Quick answer: A VAV (variable air volume) box is sized on two airflows — the maximum (cooling) CFM the zone needs on a design day, and the minimum CFM it can turn down to while still meeting ventilation and heating. Pick the inlet size so the box runs at a reasonable inlet velocity (roughly 1,500–2,500 FPM at max flow) with low pressure drop, and set minimum airflow to the larger of the ventilation requirement or the reheat air-temperature limit. Size the max CFM from the zone cooling load (sensible heat ÷ 1.08 ÷ ΔT). Start with the VAV sizing calculator.
What a VAV box does
In a variable air volume system, one air handler supplies cool air at a constant temperature to many zones, and each zone has a VAV terminal box that throttles the airflow up or down to hold that zone’s temperature. When a zone gets warm, its box opens and delivers more cool air; when it cools off, the box dampers down. This is far more efficient than the old constant-volume approach of cooling everything to the max and reheating — the fan slows down as total demand drops, saving large amounts of energy.
Because the box modulates between a maximum and a minimum, VAV sizing is really about setting those two limits correctly for each zone. Get them wrong and you either starve a zone on a hot afternoon (max too low) or over-cool and waste reheat energy (min too high).
Step 1: the maximum (cooling) airflow
The maximum CFM is set by the zone’s peak sensible cooling load — how much cool air it takes to offset the heat gain on the design day. The core airflow equation is:
CFM = Sensible load (BTU/h) ÷ (1.08 × ΔT)
where ΔT is the difference between room temperature and the supply air temperature, and 1.08 is the air constant (0.24 BTU/lb·°F × 60 min/h × 0.075 lb/ft³). With a typical 55°F supply into a 75°F room, ΔT = 20°F, so:
Max CFM ≈ Sensible load ÷ (1.08 × 20) = Sensible load ÷ 21.6
A zone with a 12,000 BTU/h sensible cooling load therefore needs about 12,000 ÷ 21.6 ≈ 555 CFM at design cooling. That maximum drives the box’s inlet size.
Step 2: choosing the inlet size
VAV boxes come in standard inlet sizes — 4, 5, 6, 7, 8, 10, 12, 14, 16 inches. You pick the inlet so that at the maximum airflow the inlet velocity lands in a good range, typically 1,500–2,500 FPM:
- Too small an inlet → high velocity, high pressure drop, and noise (a frequent complaint in occupied spaces).
- Too large an inlet → low velocity that hurts the flow sensor’s accuracy at minimum turndown, so the box can’t control precisely at low flow.
Manufacturers publish a max/min CFM range and a sound (NC) rating for each inlet size at various inlet static pressures. The goal is a box that covers your max CFM comfortably while still controlling accurately at your min CFM. The VAV sizing calculator helps you match the load-derived CFM to an inlet size and check the velocity.
| Inlet size | Typical max CFM (@ ~2,000 FPM) |
|---|---|
| 6 in | ~400 |
| 8 in | ~700 |
| 10 in | ~1,100 |
| 12 in | ~1,600 |
| 14 in | ~2,100 |
Step 3: setting the minimum airflow
The minimum CFM is where VAV design gets subtle. The box must never close below the largest of these requirements:
- Ventilation minimum — the zone still needs its ASHRAE 62.1 outdoor air even when cooling demand is low, so the min flow must deliver enough fresh air.
- Heating/reheat limit — if the box has reheat, supplying too little air very hot causes the warm air to stratify at the ceiling and never reach people; ASHRAE limits the supply-to-room temperature difference in heating, which sets a minimum heating airflow.
- Control stability — the flow sensor needs enough velocity to read accurately.
Minimums are commonly set around 20–40% of maximum, but the right number is whatever satisfies ventilation and heating for that specific zone. Setting minimums too high is a leading cause of “simultaneous heating and cooling” energy waste, where over-cooled air is reheated unnecessarily — a problem modern dual-maximum control sequences (a low cooling minimum plus a separate heating airflow) are designed to fix.
Pressure-independent vs. pressure-dependent boxes
Most modern VAV boxes are pressure-independent: they have an airflow sensor and a controller that holds the commanded CFM regardless of how the duct pressure upstream fluctuates as other boxes open and close. This is what you want — each zone gets exactly the airflow its thermostat calls for. Pressure-dependent boxes simply position the damper and let flow vary with duct pressure; they’re cheaper but can’t guarantee a zone’s airflow, so they’re rare in quality designs. Sizing assumes a pressure-independent box controlling between your min and max setpoints.
Worked example: sizing a VAV box
A private office zone has a peak sensible cooling load of 9,700 BTU/h and a ventilation requirement of 120 CFM. Supply air is 55°F into a 75°F room.
Max CFM: 9,700 ÷ (1.08 × 20) = 9,700 ÷ 21.6 ≈ 450 CFM.
Inlet size: at ~2,000 FPM, 450 CFM suits an 8-inch inlet (area 0.35 ft² → ~1,290 FPM at 450 CFM — comfortably in range, quiet).
Min CFM: the ventilation minimum is 120 CFM (about 27% of max). If the box has reheat, check the heating airflow: to deliver heat without stratifying, the min might need to rise — say to 150 CFM. Set the minimum to the larger value, 150 CFM.
Result: an 8-inch pressure-independent VAV box, 450 CFM max / 150 CFM min. That’s a complete, schedulable selection. Repeat per zone, and the sum of the box maximums (with diversity) sizes the air handler and fan.
From boxes to the air handler: diversity and fan sizing
Individual VAV boxes don’t just serve their zones — collectively they size the whole system. The air handler and supply fan are selected from the sum of the box maximum airflows, but with a crucial adjustment: diversity. Not every zone hits its peak cooling load at the same moment — east-facing zones peak in the morning, west-facing in the afternoon, interior zones stay fairly steady — so the building’s coincident peak airflow is less than the arithmetic sum of every zone’s maximum. A diversity factor (often 0.7–0.9 depending on the mix of exposures) is applied so the AHU is sized for the real simultaneous peak, not an impossible everything-at-once condition. This is one of VAV’s biggest advantages: the central equipment and ductwork can be smaller than a constant-volume system serving the same zones.
The payoff shows up in fan energy. Because fan power varies with roughly the cube of airflow, a VAV system riding down to 50% total airflow at part load draws only about one-eighth of the fan power — and buildings spend most hours well below peak. To capture this, the supply fan runs on a VFD controlling to a duct static-pressure setpoint, and modern designs use static-pressure reset (lowering the setpoint when boxes are mostly closed) to shave even more energy. Sizing each box correctly is what makes this whole efficiency chain work: undersized boxes force a higher duct pressure for every zone, erasing the fan savings.
Common VAV sizing mistakes
- Setting minimum airflow too high. Wastes reheat energy and causes simultaneous heating/cooling; use dual-maximum sequences.
- Ignoring ventilation at minimum. The box must still deliver ASHRAE 62.1 outdoor air when it dampers down.
- Undersizing the inlet. High velocity means noise complaints and excess pressure drop.
- Oversizing the inlet. Poor low-flow sensor accuracy and unstable control at minimum.
- Forgetting reheat airflow limits. Too little very-hot air stratifies and never warms the occupants.
- Adding zone maximums without diversity. Not all zones peak at once; apply a diversity factor when sizing the AHU.
Standards and references
| Reference | What it covers |
|---|---|
| ASHRAE 90.1 | VAV minimum airflow & dual-maximum control requirements |
| ASHRAE 62.1 | Zone ventilation minimums the box must maintain |
| ASHRAE Guideline 36 | High-performance VAV control sequences |
| AHRI Standard 880 | Performance rating of air terminals (VAV boxes) |
| Air constant | CFM = BTU/h ÷ (1.08 × ΔT) |
The bottom line
Sizing a VAV box means setting two airflows: a maximum from the zone’s sensible cooling load (CFM = load ÷ 1.08 ÷ ΔT) that picks the inlet size at a sensible velocity, and a minimum set by the greater of ventilation and reheat needs. Favor pressure-independent boxes, keep inlet velocity in the quiet range, and keep minimums as low as ventilation and heating allow to avoid wasted reheat. Generate a per-zone selection with the VAV sizing calculator, cross-check ventilation with the fresh air ventilation calculator, and confirm the design against ASHRAE 90.1 and 62.1 with a licensed mechanical engineer.
Frequently asked questions
How do you size a VAV box?
Size it on two airflows. The maximum (cooling) CFM comes from the zone's peak sensible cooling load, CFM = load in BTU/h divided by (1.08 times the room-to-supply temperature difference); this drives the inlet size, chosen so inlet velocity lands around 1,500 to 2,500 FPM. The minimum CFM is set by the larger of the ventilation requirement and the reheat air-temperature limit. Most boxes are pressure-independent, controlling flow between those setpoints.
How do you calculate maximum airflow for a VAV box?
Use the air-side sensible equation: CFM = sensible cooling load (BTU/h) divided by (1.08 times delta T), where delta T is the room temperature minus the supply air temperature and 1.08 is the air constant. With 55 F supply into a 75 F room, delta T is 20 F, so max CFM is roughly the load divided by 21.6. A 12,000 BTU/h zone therefore needs about 555 CFM at design cooling, which sets the inlet size.
How do you set the minimum airflow on a VAV box?
Set it to the largest of three requirements: the ASHRAE 62.1 ventilation minimum (the zone still needs outdoor air when cooling demand is low), the heating/reheat airflow limit (too little very-hot air stratifies at the ceiling and never reaches occupants), and the flow sensor's control-stability minimum. Minimums are commonly 20 to 40% of maximum, but setting them too high wastes reheat energy, which dual-maximum control sequences are designed to avoid.
How do you choose a VAV box inlet size?
Pick the standard inlet size (4 to 16 inches) so that at the maximum airflow the inlet velocity falls around 1,500 to 2,500 FPM. Too small an inlet gives high velocity, high pressure drop, and noise complaints; too large an inlet gives low velocity that hurts the flow sensor's accuracy at minimum turndown. Manufacturers publish a max/min CFM range and a sound (NC) rating per inlet size, which you match to your zone's max and min.
What is a pressure-independent VAV box?
A pressure-independent VAV box has an airflow sensor and controller that hold the commanded CFM regardless of how duct static pressure fluctuates as other boxes open and close, so each zone always gets the airflow its thermostat calls for. A pressure-dependent box only positions the damper and lets flow vary with duct pressure. Quality designs use pressure-independent boxes because they guarantee each zone's airflow and ventilation.
What is dual-maximum VAV control?
Dual-maximum control uses a low cooling minimum airflow plus a separate, higher heating airflow, instead of one high minimum used for both. It lets the box turn down to a very low airflow in cooling (saving fan and reheat energy) while still supplying enough air to distribute heat properly when heating is needed. It directly reduces the simultaneous heating and cooling waste caused by setting a single high minimum, and is promoted by ASHRAE 90.1 and Guideline 36.
Why shouldn't VAV minimum airflow be set too high?
Because a high minimum forces the box to keep delivering cool air even when the zone doesn't need cooling, which then has to be reheated — wasting both cooling and heating energy in simultaneous heating and cooling. A high minimum also over-ventilates and can overcool zones. The minimum should be only as high as ventilation and heating distribution require, which modern dual-maximum sequences achieve with a low cooling minimum and a separate heating airflow.