Construction glossary · MEP
What is VAV box in construction?
A VAV box (variable air volume box) in construction is a sheet metal terminal unit installed in the ductwork that regulates how much conditioned air flows to a zone, opening and closing an internal damper in response to the zone thermostat. Estimators count VAV boxes each (ea) from the mechanical equipment schedule, sized by design airflow in cfm, with or without a reheat coil. The boxes sit between the main supply duct and the diffusers serving each zone.
Updated June 2026 · Reviewed by the Ruh construction team
How VAV boxes are counted
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Book a walkthroughA VAV box is the workhorse terminal unit of variable air volume HVAC systems, the dominant design in US commercial office work. On the drawings it appears as a tagged box (VAV 2-04, for example) on the mechanical plans, with the real estimating data living in the equipment schedule: design cfm, inlet size, and whether the box carries a hot water or electric reheat coil. The sheet metal contractor typically furnishes and hangs the box, the controls contractor provides the actuator and zone controls, and reheat boxes pull in the hydronic piping scope (or an electrical circuit for electric reheat). That split is exactly what new estimators get wrong. A cooling-only box and a reheat box can look identical as plan symbols, but the reheat box drags a hot water runout, a control valve, insulation, and a power connection along with it, often doubling the installed cost of the line. The other classic miss is counting boxes from the floor plan and never reconciling the count against the schedule, where sizes, and therefore prices, vary widely.
VAV boxes are counted each (ea), broken out by size and type. Size is the design airflow in cfm, which maps to the manufacturer's inlet diameter; type is cooling only, hot water reheat, or electric reheat. The count comes from the mechanical floor plans, but the pricing data comes from the equipment schedule, so the takeoff should carry one line per schedule type (for example, 12 ea cooling only at 600 cfm, 8 ea hot water reheat at 1,000 cfm). Cost drivers beyond the box itself include flex or hard duct connections, hangers, the controls package, reheat coil piping, and the access clearance above the ceiling. Balancing labor is usually priced separately under testing, adjusting, and balancing (TAB), but it scales with the box count.
Worked example
An office floor's equipment schedule lists 22 VAV boxes in three types: 12 cooling-only boxes at 600 cfm, 8 hot water reheat boxes at 1,000 cfm, and 2 reheat boxes at 1,600 cfm. Using illustrative installed costs of $1,800 ea for the cooling-only boxes, $2,900 ea for the 1,000 cfm reheat boxes, and $3,400 ea for the 1,600 cfm boxes: 12 x $1,800 = $21,600, 8 x $2,900 = $23,200, and 2 x $3,400 = $6,800, an equipment subtotal of $51,600. The 10 reheat boxes each need a hot water runout and control valve at an illustrative $1,200 ea, adding 10 x $1,200 = $12,000. Zone controls at an illustrative $850 per box add 22 x $850 = $18,700. The terminal unit scope totals $51,600 + $12,000 + $18,700 = $82,300, and the reheat distinction alone accounts for a piping line a plan-only count would have missed.
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How Ruh handles VAV box
Ruh reads the mechanical plans and the equipment schedule together, counts each VAV box by tag, and carries the cfm size and reheat type onto every line so cooling-only and reheat boxes never get priced the same. Quantities are extended against the contractor's own price book, and the estimator reviews the draft, adjusts scope splits like controls and reheat piping, and signs off.
See AI construction takeoff and estimating softwareVAV box: frequently asked questions
What is the difference between a VAV box and a CAV system?+
A constant air volume (CAV) system delivers a fixed airflow and varies the supply air temperature to control the space, while a VAV system holds supply temperature roughly constant and varies the airflow through terminal boxes. VAV is the standard for multi-zone US commercial offices because each zone gets its own box and thermostat. For the estimator, the practical difference is the count: a VAV job carries dozens of terminal units, controls, and reheat connections that a CAV layout does not.
Does every VAV box have a reheat coil?+
No. Interior zones that need cooling year-round are often served by cooling-only boxes, while perimeter zones that lose heat through the envelope typically get hot water or electric reheat. The equipment schedule identifies which boxes carry coils. The distinction matters at bid time because a reheat box adds a coil, a control valve, hydronic piping or an electrical circuit, and insulation, which can roughly double the installed cost of that line.
Who installs VAV boxes on a commercial project?+
The sheet metal or mechanical contractor typically furnishes and hangs the boxes and makes the duct connections. The building automation or controls contractor mounts actuators and zone sensors, the hydronic piping crew pipes any hot water reheat coils, and the electrician powers fan-powered or electric reheat boxes. Because the scope splits across three or four subcontracts, scope sheets should name who provides, who installs, and who wires each component, or the gaps surface at buyout.
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Figures on this page are illustrative. Construction estimates depend on project-specific conditions, source documents, market pricing, and professional judgment. Ruh's AI assists the estimator and does not replace professional review: your team reviews, validates, and approves every estimate, bid, and pricing decision.