| Zone | Voz | Vpz | Zpz | Evz |
|---|
System outdoor air for multi-zone recirculating systems
When one air handler serves several zones and recirculates return air, the system outdoor air intake is not the plain sum of the zone requirements. The outdoor air is mixed into one supply stream, so a zone that needs a large share of outdoor air relative to its supply airflow can be under-ventilated even when the total intake equals the sum of the zone requirements.
ASHRAE 62.1 handles this with the system ventilation efficiency, Ev. The uncorrected outdoor air, Vou, is the zone outdoor air added up across the system, and the intake is Vot = Vou / Ev. The Pacific Northwest National Laboratory's Building America Solution Center lists the 2022 edition as the current version of the standard, which sets the procedures and methods for minimum ventilation and indoor air quality.
The zone outdoor air fraction and the critical zone
For each zone, Zpz = Voz / Vpz, where Voz is the zone outdoor airflow and Vpz is the zone primary airflow. For a VAV box, Vpz is the minimum expected primary airflow, not the cooling design airflow, because the box throttles down when the load drops and the outdoor air share rises. The zone with the highest Zpz is the critical zone. Take a conference room with Voz of 130 CFM whose box turns down to 400 CFM, so Zpz = 130 / 400 = 0.325. An open office at 250 / 1,500 gives 0.167 and a reception area at 60 / 400 gives 0.15, so the conference room sets Ev for the whole system. Use the VAV box sizing calculator to check that box minimums are high enough to keep Zpz down.
Ev from the simplified table or the Appendix A equation
The simplified table sets Ev from the highest Zpz: 1.0 up to 0.15, 0.9 up to 0.25, 0.8 up to 0.35, 0.7 up to 0.45 and 0.6 up to 0.55. Above 0.55 the table gives no value and Appendix A applies. The Appendix A equation works zone by zone: Evz = 1 + Xs − Zpz, with Xs = Vou / Vps, where Vps is the sum of the zone primary airflows, and Ev is the lowest Evz. The tool caps Ev at 1.0. In the example above, Vou is 250 + 130 + 60 = 440 CFM and Vps is 2,300 CFM, so Xs is 0.191. The table gives Ev 0.8 and Vot = 440 / 0.8 = 550 CFM, while the equation gives Evz = 1 + 0.191 − 0.325 = 0.866 and Vot of about 508 CFM. The table is the conservative choice, and the equation is worth using when the extra air costs real money.
Two limits apply. Voz here already includes the air distribution effectiveness correction, and Vou is the plain sum of the zone Voz values, so occupant diversity is not applied. Both choices err on the high side. The ASHRAE 62.1 compliance calculator handles diversity for projects that need it.
From intake airflow to energy and mixed-air conditions
Every extra CFM of outdoor air has to be heated in winter and cooled and dehumidified in summer, so a small critical zone can cost a large amount of energy. Quantify the winter load with the ventilation heat load calculator, and use the mixing air calculator to find the mixed-air temperature at the coil, since the outdoor air fraction sets how cold the mixture gets. Reducing the critical zone's outdoor air, or raising its minimum primary airflow, brings Zpz and the system penalty down.
Checking the result against Canadian requirements
Canadian building codes reference ASHRAE 62.1 for minimum ventilation, and Health Canada's guidance on improving indoor air quality in office buildings says the rates should be set according to the requirements of the applicable jurisdiction, depend on occupant numbers, floor area and activities, and be measured and brought in line with the applicable standards by a qualified HVAC professional. Confirm the edition your authority has adopted before relying on the table values.
Frequently Asked Questions
A recirculating multi-zone system mixes outdoor air into one supply stream and sends the same mix to every zone. A zone whose required outdoor air is a large share of its supply airflow gets too little unless the whole system brings in extra. ASHRAE 62.1 captures this with the system ventilation efficiency Ev, and the intake is Vot = Vou ÷ Ev, where Vou is the uncorrected outdoor air from all zones. A small, densely occupied room such as a conference room is the usual cause.
The critical zone has the highest primary outdoor air fraction, Zpz = Voz ÷ Vpz. Voz is the zone outdoor airflow and Vpz is the zone primary airflow, which for a VAV box means the minimum expected primary airflow, not the cooling design airflow. Entering the design airflow understates Zpz and the intake. Zone Voz comes from the ventilation rate calculator or the minimum ventilation calculator after applying air distribution effectiveness.
The simplified table in ASHRAE 62.1 gives Ev by the highest Zpz: 1.0 up to 0.15, 0.9 up to 0.25, 0.8 up to 0.35, 0.7 up to 0.45 and 0.6 up to 0.55, with no value above 0.55. The Appendix A equation is Evz = 1 + Xs − Zpz for each zone, where Xs = Vou ÷ Vps, and Ev is the lowest Evz. It is usually less severe than the table. For the example zones the table gives Ev 0.80 and an intake of 550 CFM, while the equation gives 0.87 and about 508 CFM. This tool offers both and falls back to the equation when Zpz exceeds 0.55.
No. It sets the uncorrected outdoor air Vou equal to the sum of the zone Voz values, which assumes every zone is at its design occupancy at the same time. That is conservative, because ASHRAE 62.1 allows the people part of Vou to be scaled by an occupant diversity factor D. Use the ASHRAE 62.1 compliance calculator when you need diversity.