A terminal unit is a device at the end of an HVAC duct system that controls the temperature, airflow, or pressure of air delivered to a specific room or zone in a building. It’s the last piece of equipment air passes through before entering the space you occupy. In most commercial buildings, every conference room, office, or open floor area has its own terminal unit, allowing each zone to be heated or cooled independently.
How a Terminal Unit Fits Into an HVAC System
A central air handling unit (AHU) conditions air for an entire building or floor, then pushes that air through a network of ducts. The terminal unit sits between the ductwork and the room. Its job is to take that centrally conditioned air and fine-tune it for the specific needs of the zone it serves. If your office is too warm but the conference room next door is comfortable, the terminal unit in your zone adjusts independently to fix the problem.
ASHRAE, the organization that sets HVAC standards, defines an air terminal unit as a distribution assembly that can control airflow rate, velocity, pressure, or temperature. It can also mix streams of air at different temperatures or blend supply air with air already in the room. These functions can be automatic or manual, though nearly all modern systems use automatic controls tied to a thermostat or temperature sensor in the zone.
The Most Common Type: VAV Boxes
The variable air volume (VAV) box is by far the most widely used terminal unit in commercial buildings. Rather than delivering a constant stream of air and changing its temperature, a VAV box changes how much air flows into the room. When a zone needs more cooling, the box opens its damper to let more conditioned air through. When the temperature is satisfied, the damper closes to a minimum position that still provides enough fresh air for ventilation.
A typical VAV box contains several key components:
- Airflow sensor: Measures incoming air volume at the inlet and feeds that data to the controller, allowing the box to maintain precise flow rates regardless of pressure changes in the duct system.
- Damper: A motorized blade that opens or closes to increase or decrease airflow based on what the zone needs.
- Reheat coil (optional): Warms the air leaving the box when a zone needs heat. This coil can be electric or hydronic (hot water). Perimeter zones near windows commonly have reheat coils because they lose more heat to the outdoors.
- Controller: Receives signals from the zone thermostat and the airflow sensor, then positions the damper and activates heating as needed. Modern systems use direct digital controls (DDC), though older buildings may still have pneumatic or analog electronic controls.
Three Modes of VAV Operation
A pressure-independent VAV box, the standard in modern buildings, operates in three distinct modes. In cooling mode, the damper opens progressively to deliver more cold air as the room temperature rises above the setpoint. In dead-band mode, the zone temperature is satisfied and the box holds airflow at its minimum setting, providing just enough outside air to meet ventilation requirements. In heating mode, when the zone drops below its heating setpoint, the reheat coil activates to warm the supply air before it enters the room.
This three-mode approach is what makes VAV systems energy efficient. The central system doesn’t waste energy cooling air that then needs to be reheated. Instead, each zone gets only the amount of conditioned air it actually needs at any given moment.
Fan-Powered Terminal Units
Some VAV boxes include a small built-in fan. These fan-powered terminal units come in two configurations. A series fan runs continuously and supplements the airflow from the central duct system, drawing additional air from the ceiling plenum (the space above the drop ceiling) and mixing it with the primary supply air. A parallel fan only kicks on when the zone needs extra heating, pulling warm air from the plenum to reduce the load on the reheat coil.
Fan-powered boxes typically include a small filter at the fan inlet to clean the plenum air before mixing it into the supply stream. They’re common in interior zones of large buildings where consistent airflow and mixing are important for occupant comfort.
Active Chilled Beams
A newer type of terminal unit, the active chilled beam, takes a different approach entirely. Instead of using a damper to control airflow volume, it uses a principle called induction. A small stream of conditioned primary air is pushed through nozzles inside the beam, creating a pressure difference that pulls room air up through a cooling or heating coil built into the unit. The treated room air then mixes with the primary air and flows back into the space.
The key advantage is that this entrainment effect moves a large volume of air across the coil without needing a fan, reducing energy use and noise. The ratio of room air drawn in to primary air supplied (called the entrainment ratio) is an important performance metric for these units. Active chilled beams are typically mounted in the ceiling and are common in labs, hospitals, and office buildings where quiet operation and good air quality are priorities.
How Zone Temperature Control Works
Regardless of the terminal unit type, the control logic follows the same basic pattern. A temperature sensor or thermostat in the zone sends a signal to the terminal unit’s controller. The controller compares the actual room temperature to the desired setpoint, then adjusts the damper position, fan speed, or coil output to close the gap. In a well-functioning system, this happens continuously and automatically, keeping room temperatures within one or two degrees of the setpoint.
The thermostat on your office wall, or the temperature sensor mounted near the ceiling, is communicating directly with the terminal unit serving your zone. When you adjust the setpoint on a building thermostat, you’re telling that specific terminal unit to change its behavior, not the entire building’s air system.
Electric vs. Hydronic Reheat
When a terminal unit includes a reheat coil, the two main options are electric resistance heating and hydronic (hot water) heating. Electric coils are simpler to install because they only need a power connection, not piping. Hydronic coils circulate hot water through a finned tube, transferring heat to the air passing over it. Hydronic systems are generally more energy efficient because moving heat through water requires less energy than generating it with electric resistance. In buildings with both heating and cooling needs, the hydronic piping network can serve both the central cooling coils and the terminal reheat coils, consolidating the distribution system.
Some terminal units also include cooling coils for full dehumidification at the zone level. In these configurations, a heating coil sits downstream of the cooling coil to reheat the air after moisture has been removed, preventing the space from becoming too cold while still controlling humidity.