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Understanding minimum outdoor air for a zone
The ASHRAE 62.1 Ventilation Rate Procedure builds a zone's minimum outdoor air from two additive parts. The people component (Rp) covers bioeffluents and other contaminants that occupants produce, so it scales with headcount. The area component (Ra) covers contaminants that come off building materials, furnishings, carpet and finishes all the time, so it scales with floor area whether or not anyone is in the room.
The combined result is Vbz = (Rp × Pz) + (Ra × Az), where Pz is the zone population and Az is the zone floor area. Vbz is called the breathing zone outdoor airflow. It is the starting point for the rest of the procedure, which then corrects for how well the supply air reaches the occupants and for how several zones share one air handler.
Where the preset rates come from
The presets are the Rp and Ra values for each occupancy category in Table 6.2.2.1 of ASHRAE Standard 62.1. An office uses 5 CFM per person (2.5 L/s) and 0.06 CFM/ft² (0.3 L/s·m²). Retail is 7.5 CFM per person and 0.12 CFM/ft², a classroom for ages 9 and up is 10 and 0.12, a health club or aerobics room is 20 and 0.06, and restaurant dining is 7.5 and 0.18. The metric table values are rounded in their own right, so the metric presets give a result a few percent different from converting the imperial ones. Rates change between editions and the edition your authority has adopted governs, so check the rates against it, and use the custom option for a space type that is not listed.
Worked example: an office with 20 people and 1,500 ft² needs (5 × 20) + (0.06 × 1,500) = 100 + 90 = 190 CFM, which is 90 L/s.
Why both components matter, even in a lightly occupied space
A large retail floor with few people can have a minimum driven mostly by the area term, while a small conference room is driven mostly by the people term. Leaving out either term understates the true minimum, which is why the two are added rather than taking the larger one. Densely occupied rooms also carry a large heat gain from their occupants, so run the same headcount through the heat gain people calculator to size the cooling that goes with this outdoor air.
From zone minimum to system outdoor air intake
This tool stops at Vbz for one zone. The next step divides it by the zone air distribution effectiveness, Ez, to get the zone outdoor airflow: Voz = Vbz / Ez. Ez is 1.0 for a typical ceiling supply of cool air and lower for configurations that short-circuit supply air to the return, so a zone with Ez of 0.8 needs 25 percent more outdoor air than its Vbz. The ventilation effectiveness calculator works through that correction. For a system serving several zones, the full ASHRAE 62.1 compliance calculator applies occupant diversity and the system ventilation efficiency to size the intake at the air handler.
Canadian codes and residential projects
The Canadian Centre for Occupational Health and Safety notes that Canadian building codes and health and safety legislation generally refer to a version of ASHRAE 62.1, so confirm which edition your province or municipality has adopted. This calculator covers commercial occupancy categories only. Houses and apartments follow a different method, so use the whole-house ventilation calculator for residential projects.
Frequently Asked Questions
This calculator gives Vbz, the breathing zone outdoor airflow, from the people and area rates. The full procedure divides Vbz by the zone air distribution effectiveness (Ez) to get the zone outdoor airflow Voz, then, for a system serving several zones, accounts for the share of air that is outdoor air and for occupant diversity to find the intake at the air handler. Use this tool for the zone figure and the ASHRAE 62.1 compliance calculator for the full multi-zone calculation.
The people component (Rp) covers bioeffluents and other contaminants that occupants produce, so it scales with headcount. The area component (Ra) covers contaminants that come off building materials, furnishings and finishes whether or not anyone is present, so it scales with floor area. Adding both means a space is ventilated adequately when it is full, partly occupied or empty.
For design occupancy under the Ventilation Rate Procedure, no, this is the zone floor. Lower outdoor air flows are possible in operation through demand controlled ventilation, which resets the rate to the people actually present, through occupant diversity at the system level, or by designing to the separate Indoor Air Quality Procedure. Each carries its own requirements, so see the demand controlled ventilation calculator before assuming a saving.
No. It uses the commercial ASHRAE 62.1 people-plus-area method for offices, retail, classrooms and similar spaces. Houses and apartments are ventilated to a different method, with Canadian codes setting requirements by room type and the US ASHRAE 62.2 standard using floor area and bedroom count. Use the whole-house ventilation calculator for residential projects.