📊 Airflow Reduction
💰 Annual Savings
Design vs. Typical Occupancy
Full Calculation Table
| Parameter | Fixed-Rate Baseline | Demand-Controlled |
|---|
How to Use the Demand-Controlled Ventilation Calculator
Choose the space type to load default Rp and Ra rates, or enter your own under Other / Custom.
Enter the design population the zone was sized for, and your honest estimate of the percentage of that population the space typically holds while occupied.
Pick the nearest city for a default heating-season outdoor temperature, then enter the occupied hours that fall in the heating season and what a kWh of heat costs you.
Click Calculate DCV Savings to see the outdoor air reduction and an estimated annual heating cost saving from running at typical occupancy instead of at the fixed design rate.
How Demand-Controlled Ventilation Saves Energy
A fixed-rate system delivers outdoor air for the zone's design population every occupied hour, even though most rooms sit below that number most of the time. Demand-controlled ventilation reads a CO2 sensor, or sometimes an occupancy sensor, and lowers outdoor air when the room is thin and raises it again as people arrive. ASHRAE 62.1 allows this in its dynamic reset provisions (Section 6.2.7), with one hard floor: outdoor air can never fall below the area-based rate. In a Canadian winter every litre per second of outdoor air you do not bring in is a litre per second of cold air the heating plant does not have to warm from outdoor temperature to room temperature.
Why the Savings Are Capped
Only the people term, Rp times occupants, moves with occupancy. The area term, Ra times floor area, stays put because it dilutes what the building itself emits. Take the default case of 20 people in 80 m² at Rp 2.4 and Ra 0.30 L/s: the people term is 48 L/s and the area term is 24 L/s, so the design requirement is 72 L/s. At half occupancy the people term halves to 24 L/s and the total falls to 48 L/s, a 33% cut, not 50%. A classroom, as in the school HVAC calculator, has a high Rp and so keeps more of its savings; a lightly occupied space with a large floor area keeps very little.
Setting the CO2 Setpoint
DCV controls to a CO2 differential above outdoors, so the outdoor reading matters. NOAA's Global Monitoring Laboratory reported 426 ppm at Mauna Loa for September 2026, and 425 ppm is a fair planning value for fresh air in most places, a little higher near traffic or in a dense city core. A seated adult adds about 0.0052 L/s of CO2, so at 7.5 L/s of outdoor air per person the room settles about 690 ppm above outdoors, near 1,100 ppm. That is a result of the airflow, not a limit set by the standard. Use the CO2 calculator to turn your own occupant count and airflow into a setpoint.
This Estimate's Assumptions
The avoided heat is 1.2 × the airflow reduction in L/s × the temperature difference between a 21 °C room and the outdoor air, giving watts of sensible heat. That figure is multiplied by your heating-season occupied hours and by the cost of a kWh of heat. For the default case, 24 L/s × 26 K × 1.2 is 749 W, which over 1,600 hours is about 1,198 kWh, or $156 at $0.13 per kWh. A single average temperature stands in for the whole season, so a degree-day calculation is the better route when you want to follow the actual weather. Cooling savings, humidity load and any heat already recovered by an HRV or ERV are left out, and heat recovery in particular makes the true saving smaller. Treat the result as a planning figure, not a utility bill forecast.
Sizing the Rest of the System
The baseline here is the zone breathing-zone requirement, Vbz, before any zone air distribution correction. Build it first in the ASHRAE 62.1 calculator, then see what a heat recovery core takes off the remaining load with the HRV calculator or the ERV calculator. Pairing DCV with recovery saves less than the sum of the two, since the recovery core was already reclaiming some of the heat that DCV avoids.
Where DCV Makes the Most Sense
Spaces whose occupancy swings widely, such as classrooms, meeting rooms, gyms, theatres, and places of worship, give DCV the most to work with. A retail floor that stays near design occupancy all day gives it little. A review by Lawrence Berkeley National Laboratory of DCV specifications for California's Title 24 describes CO2 sensors as the usual control signal and reports modelled cooling savings of up to 20%, concentrated in spaces with high and variable occupancy. Sensor drift is the practical risk, so budget for calibration checks.
Frequently Asked Questions
Demand-controlled ventilation, or DCV, uses a CO2 sensor or an occupancy signal to adjust outdoor airflow to the number of people actually in a space, instead of supplying a fixed rate sized for maximum design occupancy at all times. When a room runs below its design population, which is common in classrooms, gyms, and conference rooms, DCV lowers the people-based portion of outdoor air and cuts the heating or cooling load that comes with it.
It depends on how far below design occupancy the space typically runs and on the climate. With the calculator's default conference room (20 people, 80 m², running at 50% of design), outdoor air drops from 72 to 48 L/s, a 33% reduction. In a cold climate that can mean a few hundred dollars a year in avoided heating, since only the people-based component scales down while the area-based component stays fixed.
No. The area-based rate, Ra, in ASHRAE 62.1 dilutes contaminants from the building itself, such as off-gassing from finishes, and it does not depend on occupancy. DCV only reduces the people-based component, Rp times the number of occupants, so outdoor air never falls below Ra times floor area. A zone with a large Ra relative to Rp will see a smaller percentage saving.
ASHRAE 62.1 does not set a CO2 limit. DCV normally controls to a differential above outdoor CO2. With about 7.5 L/s (15 CFM) of outdoor air per person in total, people at office activity raise CO2 roughly 700 ppm, so with outdoor air near 425 ppm the setpoint lands near 1,100 ppm. Work out the setpoint from the zone's actual outdoor air per person, and calibrate the sensor against a measured outdoor reading. The CO2 calculator shows the arithmetic.