Commercial buildings waste significant energy ventilating for design occupancy that rarely exists. A lecture hall designed for 65 people may hold 20 for most of the day, yet the HVAC system delivers outdoor air for 65. ASHRAE Standard 62.1 addresses this with its dynamic reset provision, which allows ventilation rates to adjust based on actual occupancy rather than design maximums (Trane/ASHRAE).
The ASHRAE 62.1 ventilation formula
Since 2004, ASHRAE 62.1 has required breathing-zone outdoor airflow calculated as:
\[ V_{bz} = (R_p \times P_z) + (R_a \times A_z) \]
Where \(R_p\) is the per-person rate (cfm/person), \(P_z\) is zone population, \(R_a\) is the per-area rate (cfm/sq ft), and \(A_z\) is zone floor area. For a lecture classroom: 7.5 cfm per person plus 0.06 cfm per square foot. At design occupancy of 65 people in 1,000 sq ft, the required outdoor air is 550 cfm. At 20 people, it drops to 210 cfm — a 62% reduction in outdoor air delivery. The per-person component decreases by 7.5 cfm for every person who leaves the zone (Trane/ASHRAE).
Three methods for occupancy assessment
| Method | How It Works | Accuracy | Cost | Best For |
|---|---|---|---|---|
| Occupancy schedules | BAS predicts population by time of day | Low — assumes fixed patterns | Lowest — no hardware | predictable schedules |
| Occupancy sensors | PIR, ultrasonic, or mmWave detect presence/count | High — real-time detection | Medium — sensor per zone | Variable occupancy spaces |
| CO2 sensors | Measure CO2 produced by occupants, diluted by outdoor air | Medium — proxy for occupancy | Medium — sensor per zone | Densely occupied spaces |
Standard 62.1 identifies three types of dynamic reset: based on occupancy, ventilation efficiency, and economizer operation. Occupancy can be assessed through schedules, sensors, or CO2 — the choice depends on space variability, budget, and required precision (Trane/ASHRAE).
Energy savings: what the data shows
A 2013 PNNL building-modeling study found that advanced occupancy sensors and controls could save an average of 18% of HVAC energy across U.S. commercial buildings — a potential of 670.4 TBtu. Demand-controlled ventilation specifically accounts for 15-20% savings (372 TBtu) across all building types except large offices and healthcare facilities (DOE).
NIST research quantified savings by space type. CO2-based DCV produced ventilation energy load reductions ranging from 10% to 80%, depending on space type, climate, and occupancy schedule:
- Conference rooms and lecture halls: More than 50% reduction in all cases, as high as 80% — the highest savings because occupancy varies most dramatically
- Classrooms: 30-50% reduction, depending on city and schedule alignment
- Fast food restaurants: 40-75% reduction, reaching 86% in Los Angeles's mild climate
- Offices: Approximately 20% lower than fixed-rate ventilation
The NIST study found that spaces with variable occupancy — conference rooms, lecture halls, and restaurants — had significantly larger savings than offices with stable occupancy patterns. Savings were also greater in mild climates than extreme climates, because ventilation energy is a larger fraction of total HVAC load when heating and cooling demands are lower (NIST).
Residential DCV is emerging.NIST is currently testing demand-controlled ventilation performance at its Residential Test Facility, using CO2 sensors to control an energy recovery ventilator (ERV) in real time. The study evaluates control algorithms against ASHRAE Standard 62.2-2019 annual relative exposure limits, measuring outdoor air rates, ozone, and formaldehyde concentrations. Consumer-grade CO2 sensors from two brands showed different accuracies — a reminder that sensor quality matters as much as the control strategy (NIST).
Choosing the right sensor technology
- PIR (passive infrared): Detects body heat motion. Low cost, widely available, but cannot count people — only presence/absence. Best for private offices and restrooms.
- Ultrasonic: Detects motion via Doppler effect. More sensitive than PIR to small movements, but can false-trigger from air currents. Best for restrooms and corridors.
- mmWave (millimeter wave radar): Detects presence and can count occupants. Newest technology, highest accuracy, highest cost. Best for conference rooms and classrooms where precise occupant counting enables per-person ventilation reset.
- CO2 sensors (NDIR): Measure actual CO2 concentration as a proxy for occupancy density. Required by some code jurisdictions for densely occupied spaces. Must be calibrated periodically to maintain accuracy.
Integration with ASHRAE Guideline 36
A PNNL study for DOE used EnergyPlus to simulate occupancy-based control (OBC) in medium-sized office buildings across five U.S. climate zones, comparing against baselines from ASHRAE Standard 90.1 and Guideline 36 (high-performance sequences of operation). The proposed OBC strategies used room temperature reset, system minimum outdoor airflow reset, and zone minimum supply airflow reset — all driven by occupancy data. Results indicated significant energy savings potential (DOE/OSTI).
Naming a product or manufacturer in this article is for identification purposes only and does not constitute an endorsement by HVAC Zone Inc. We are a multi-brand dealer and remain neutral on brand preference.
Want to reduce ventilation energy waste with occupancy-based controls? Request a consultation — we assess your building's occupancy patterns and recommend the right sensor strategy for your space.