Ez Comparison Across Distribution Types
Full Results Table
| Parameter | Value | Notes |
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How to Use This Calculator
Choose the configuration that matches your system: ceiling supply with ceiling or floor return, floor supply or underfloor air distribution, displacement ventilation, or makeup air supply. Each has a published ASHRAE 62.1 default Ez value.
Enter the outdoor air flow rate required at the breathing zone from your ASHRAE 62.1 ventilation rate procedure calculation, before any Ez correction is applied.
Warm supply air lowers Ez for several layouts: ceiling supply falls to 0.8 because warm air stratifies near the ceiling, and floor supply with a ceiling return falls to 0.7. It doesn't change the makeup air entries.
The results show the Ez-corrected outdoor airflow that must actually be supplied to meet the breathing zone requirement, plus a comparison table showing how different distribution types would change this requirement.
Zone Air Distribution Effectiveness in ASHRAE 62.1 Design
Zone air distribution effectiveness is one of the more consequential factors in ASHRAE 62.1's ventilation rate procedure that gets overlooked in quick sizing exercises but matters significantly for actual delivered indoor air quality. Two systems supplying identical outdoor air volumes can deliver very different air quality outcomes at the breathing zone depending entirely on how that air is distributed through the space. The standard's symbol for it is Ez, while Ev is a different quantity, the system ventilation efficiency used for multiple-zone recirculating systems.
What Ez Represents
The zone air distribution effectiveness factor Ez estimates how effectively outdoor air supplied to a zone reaches the breathing zone compared to a fully mixed reference case, which is assigned Ez of 1.0. ASHRAE Standard 62.1 publishes default Ez values by air distribution configuration specifically so designers don't need to model airflow patterns for every project. When Ez is less than 1.0, some of the supplied outdoor air short-circuits from supply to return without adequately reaching the occupied zone, meaning more total supply air is needed to achieve the same breathing zone air quality. When Ez exceeds 1.0, the distribution pattern delivers fresh air to the breathing zone more efficiently than full mixing would. The highest value in the table is 1.2, and the lowest is 0.5.
Ceiling Supply Systems: The Common Case
The majority of Canadian commercial and institutional buildings use ceiling supply diffusers with ceiling or high sidewall return. With cool supply air, ASHRAE 62.1 assigns ceiling supply the reference Ez of 1.0, because cool air is denser than room air and falls into the occupied zone, achieving reasonably complete mixing before being drawn back to the ceiling return. With warm supply air the table depends on the temperature difference and the jet. Ceiling supply of warm air 8°C or more above space temperature with a ceiling return gets 0.8, meaning 25% more outdoor air must be supplied. If the warm supply is less than 8°C above space temperature and the jet reaches within 1.4 m of the floor at 0.8 m/s or more, the value is 1.0, and at lower jet velocity it is 0.8. ASHRAE's 2019 errata for the zone air distribution effectiveness table restates those rows, and warm ceiling supply with a floor return stays at 1.0. This calculator applies 0.8 to any warm-air ceiling supply, which is conservative.
Displacement and Underfloor Systems
Displacement ventilation supplies cool air at low velocity near floor level, allowing it to spread across the floor and rise through the occupied zone via natural buoyancy as it picks up heat from occupants and equipment, eventually exiting near the ceiling. ASHRAE 62.1 permits an Ez of 1.2 for cool floor supply with a ceiling return only when low-velocity displacement achieves unidirectional flow and thermal stratification. Most underfloor air distribution systems are rated 1.0 instead, provided the supply jet reaches at least 1.4 m above the floor at 0.8 m/s, and floor supply of warm air with a ceiling return drops to 0.7. Displacement systems require more careful design around throw distances, floor-level diffuser placement, and stratification management, and they're not well suited to spaces with highly variable occupancy or heat load patterns. The displacement ventilation calculator covers that design side.
Applying Ez in Practice
Once the zone's Ez is determined, the corrected zone outdoor airflow, Voz in the standard, equals the breathing zone outdoor airflow requirement Vbz divided by Ez. For a zone requiring 150 L/s at the breathing zone with Ez of 0.8 (ceiling supply, warm air), the corrected requirement becomes 187.5 L/s, so the system must actually supply that much outdoor air to guarantee 150 L/s effectively reaches occupants. The ventilation rate calculator covers working out the breathing zone requirement that goes into this tool. This calculation directly affects fan sizing, ductwork sizing, and the energy cost of conditioning outdoor air, making Ez a meaningful economic factor in Canadian climates where conditioning outdoor air from winter design temperatures represents significant heating energy cost. See the CO2 level calculator for how ventilation rate translates to indoor CO2 outcomes.
Zone-Level vs. System-Level Application
Ez and the corrected zone outdoor airflow apply at the individual zone level in multi-zone systems. The zone results then feed into the system ventilation efficiency, Ev, which sets the outdoor air intake for a multiple-zone recirculating system, with the uncorrected intake Vou divided by Ev to give Vot. A building with mostly Ez 1.0 zones but one problematic 0.8 zone, perhaps a space with an unconventional diffuser layout, needs that zone's higher corrected requirement reflected in the system-level calculation, which is a common source of under-ventilation in buildings where zone-level corrections were skipped during design.
Frequently Asked Questions
Zone air distribution effectiveness (Ez) is a zone-level design factor in ASHRAE 62.1's ventilation rate procedure accounting for how well outdoor air reaches the breathing zone relative to full mixing. Air change effectiveness (ACE) is a related field-measured metric, determined under ASHRAE Standard 129 and often verified with tracer gas testing after installation. The standard allows Ez to be taken as equal to a measured air change effectiveness for all configurations except unidirectional flow. Ez is design-stage prescriptive guidance, while ACE is an empirical measurement of actual performance. Ev, by contrast, is the system ventilation efficiency used for multiple-zone recirculating systems.
Ez depends on how well supply airflow reaches the breathing zone before mixing with or bypassing room air. Ceiling supply of cool air achieves 1.0, the reference case. Cool floor supply with a ceiling return gets 1.0 for most underfloor systems and 1.2 only when low-velocity displacement achieves unidirectional flow and thermal stratification. Warm floor supply with a ceiling return drops to 0.7. Makeup air outlets located near the return or exhaust rate 0.5, and 0.8 when they are more than half the length of the space away. Short-circuit paths therefore require more outdoor air to compensate.
Under the ventilation rate procedure, the breathing zone outdoor airflow is divided by Ez to get the corrected zone outdoor airflow that must actually be supplied. A zone with Ez of 0.8 needs 25% more outdoor air supplied than the breathing zone requirement, since only 80% of supplied air effectively reaches occupants under that configuration. A zone with Ez above 1.0 can meet the same requirement with less supplied air. Use this calculator to find your corrected requirement for any distribution type.
For ceiling supply with ceiling or high sidewall return, which describes the majority of Canadian office and classroom installations, ASHRAE 62.1's default Ez is 1.0 with cool supply air. If the supply air is warm, Ez is 0.8 when it is 8°C or more above the space temperature, and also when the jet does not reach within 1.4 m of the floor at 0.8 m/s, and it is 1.0 when the warm supply is less than 8°C above the space and the jet does reach. This calculator uses 0.8 for any warm-air ceiling supply, which is conservative. See the CO2 level calculator to check ventilation adequacy for your space type.
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