Section 6.2.3/Zone Air Distribution Effectiveness
ASHRAE 62.1 Section 6.2.3 provides the Ez factor that adjusts outdoor air based on supply air configuration (ceiling vs. floor).
The zone outdoor airflow must be adjusted by the zone air distribution effectiveness factor Ez from Table 6.2.3.1. The breathing zone outdoor airflow (Vbz) is divided by Ez to determine the zone outdoor airflow (Voz = Vbz / Ez). For ceiling supply with ceiling or wall return and a supply-to-room temperature differential of 15 degrees F or less, Ez = 1.0 (no adjustment). For ceiling supply with floor return delivering warm air 15 degrees F or more above room temperature, Ez = 0.8 (requiring 25% more outdoor air). For floor supply with ceiling return delivering cool air, Ez = 1.2 (allowing a 17% reduction). The factor reflects how well the supply air mixes with room air at the breathing zone height.
Why this section exists
The ventilation rate calculation assumes that outdoor air is perfectly mixed with room air at the breathing zone (4 to 6 feet above the floor). In reality, supply air distribution affects mixing. Warm air supplied from the ceiling tends to stratify above the breathing zone, delivering less outdoor air to occupants than expected. Cool air from floor diffusers rises through the breathing zone, providing better outdoor air delivery. The Ez factor corrects the calculation for these real-world distribution effects.
What plan reviewers look for
Plan reviewers check the ventilation calculation for the Ez factor at each zone. They verify the supply air configuration (ceiling vs. floor supply, ceiling vs. floor return) from the mechanical drawings. They check the supply air temperature differential to determine which Ez value applies. They verify the zone outdoor airflow is calculated as Vbz / Ez, not just Vbz.