ASHRAE 62.1 Ventilation Rate Procedure (Multiple-Zone, Critical Zone, System Efficiency)

The problem the ventilation rate procedure solves is that a single air handler delivers ONE outdoor air fraction to every zone it serves. A zone with many people and little supply air receives the same percentage of outdoor air as a zone with few people and lots of supply -- so the zone with the highest ratio of required outdoor air to primary air, the critical zone, sets the fraction the whole system has to run at, and every other zone is over-ventilated as a consequence. The penalty that produces is the number this exists to report, because it is invisible in a zone-by-zone calculation. The procedure builds upward. Each zone's breathing zone rate is the per-person rate times the people plus the per-area rate times the area; dividing by the zone air distribution effectiveness gives the outdoor air that zone needs at its diffuser, and dividing that by the zone's primary airflow gives its outdoor air fraction. The system's uncorrected intake sums those requirements, the system fraction is that over the total primary air, and the simplified system ventilation efficiency is one plus the system fraction minus the critical zone's fraction. The intake is the uncorrected total divided by that efficiency. Where the fix lies follows directly and it is not more outdoor air. REDISTRIBUTING primary air toward the critical zone, taken from the zones the design over-ventilates and holding the system total where it is, lowers the critical fraction without lowering the system fraction alongside it -- so the efficiency rises and the intake comes down, usually far cheaper than conditioning the extra outdoor air for the life of the building. The distinction matters and it is easy to get wrong: simply ADDING primary air to the critical zone is a different move, because it raises the system's total primary flow and therefore lowers the system fraction as well as the zone fraction, and the efficiency can fall rather than rise. A design that responds to a low efficiency by increasing the INTAKE has treated the symptom. Diversity applies to the people term only, and it is entered rather than assumed, because the population of a whole building is rarely the sum of its zones' design populations while its floor area always is. This handles up to three zones on one air handler at a single design condition: it does not size equipment, address multiple systems or secondary recirculation, apply the IAQ procedure or the natural ventilation procedure as alternatives, select the zone air distribution effectiveness (which depends on supply air temperature and diffuser type), or verify anything in operation. ANSI/ASHRAE 62.1, the mechanical code in force, and the engineer of record govern.

per breathing zone Vbz = Rp x people + Ra x area and Voz = Vbz / Ez; Zp = Voz / primary air; Xs = Vou / Vps; Ev = 1 + Xs - max(Zp); Vot = Vou / Ev.

The ASHRAE 62.1 Ventilation Rate Procedure for a multiple-zone recirculating system by name, in its simplified single-supply form. Rp, Ra and Ez are ENTERED from the standard's own occupancy-category and air-distribution tables rather than reproduced here, because those tables are the part that changes between editions.

The procedure's algebra; no table from the standard is reproduced.

Estimate. AHJ and licensed professional govern.

Field names used by the API: rp_cfm_per_person, ra_cfm_per_ft2, ez, people_1, area_1_ft2, primary_1_cfm, people_2, area_2_ft2, primary_2_cfm, people_3, area_3_ft2, primary_3_cfm, diversity, vou_cfm, xs, zp_max, ev, vot_cfm, penalty_cfm

• Single-supply simplified form multiple air handlers or systems are computed separatelyASHRAE 62.1
• Rp, Ra and Ez are entered from the standard's occupancy-category and distribution tablesthe applicable edition
• Design, not measurement a measured intake is a different questiona balancing report