2.7 of ASHRAE 62.1 requires DCV for zones with design occupant density exceeding 25 people per 1000 ft² and systems with design supply airflow exceeding 3000 cfm serving those zones. IMC (International Mechanical Code) Section 403 references ASHRAE 62.1 for minimum ventilation rates. ASHRAE 90.1 Section 6.4.3 also requires DCV in certain occupancy types to reduce energy consumption from unnecessary ventilation.
The CO2 setpoint for DCV should target approximately 700–1100 ppm above outdoor levels, which corresponds to 1100–1500 ppm absolute with typical 400 ppm outdoor air. Setting the DCV setpoint too high (above 1100 ppm) indicates marginal per-person ventilation that may not meet ASHRAE 62.1 requirements at design occupancy. CO2 sensors require annual calibration to maintain ±50 ppm accuracy; sensor drift can cause the DCV system to under-ventilate without occupant awareness. The area-based component Ra × Az is constant regardless of occupancy and should be maintained even when the space is unoccupied if air quality concerns exist.
Select the space type to load default Rp and Ra values from ASHRAE 62.1 Table 6.2.2.1, then adjust occupant count and floor area for your zone. The outdoor CO2 concentration defaults to 400 ppm; adjust for local conditions. The CO2 generation rate defaults to 0.0084 cfm/person (sedentary/office activity). Results show the required Vbz in cfm, the people and area components separately, and the DCV CO2 setpoint. A green status indicates the DCV setpoint is below 1100 ppm, meaning the per-person ventilation rate is adequate.
The typical DCV CO2 setpoint ranges from 900 to 1100 ppm indoors, corresponding to 500–700 ppm above outdoor ambient (~400 ppm). ASHRAE 62.1 does not mandate a specific CO2 setpoint — it requires adequate outdoor air per the Ventilation Rate Procedure; the CO2 setpoint is derived from that ventilation requirement.
ASHRAE 62.1 Section 6.2.7 requires DCV for zones with occupant density greater than 25 people per 1000 sq ft (0.025 people/ft²) when the system design supply airflow exceeds 3000 cfm. Classrooms, conference rooms, auditoriums, and assembly spaces typically exceed this density threshold.
ASHRAE 62.1 Annex C provides CO2 generation rates by metabolic activity. Sedentary office work is approximately 0.0084 cfm/person (0.24 L/min). Light activity (retail, light manufacturing) is 0.0115 cfm/person. High-activity spaces such as gyms may reach 0.035 cfm/person or higher, requiring much higher ventilation rates.
Yes. The Ra × Az component accounts for off-gassing from building materials, furnishings, and other non-occupant sources. ASHRAE 62.1 requires this component to be provided continuously regardless of occupancy, though some exceptions apply for unoccupied periods with adequate flush-out.
CO2 sensors control the DCV system, but airflow measurement (via airflow stations or VAV box tracking) is still required to demonstrate compliance with minimum ventilation rates. The CO2 sensor modulates the OA damper; the airflow measurement verifies that the minimum Vbz is actually being delivered to the breathing zone.
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