VAV terminal airflows and inlet size from zone loads and supply air temperatures.
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Zone Loads
Enter the sensible cooling load only — a total (sensible + latent) load oversizes the airflow by the latent share.
Design Conditions
VAV Box CFM Requirements
Design (Cooling) Max CFM
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CFM
Deadband Minimum CFM
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CFM
Heating Load CFM
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CFM
Heating Maximum CFM
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CFM (dual max)
Box Size Selection (inlet velocity at design max ≤ 1,200 FPM, low-noise target)
Smallest Box at ≤ 1,200 FPM
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Inlet
Inlet Area (ft²)
CFM at 900 FPM
CFM at 1200 FPM
CFM at 2500 FPM
FPM at Max
FPM at Min
Status at Max
Inlet velocity bands are rules of thumb: 900–1,200 FPM at design max is a conservative low-noise selection (the AIM Works app’s default limit); up to about 2,000–2,500 FPM is common in catalogs if NC and pressure drop check out. At the deadband minimum, keep enough velocity for the flow sensor — below roughly 300 FPM (amber) check the controller’s minimum controllable airflow. Verify the reheat coil at the heating maximum separately. Formula: CFM = Load ÷ (1.08 × ΔT), with 1.08 corrected for elevation.
Inputs, results, and method — as an Excel workbook or plain CSV.
Doing this for a whole building? See this step inside a connected AIM Works project — loads, ventilation, equipment, and Design Review in one place.
This VAV box sizing calculator turns zone cooling and heating loads into the airflow values you need to select a variable-air-volume terminal unit. Enter the zone sensible cooling load and heating load in BTU/h, the room setpoint, the cooling supply-air and heating discharge temperatures, the zone outdoor airflow (Voz), and the site elevation, and the tool returns the design maximum, the deadband minimum, the heating-load airflow, and the dual-maximum heating airflow, checked against the ASHRAE 90.1-2022 §6.5.2.1 reheat limits.
It then lists every standard round inlet with its velocity at the maximum and at the deadband minimum, and picks the smallest inlet at or below 1,200 FPM at design airflow. That balances noise and pressure drop against controllability — an oversized box controls poorly at its minimum, while an undersized box runs noisy and high in pressure — and the tool tells you when a smaller box would control the minimum better.
The constant 1.08 is the sea-level sensible-heat factor for air: 60 min/h × 0.075 lb/ft³ × 0.24 BTU/lb·°F; it is scaled by the ASHRAE standard-atmosphere pressure ratio at the site elevation (zm in metres), the same correction the AIM Works app applies. ASHRAE 90.1-2022 §6.5.2.1 limits the airflow reheated in the deadband to the larger of the zone’s 62.1 Voz and 20% of its design supply, and the dual-maximum heating airflow to the larger of 50% of design and the deadband minimum; the tool screens both, and a minimum above the limit can still be justified by an energy analysis or by code-required pressurization. The 20% default is that limit, not a recommended setpoint. The inlet-velocity bands — 900–1,200 FPM low-noise, about 2,500 FPM typical catalog maximum, about 300 FPM minimum for the flow sensor — are rules of thumb; confirm NC, pressure drop, and minimum controllable airflow with the box manufacturer.
Frequently Asked Questions
How is the cooling maximum CFM for a VAV box calculated?
The design (cooling) maximum airflow is the zone sensible cooling load divided by the supply temperature difference: CFM = sensible load divided by (1.08 times delta-T), where delta-T is the room setpoint minus the cooling supply-air temperature. The constant 1.08 is the sea-level sensible-heat factor for air (60 minutes per hour times 0.075 lb per cubic foot times 0.24 BTU per pound per degree F); the tool scales it by the standard-atmosphere air density at the elevation you enter. For example, an 18,000 BTU/h sensible load with a 72 degree F room and 55 degree F supply air needs about 980 CFM at sea level and about 1,180 CFM at 5,000 ft.
How is the minimum CFM set on a VAV terminal?
The deadband minimum is the larger of a percentage of the design maximum and the zone outdoor airflow Voz from ASHRAE 62.1. ASHRAE 90.1-2022 section 6.5.2.1 limits the airflow that may be reheated in the deadband to the larger of Voz and 20 percent of the zone design supply, unless a higher rate is justified, so the tool defaults to 20 percent and checks your entry against that limit. The 20 percent is a ceiling, not a target: a lower minimum that still meets Voz and the box’s minimum controllable airflow saves reheat energy.
What inlet velocity should a VAV box be sized for?
There is no code value; it is a trade-off. Selecting for about 900 to 1,200 FPM at the design maximum is a conservative low-noise choice and is the default limit the AIM Works app uses, and catalogs commonly run single-duct boxes to about 2,000 to 2,500 FPM when the NC and pressure drop are acceptable. A larger box is quieter, but at the deadband minimum its inlet velocity may fall below roughly 300 FPM, where many flow sensors cannot measure accurately, so the tool shows the velocity at both the maximum and the minimum and suggests a smaller box when the minimum is too low.
How is the heating airflow found?
The heating airflow is the heating load divided by 1.08 times the heating discharge temperature rise (discharge temperature minus room setpoint). With dual-maximum control the box first raises the discharge temperature at the deadband minimum and then increases airflow up to a heating maximum, which the tool takes as the larger of the deadband minimum and the heating-load airflow. ASHRAE 90.1-2022 section 6.5.2.1 caps that heating maximum at the larger of 50 percent of design supply and the deadband minimum, and the tool flags a zone that would exceed it. Warm air supplied 15 degree F or more above the room from the ceiling also lowers the zone air distribution effectiveness to 0.8 under ASHRAE 62.1 Table 6.2.2.2, which the tool notes.
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AIM Works brings peak loads, ASHRAE 62.1 ventilation, equipment and plant sizing, compliance checks, Design Review, and engineering deliverables into one connected mechanical design workspace.
Peak loads (RTS or CLTD/CLF) and ASHRAE 62.1-2019 ventilation
Equipment and plant sizing against ASHRAE 90.1-2022 efficiency floors
Design Review that cites the check behind every finding
An AI co-pilot that drafts changes — you approve each one
Results are design estimates for preliminary sizing. Verify final designs against applicable codes and standards — engineering judgment and a licensed professional engineer’s review are required.