Air Terminal Unit (ATU): What It Is and How It Works in HVAC Systems
An AHU conditions air for a whole floor. It has no idea that the west-facing meeting room is full of people at 4pm while the north corner is empty. The air terminal unit is what makes that distinction, and the setting that matters most on it is not the one most people check.
An air terminal unit (ATU) is a device in the ductwork close to the space it serves that regulates how much conditioned air each zone receives, and sometimes its temperature. It sits between the air handling unit and the diffuser. The most common type is the VAV box, which modulates a damper to vary airflow while supply temperature stays roughly constant. Other types are constant volume, dual duct, and fan powered units. What separates a real terminal unit from a motorised damper is pressure independence: the unit measures the airflow actually passing through it and adjusts the damper until measured flow matches setpoint, so the zone holds its condition even as duct pressure changes around it.
A single-duct VAV box with reheat, installed above a ceiling. Note the straight duct run at the inlet, which the flow sensor needs in order to read accurately. [REPLACE: image to be supplied.]
TL;DR
Key takeaways
- An ATU regulates airflow at zone level, sitting between the AHU and the diffuser. A VAV box is the most common type of ATU, not a separate thing.
- Pressure independence is the defining feature. The unit measures flow and corrects its damper, so the zone holds setpoint even as other boxes open and close.
- The minimum airflow setpoint matters more than the maximum, because it sets ventilation compliance, reheat energy and fan energy for the entire time the zone is not at peak load, which is most of the year.
- Dual maximum control uses separate heating and cooling maximums so minimum flow can be lower. On our worked figures that halves reheat energy in the low-heating range.
- Series fan-powered units run the fan continuously for constant air motion; parallel units run it only when heating is needed, so they use less fan energy.
- Selection is on airflow, pressure drop and NC rating together. A box selected on airflow alone is how you get a quiet office with an audible hiss.
What Is an Air Terminal Unit?
An air terminal unit is a device installed in the ductwork close to the space it serves, which regulates the volume, and sometimes the temperature, of conditioned air delivered to that zone.
ASHRAE's own definition is broader than most people assume. It describes an air-distribution assembly with inlet and outlet duct connections that performs one or more of: controlling the rate of airflow, controlling velocity, pressure or temperature, mixing primary streams of different temperature or humidity, or mixing primary air with air drawn from the treated space. It may contain dampers, filters, valves, heating or cooling coils, sound attenuation, nozzles or fan assemblies. That last clause is worth noting: sound attenuation and filtration are part of the recognised scope of a terminal unit, not accessories bolted on afterwards.
The clearest way to understand it is by position in the system.
Terminology worth getting right
Three terms get used as if they were interchangeable, and they are not. Air terminal unit is the general category. VAV box is one type of ATU, the variable air volume type. Air terminal device, confusingly, usually means the diffuser or grille, the thing you can see in the ceiling. So a VAV box is an air terminal unit, and it feeds an air terminal device. Because variable air volume dominates commercial practice, most ATUs you meet are VAV boxes, which is how the terms blurred in the first place.
Types of Air Terminal Units
There are six types in common use: single-duct VAV, single-duct VAV with reheat, dual-duct VAV, series fan-powered, parallel fan-powered, and constant air volume. Single-duct VAV with reheat is the default on most commercial work; the others exist to solve specific problems it cannot.
| Type | How it works | Reheat | Best suited to |
|---|---|---|---|
| Single-duct VAV | One inlet, one damper. Varies airflow to the zone; supply temperature stays roughly constant | Optional coil at discharge | The default. Interior and perimeter zones in most commercial buildings |
| Single-duct VAV with reheat | As above, plus a hot water or electric coil to warm the air when the zone needs heat | Yes | Perimeter zones, and anywhere a zone can call for heating while the system supplies cool air |
| Dual-duct VAV | Two inlets, hot deck and cold deck, with separate dampers mixing to the required temperature | Not needed, the hot deck provides it | Buildings with a dual-duct distribution system, now largely legacy on new work |
| Fan-powered (series) | Integral fan runs continuously, mixing primary and plenum air. Constant discharge volume | Usually yes | Spaces needing constant air motion and steady discharge temperature |
| Fan-powered (parallel) | Integral fan in a side branch, runs only when heating is needed | Usually yes | Perimeter zones with intermittent heating demand |
| Constant air volume (CAV) | Holds a fixed airflow regardless of duct pressure. No modulation with load | Sometimes | Spaces with a fixed ventilation requirement, such as labs and some clinical areas |
What drives the choice: whether the zone needs independent control at all, whether it can call for heating while the system is cooling (which forces reheat or fan-powered), how much fan energy the design can accept, and budget. Fan-powered units cost more, need power and a filter at each unit, and add maintenance points in the ceiling, so they are specified where the zone genuinely needs them rather than as a default.
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How VAV Terminal Units Control Airflow
A VAV box holds airflow to a setpoint by measuring the flow through itself and correcting the damper until measured flow matches the target. The zone thermostat does not command a damper position; it commands an airflow.
The control loop, in order
Inside a single-duct VAV box. The flow sensor sits at the inlet, upstream of the damper, which is why the unit needs a straight duct run in front of it. [REPLACE: image to be supplied.]
- The zone thermostat compares room temperature to setpoint and produces a cooling or heating demand signal.
- The controller converts that demand into an airflow setpoint, somewhere between the zone's minimum and maximum.
- The inlet flow sensor, typically a multi-point averaging probe, measures velocity pressure and reports actual airflow.
- The controller compares actual against setpoint and drives the damper actuator until they match.
- If the zone still calls for heat at minimum airflow, the reheat coil modulates.
Pressure independence: the thing that makes it a terminal unit
Step 3 is what distinguishes a VAV box from an expensive motorised damper. Duct static pressure is not constant. Every time another box on the same system opens or closes, pressure at your branch changes. A pressure-dependent unit, positioned by damper angle alone, would then deliver more or less air without knowing it, and the zone would drift every time a neighbouring zone changed. A pressure-independent unit measures flow and re-corrects, so it delivers the airflow it was asked for regardless of what the rest of the system is doing. Nearly all modern VAV boxes are pressure independent, and it is worth confirming rather than assuming on a specification, because the difference shows up as unexplained zone instability that gets blamed on the controls contractor.
The limit of that sensor, which decides your minimum
The flow sensor works on velocity pressure, and velocity pressure falls with the square of velocity. Halve the airflow and the signal drops to a quarter; at very low flow the signal becomes too small to measure reliably. Every VAV box therefore has a controllable minimum below which it cannot regulate accurately, and setting a minimum airflow below that figure does not give you a lower flow, it gives you an uncontrolled one. This is a hardware limit of the selected box, published by the manufacturer, and it is one of the constraints on how low the minimum airflow setpoint can go.
BMS integration
Modern boxes carry a networked DDC controller reporting to the building management system over BACnet or similar. The BMS reads zone temperature, airflow setpoint, measured airflow, damper position and reheat valve position, and writes setpoints and occupancy modes back. That two-way link is what makes system-level optimisation possible: the AHU can reset duct static pressure based on how far open the boxes actually are, rather than holding a fixed pressure all day, which is one of the larger fan energy savings available on a VAV system.
Single Maximum vs Dual Maximum Control
Single maximum control pins airflow at one minimum and modulates reheat against it. Dual maximum uses separate heating and cooling maximums, letting the minimum sit lower and raising airflow only once reheat alone cannot meet the load. This is where VAV design gets genuinely interesting, and where a lot of energy is quietly wasted. Drag the slider from full heating to full cooling and compare the two sequences.
Why the minimum setpoint is the number that matters
Designers scrutinise the cooling maximum because it sizes the box. The minimum decides the energy bill. A zone spends a small number of hours a year at peak cooling and a very large number near setpoint, and at every one of those hours the box is delivering its minimum airflow, which then has to be reheated if the zone wants warmth. Halving the minimum from 30% to 15% of design flow halves the air being reheated in that condition. Because fan power varies with roughly the cube of flow, the same reduction also cuts that zone's share of fan power substantially. ASHRAE Guideline 36 codifies dual maximum logic for exactly this reason, and ASHRAE Standard 90.1 requires separate heating and cooling maximum setpoints on VAV reheat systems.
What stops you setting the minimum lower still
Three limits, and all three are real. Ventilation: the zone must still receive its outdoor air requirement under ASHRAE Standard 62.1, and on a VAV system that outdoor air arrives inside the primary airflow, so the minimum cannot fall below what ventilation demands. Sensor range: the flow signal is velocity pressure, and below the box's controllable minimum the reading stops being trustworthy. Air distribution: too little air and the supply will not throw far enough to mix properly, leaving stratification and a cold or stale pocket in the room. The correct minimum is the highest of those three constraints, not the lowest number the controller will accept.
Fan-Powered Terminal Units: Series vs Parallel
A fan-powered terminal unit contains its own fan, which draws warm air from the ceiling plenum and mixes it with cool primary air. This lets a zone keep air moving and stay warm even when primary airflow has been turned right down, which is useful in perimeter zones. The two configurations differ in where the fan sits and when it runs.
Series against parallel. In the series unit the fan sits in the main air path and handles everything; in the parallel unit it sits in a side branch and handles only the plenum air it induces. [REPLACE: diagram to be supplied.]
A ventilation trap specific to parallel units
On a parallel unit, part of the air reaching the room comes from the ceiling plenum, and plenum air is recirculated room air, not outdoor air. The ventilation requirement must therefore be satisfied by the primary airflow alone, not by the total volume leaving the unit. It is an easy mistake to look at the discharge volume, see a healthy number, and assume the zone is well ventilated when the outdoor air component is coming only from the smaller primary stream. ASHRAE Guideline 36 makes the point explicitly for parallel fan-powered zones: the designer must ensure the combination still meets Standard 62.1. Check ventilation against primary air, always.
Where Air Terminal Units Are Used
Air terminal units appear wherever one air handling unit serves zones with genuinely different loads: multi-zone offices above all, plus hospitals, hotel public areas and laboratories. The test is simple, and it is in the last bullet below.
- Multi-zone commercial offices. The core application. One AHU system per floor, a terminal unit per zone, and independent control of perimeter and interior areas that have entirely different load profiles through the day.
- Hospitals and healthcare. Widely used, but with tighter constraints: many clinical spaces have minimum air change requirements and pressure relationships that fix the minimum airflow far higher than an office would, and some critical spaces use constant volume units for exactly that reason.
- Hotels and mixed-use. For public areas, meeting rooms and back-of-house, though guest rooms more often use fan coil units on a water system instead.
- Laboratories. Usually constant volume or specialised venturi valves rather than standard VAV, because containment and pressure relationships depend on flows that do not drift.
- Any building with genuinely different zone loads. The test is simple: if a west-facing room and a north-facing room on the same floor can never be comfortable at the same time on one air stream, you need terminal units.
Sizing and Selection Basics
Selection balances three things at once: airflow, pressure drop and noise. Optimise any one alone and one of the others fails.
| Input | What it comes from | What happens if you get it wrong |
|---|---|---|
| Design airflow | Zone sensible load and supply temperature difference, from the HVAC load calculation | Oversized box runs near its low limit and controls badly; undersized box cannot meet peak |
| Minimum airflow | The highest of: ventilation requirement, sensor controllable minimum, and air distribution needs | Too low, ventilation or control fails; too high, wasted reheat and fan energy all year |
| Inlet size and velocity | Design airflow. Manufacturer tables give the range for each inlet size | Velocity too high generates noise; too low and the flow sensor loses accuracy |
| Pressure drop | Selection at design flow, including the reheat coil and any attenuator | Adds directly to the AHU external static pressure and therefore to fan energy for the life of the system |
| NC rating | Manufacturer data at the actual inlet static pressure, checked against the target for the space | The complaint that surfaces after handover and is expensive to fix |
| Reheat coil capacity | Zone heating load at the heating airflow, not the minimum | Coil sized on minimum flow cannot deliver when the box steps up to heating maximum |
The two selection mistakes that recur
First, selecting on airflow alone and treating NC as an afterthought. Manufacturer NC data is quoted at a stated inlet static pressure. If your actual duct pressure is higher than the catalogue condition, the real noise is higher than the published number, and a box that looked acceptable on paper produces an audible hiss in a quiet office. Check NC at your actual inlet pressure, not the catalogue default, and remember that inlet pressure is highest at the boxes nearest the AHU.
Second, oversizing "for safety". A box selected two sizes up does not fail safe. It spends its life operating near the bottom of its range, where flow measurement is least accurate and control is least stable, and it will hunt. In terminal unit selection, generous sizing makes performance worse rather than more forgiving, which is the opposite of the instinct most engineers bring from equipment selection elsewhere.
Glossary
- Air terminal unit (ATU)
- A device in the ductwork close to the space served that regulates the volume, and sometimes the temperature, of conditioned air delivered to a zone. Sits between the AHU and the diffuser.
- VAV box
- A variable air volume terminal unit. The most common type of ATU, varying airflow to the zone while supply temperature stays roughly constant.
- Air terminal device
- Confusingly, the diffuser or grille itself, not the box. An ATU feeds an air terminal device.
- Pressure independent control
- The unit measures its own airflow and corrects the damper to hold setpoint, so delivered flow stays correct as duct pressure changes. The feature that distinguishes a terminal unit from a motorised damper.
- Minimum airflow setpoint
- The lowest airflow the box is allowed to deliver, set by the highest of the ventilation requirement, the sensor's controllable minimum, and air distribution needs.
- Dual maximum control
- Separate maximum airflow setpoints for heating and cooling, allowing a lower minimum than a single maximum sequence where minimum equals heating airflow.
- Series FPTU
- Fan-powered unit with the fan in line, running continuously and handling total airflow, giving constant discharge volume.
- Parallel FPTU
- Fan-powered unit with the fan in a side branch, running only when heating is needed and handling only induced plenum air.
- NC rating
- Noise Criteria rating, a single number describing equipment sound across octave bands, checked against the target for the space served.
Frequently Asked Questions
Sources
- ASHRAE Terminology, terminology.ashrae.org, for the formal definition of air terminal unit quoted above, including the functions it may perform and the components it may contain.
- ASHRAE Handbook: HVAC Systems and Equipment, ASHRAE, chapters on air distribution and terminal units, for unit types, configurations and application guidance.
- ASHRAE Guideline 36, High Performance Sequences of Operation for HVAC Systems, for dual maximum VAV control logic, the requirement that reheat be controlled to a discharge air temperature setpoint rather than space temperature alone, and the ventilation caution for parallel fan-powered zones.
- ASHRAE Standard 90.1, for the requirement that VAV reheat systems use separate maximum airflow setpoints for heating and cooling.
- ASHRAE Standard 62.1, for the outdoor air requirements that constrain how low a VAV minimum airflow setpoint can be set.
- Manufacturer selection data for inlet sizes, controllable minimum airflow, pressure drop and NC ratings at stated inlet static pressures.
Basis of this article's control comparison
The sequence chart models a single maximum sequence with a 30% minimum airflow setpoint against a dual maximum sequence with a 15% minimum and a 50% heating maximum. Those values are representative for illustrating the difference in shape, not design values. Real setpoints follow from the zone's ventilation requirement under Standard 62.1, the selected box's controllable minimum from manufacturer data, and the heating load, and they vary widely by space type. The reheat energy comparison assumes the same temperature rise in both cases, so the ratio reflects the airflow ratio alone; real reheat energy also depends on supply and discharge temperatures. Use published control sequences and manufacturer selection data for issued designs rather than the indicative figures here.
This article was last reviewed on 1 August 2026.
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