Takeoff & Estimating — Mechanical (HVAC)

VAV (Variable Air Volume)

A system that controls zone temperature by changing how much air a box delivers.

Quick Answer

Variable air volume (VAV) is an air distribution method in which a terminal box in each zone varies the amount of supply air, using a damper and a controller, to hold the space temperature. VAV boxes, often with a reheat coil, are counted EA by size from the mechanical equipment schedule.

The Full Picture

A central air handler or rooftop unit supplies cool air at a fairly steady temperature, and each zone has a VAV terminal box that opens or closes a damper to change how much air reaches the room. When a zone is warm the damper opens, and when it is cool the damper closes toward a minimum position. Many boxes include a reheat coil, hot water or electric, so a zone can be warmed at its minimum airflow. Because the central fan can slow as the dampers close, the system uses less fan energy than a constant volume system. The EPA notes that constantly changing VAV systems can cause under or over ventilation if outdoor air is not controlled, which is why controls matter.

On the documents, VAV boxes appear on the mechanical plan as tagged symbols in the ceiling space, such as VAV-1 or VAV-2, tied by duct to the main supply. The VAV box schedule lists inlet size, maximum and minimum airflow, whether the box has a reheat coil, coil capacity, electrical data and the controls type. Sometimes the schedule also gives sound limits. The controls drawings and sequence of operation say how each box behaves. The specification in Division 23 gives the equipment requirements, and the controls scope is often written in a separate section.

The takeoff unit for a VAV box is EA by size and by type, typically grouped by inlet diameter and by cooling-only, hot water reheat or electric reheat. What follows each box is counted separately: the connecting duct, the flexible duct to the diffusers, the hot water pipe connections and valves on reheat boxes, the electric power for electric heat, the thermostat or zone sensor, and the controller. Controls are frequently provided by the building automation contractor even when the box itself is furnished by the mechanical contractor.

The pricing risks come from who supplies what. Boxes ship with factory controllers on some jobs and with none on others, and the schedule may not say which. Reheat coils need pipe, valves and balancing that are easy to miss when the estimator only counts boxes. Access above the ceiling also matters: a box that needs a service panel clear of other trades affects both coordination and labor. Cross-check the count against the zones on the plan and against the airflow in the schedule.

Real Examples

→Worked takeoff: VAV box schedule: A schedule lists 12 VAV boxes: four with a 6 in inlet, five with an 8 in inlet and three with a 10 in inlet, so 4 + 5 + 3 = 12 EA. Seven of the twelve have hot water reheat. Each reheat coil has two pipe connections, giving 7 x 2 = 14 connections, and each needs an isolation valve at the supply and the return, so 14 valves. The estimator counts 12 thermostats or zone sensors from the controls drawings.

→Controls supply split: The schedule shows the box and a DDC controller, but the specification places controls in a separate section. The estimator carries the box in the mechanical price and gets confirmation on who furnishes and mounts the controllers, so they are not counted twice or left out.

→Zones versus boxes: A floor plan has 18 zones but the schedule lists 16 boxes, because two small offices share one box. The estimator follows the schedule and the duct layout, not the zone count, for the quantity.

Common Misconceptions

People assume: A VAV box is just a damper in a duct.

Actually: A VAV box is a terminal unit with a damper, an airflow sensor, a controller and sometimes a reheat coil and a fan. Each of those parts has its own connections, controls and installation labor.

People assume: VAV systems are priced from the box count alone.

Actually: The box is one line. Duct, reheat piping, valves, controls and balancing can each exceed the cost of the box. A count with no connections is incomplete.

Does MeltPlan Solve This?

Partially — adjacent

VAV boxes are counted by tag and size from the mechanical plans and schedule, and MeltPlan's takeoff service covers that counting work: AI pulls quantities from plans and US-based experienced estimators review and adjust them. Reheat piping design, controls scope and who supplies the controllers stay with your team.

Get VAV boxes counted from your plans and schedule →

Frequently Asked Questions

What does VAV stand for?

VAV stands for variable air volume. It describes a system in which each zone's terminal box varies the amount of supply air to maintain temperature.

What is a VAV box?

A VAV box is a terminal unit in the ductwork with a damper, an airflow sensor and a controller, and often a reheat coil. It meters supply air into one zone.

How is a VAV box counted in a takeoff?

Count each box EA from the mechanical schedule and group by inlet size and by reheat type. Count duct connections, pipe connections, valves and controls as their own items.

What is VAV reheat?

A reheat coil, hot water or electric, warms the air at the box when the zone needs heat but the damper is at its minimum. That coil adds piping or power to the takeoff.