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ASHRAE 62.1 ventilation rate calculator
Outdoor air for ventilation is sized two ways at once: enough for the people and enough for the space itself, and ASHRAE 62.1 adds them. The breathing-zone outdoor airflow Vbz equals the per-person rate Rp times the number of people Pz, plus the per-area rate Ra times the zone area Az. Enter the four values. Typical office numbers are about 5 cfm per person and 0.06 cfm per square foot, but the correct rates come from the occupancy-category table in the standard, which varies a lot by space type (classrooms, gyms, labs, and patient rooms are all different). The result is the breathing-zone requirement. To get the zone outdoor airflow you divide by the zone air distribution effectiveness Ez, which is around 0.8 when warm air is supplied from the ceiling and up to 1.0 with good mixing, so poor distribution means more outdoor air. At an air handler serving several zones, the multiple-spaces equation and the system ventilation efficiency adjust the total again, usually downward but never below any single zone's need. Use the rates, effectiveness values, and full procedure from the adopted edition of ASHRAE 62.1, and confirm the design with the mechanical engineer.
Result
ASHRAE 62.1 breathing-zone outdoor air: Vbz = Rp × Pz + Ra × Az, the people-based rate (Rp per person times the number of people Pz) plus the area-based rate (Ra per square foot times the zone area Az). Enter the four values; typical office figures are about 5 cfm per person and 0.06 cfm per square foot, but the right numbers come from the occupancy-category table in the standard. The result is the breathing-zone outdoor airflow. To get the zone outdoor airflow you divide by the zone air distribution effectiveness Ez (around 0.8 for ceiling supply of warm air, up to 1.0 for good mixing), and at the air handler serving several zones the multiple-spaces equation and the system ventilation efficiency reduce the total further. Use the rates, effectiveness values, and procedure from the adopted edition of ASHRAE 62.1, and confirm the design with the mechanical engineer.
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Ventilation rate FAQ
What is the ventilation rate procedure?
The ventilation rate procedure is the prescriptive method in ASHRAE 62.1 for setting outdoor air. The breathing-zone airflow is a per-person rate times the population plus a per-area rate times the floor area, then corrected by the zone effectiveness Ez and the system efficiency Ev to the intake airflow.
How much outdoor air does a building need?
It depends on the occupancy and the area. Under ASHRAE 62.1 the breathing-zone outdoor air is the per-person rate times the people plus the per-area rate times the square footage. A 20-person, 2,000 square foot office at 5 cfm per person and 0.06 cfm per square foot needs about 220 cfm, before the Ez and Ev corrections.
What is ASHRAE 62.1?
ASHRAE Standard 62.1, Ventilation for Acceptable Indoor Air Quality, is the ventilation standard for commercial and institutional buildings. It sets the minimum outdoor air rates, the calculation methods, the filtration minimum, and the DCV rules. It is a standard until a jurisdiction adopts it, usually through the mechanical code, so the adopted edition controls.
What is demand-controlled ventilation?
Demand-controlled ventilation modulates the outdoor air with actual occupancy instead of holding the design-maximum rate. Usually a CO2 sensor drives the outdoor air damper, opening it as people arrive and closing it as they leave. It recovers the people component of the rate, saving conditioning energy in spaces whose occupancy swings, like conference rooms and theaters.
What is the difference between ventilation and infiltration?
Ventilation is intentional outdoor air brought in through a damper or unit in a measured volume the design controls, and it is what ASHRAE 62.1 governs. Infiltration is uncontrolled leakage through the envelope, driven by wind and stack effect. Tight modern construction has cut infiltration to almost nothing, so the standard does not let you count it toward the rate.