VAV vs CAV air distribution systems field guide for HVAC

What an air distribution system does, and the split between CAV and VAV

An air distribution system takes conditioned air from a central air handler and delivers it through ductwork to the spaces that need it. The air handling unit, the AHU, cools or heats and filters the air, the supply fan pushes it down the duct, and each zone pulls off what it needs. The question that separates the two main system types is simple: what do you hold steady, and what do you let vary?

A constant air volume (CAV) system holds the airflow steady and changes the supply air temperature to meet the load. A variable air volume (VAV) system does the opposite. It holds the supply air at a constant cold temperature, commonly around 55°F, and varies the airflow to each zone to meet the load. That one control choice, vary the temperature or vary the flow, drives everything downstream: the fan, the controls, the energy bill, and how evenly the building stays comfortable.

VAV is the standard for multi-zone commercial buildings, and CAV holds on in single-zone and constant-volume work where steady airflow is the point. The rest of this guide walks the difference and where each one belongs. Two pieces it leans on, setting the box airflows and sizing the duct, have their own guides on VAV box commissioning and on duct design friction rate.

What is a constant air volume system?

A constant air volume system runs the supply fan at a fixed airflow and meets the zone load by changing the temperature of the air it delivers. The fan moves the same cubic feet per minute whether the space is calling for full cooling or barely any, and the system modulates a cooling coil, a heating coil, a reheat coil, or a face-and-bypass damper to land the supply temperature that holds the thermostat. The volume is constant. The temperature does the work.

The simplest form is single-zone CAV: one AHU, one thermostat, one space, the supply temperature riding up and down to hold that one space. A packaged rooftop unit serving an open warehouse or a single retail box is single-zone CAV in its plainest form, and for that job it is hard to beat on cost and simplicity. Multi-zone CAV tries to serve several spaces from one constant-volume unit, and that is where the trouble starts.

CAV is the older approach, and for a single load it is still the right one. The fan logic is uncomplicated, the controls are few, and there is little to commission beyond setting the airflow once and confirming the temperature control. Set it and forget it is close to true on a single-zone CAV unit, which is exactly why it survives where the load is one space with one setpoint.

Why did VAV replace most CAV systems?

Multi-zone CAV has two problems that VAV solves, and they are the reason the trade moved. The first is comfort. One thermostat controls the supply temperature for the whole CAV zone, so when the corner office is hot and the interior room is cold, the system can satisfy one but not both. Whichever space holds the thermostat wins, and the rest live with whatever that setting gives them. The classic CAV complaint is the perimeter office that bakes while the interior freezes, or the reverse.

Designers tried to fix this with more hardware. Multizone CAV units built a hot deck and a cold deck inside the unit and blended them per zone at the unit face. Dual-duct CAV ran two full ducts, hot and cold, out to each zone and mixed them at a mixing box near the space. Both gave per-zone control, and both did it by conditioning air two ways and then mixing it back, which wastes energy by design and takes two duct systems or a complicated unit to pull off.

The second problem is the fan. A constant-volume fan moves design airflow every hour the system runs, even at three in the morning when the building is empty and the load is a fraction of design. The energy in that air, moved and conditioned for no reason, never comes back. VAV attacks both problems at once by varying the airflow per zone and letting the fan back off when the building does not need the air. That is the win, and it is why VAV took over commercial design.

What is a variable air volume system?

A variable air volume system holds the supply air at a constant cold temperature and varies the volume delivered to each zone to hold that zone's setpoint. The central AHU makes cold supply air, commonly around 55°F, and pushes it down the duct at that one temperature for the whole building. At each zone sits a VAV box, a terminal unit with a damper, that throttles the airflow into the space. A zone that needs more cooling opens its damper for more cold air. A zone that needs less closes down toward its minimum.

Because every zone has its own box and its own controller, every zone gets independent control off a single supply duct. The interior room and the corner office no longer fight over one thermostat. Each one takes the air it needs and no more. This is zone-by-zone airflow control, and it is the core idea that makes VAV work where multi-zone CAV struggles.

Holding the airflow variable also lets the supply fan slow down. When most boxes are throttled back, the total air the fan has to move drops, and a variable-speed fan rides that demand down instead of plowing through design airflow all day. The cold deck stays cold, the boxes meter it out, and the fan and the cooling plant both coast at part load. The box airflows that make this work, the minimum and the maximum at each terminal, are set during commissioning, covered in the VAV box commissioning guide.

The VAV box

The VAV box is the terminal unit that does the throttling. At its plainest it is a sheet-metal box with an inlet damper, an airflow sensor, and a controller, sitting above the ceiling between the trunk duct and the diffusers it feeds. The controller reads the zone temperature, compares it to setpoint, and drives the damper between a minimum and a maximum airflow. A cooling-only box does just that. A box with reheat adds a coil for heating, covered in the next section. A fan-powered box adds a small terminal fan: a parallel fan-powered box runs its fan only on a heating call, pulling warm plenum air to temper the zone while the primary damper sits at minimum; a series fan-powered box runs its fan continuously, blending primary and plenum air for constant discharge volume, common where steady airflow or perimeter heating matters.

Two airflow setpoints define how the box behaves. The cooling maximum is the design airflow for the zone, the most air it will deliver on a full call. The minimum is the floor, set for ventilation and, on a reheat box, for the lowest flow the system will reheat. Between those two numbers the box modulates. Get those setpoints wrong and the zone is either starved or overblown no matter how good the rest of the system is.

Selecting, sizing, and commissioning the box is its own discipline, including calibrating the flow sensor to a measured hood reading and proving the reheat sequence. That work lives in the VAV box commissioning guide, so this guide keeps the box brief and points there for the setpoints, the K-factor, and the functional test.

VAV with reheat

Many VAV boxes carry a reheat coil, hot water or electric, downstream of the damper. The reason is that the central system makes one temperature of air, cold, for the whole building. A zone that needs heating cannot get it from a cold supply duct, so the box first throttles the cold air down to its minimum, and if the zone still calls for warmth, it opens the reheat coil to temper that minimum airflow up to a heating supply temperature. Perimeter zones in winter, with skin load and cold glass, are where reheat earns its place.

The caution is simultaneous heating and cooling. The central plant is spending energy to cool the supply air to 55°F, and the box is then spending more energy to heat that same air back up. If the box minimum is set too high, the zone gets more cold air than it needs at low load, the reheat coil fights it, and you pay twice, once to cool and once to reheat, for air the space did not want. That is the single biggest energy waste in a poorly set VAV system, and it traces straight back to a minimum airflow that is higher than the ventilation requirement demands.

The modern fix is the dual-maximum reheat sequence in ASHRAE Guideline 36, which drops the box to a low minimum airflow for ventilation in the deadband, then raises airflow on a heating call rather than just blasting reheat into the minimum flow. It cuts the reheat penalty hard. The minimum setting and the reheat logic are project- and sequence-specific, so confirm them against the design and the Guideline 36 sequence in use.

The variable-speed supply fan

The variable-speed fan is what turns the variable airflow into saved energy. A variable frequency drive (VFD) on the AHU supply fan ramps the fan speed up and down with the total demand of all the boxes. When most zones throttle back, the air the fan has to move drops, the drive slows the motor, and the fan power falls with it. A constant-volume fan cannot do this. It runs at one speed and burns the same power all day.

The fan does not chase a flow target directly. It chases a duct static pressure setpoint. A pressure sensor out in the supply duct tells the controller how much pressure the system is holding, and the VFD modulates fan speed to keep that pressure at setpoint as boxes open and close. Open boxes drop the pressure and the fan speeds up. Closing boxes raise it and the fan slows down. The static pressure control loop is the link between what the zones are doing and what the fan does, and it is covered in more detail in the static pressure section.

How far the fan backs off depends on the static pressure setpoint, which is why resetting that setpoint matters so much for the savings. A fan held to a high fixed pressure cannot slow down as much as the airflow alone would allow. The relationship between airflow, pressure, and fan power, and why a high fixed setpoint leaves savings on the table, ties into the duct design and the friction rate the system was built around, covered in the duct design friction rate guide.

Why is VAV more efficient than CAV?

VAV is more efficient than CAV for two reasons that stack: it saves fan energy at part load, and it avoids cooling air down only to heat it back up. The fan saving comes from the physics of moving air. Fan power follows the affinity laws, and in the ideal case the power needed falls with the cube of the airflow. Move half the air and the fan, in the ideal case, needs roughly one-eighth the power. A VAV system spends most of its hours at part load, where that cube relationship is paying out, while a CAV fan moves full design airflow every hour it runs.

The cube law is the ceiling, not what you actually bank. Because the fan holds a duct static pressure setpoint rather than dropping pressure to zero, the real saving is smaller than the perfect cube, and that gap is exactly why static pressure reset and supply air temperature reset exist. Even with that haircut, the part-load fan saving over constant volume is large. Published comparisons commonly put VAV fan energy 30 to 50 percent below an equivalent constant-volume system, though the real number depends on the building, the load profile, and the controls.

The second saving is structural. A multi-zone CAV system, and especially a dual-duct or multizone unit, conditions air two ways and mixes it, which means it routinely overcools and then reheats. VAV serves each zone with only the cold air it needs and reheats only the minimum, only when a zone actually calls for heat. Less air moved, less air overcooled and reheated. That combination is why VAV became the standard for multi-zone commercial buildings, and why energy codes are written around it for systems above a size threshold.

Zoning: room-by-room control versus coarse zones

Zoning is where VAV pulls ahead on comfort, not just energy. A VAV system can put a box on every room or every small group of rooms, so each one holds its own setpoint independently. A 30-zone floor is 30 boxes, 30 thermostats, and 30 independent loops. The corner conference room on the sunny side and the interior copy room get exactly the airflow each needs at the same time, because nothing forces them to share a control.

Multi-zone CAV cannot match that without the heavy dual-duct or multizone hardware, and even then the count of zones is limited by the unit. Single-zone CAV has exactly one zone by definition. When a building has many spaces with different loads, orientations, and schedules, that diversity is the whole argument for VAV. The spaces do not peak at the same time, and a system that can serve each one independently rides that diversity instead of fighting it.

The practical limit is that more zones mean more boxes, more controllers, and more points to commission and maintain. Fine-grained zoning is a comfort tool you pay for in first cost and in commissioning labor. The right number of zones follows the building's actual load variation, not a reflex to box everything separately.

Duct static pressure control and reset

Duct static pressure is what the supply fan controls to in a VAV system. A static pressure sensor sits out in the supply duct, often around two-thirds of the way down the longest run, and the VFD modulates the fan to hold the measured pressure at setpoint. Hold it too low and the boxes farthest out cannot get their design airflow when they open. Hold it too high and the fan burns energy it does not need and the boxes throttle hard against extra pressure, which can make them noisy and unstable.

A fixed static pressure setpoint sized for the worst-case hour wastes energy every hour that is not the worst case, which is almost all of them. Static pressure reset fixes that. The control logic watches how open the box dampers are and trims the pressure setpoint down until a box or two start asking for more, then nudges it back up just enough to satisfy them. This trim-and-respond approach, used in ASHRAE Guideline 36, keeps the setpoint at the lowest pressure that still serves the hungriest zone.

The mechanism is small, repeated steps. The logic drops the setpoint by a small increment on a timer when few or no zones are requesting more pressure, and bumps it up by a larger step when enough zones request it. The exact step sizes, timing, and pressure limits are part of the published sequence and the project's controls programming, so verify them against the sequence in use rather than assuming a number. Done right, the fan finds the floor on its own, and that is where the real part-load saving lives.

Minimum airflow and ventilation

Every VAV box has a minimum airflow it will not go below, and that floor is mostly about ventilation. ASHRAE Standard 62.1 sets the outdoor air a space needs for acceptable indoor air quality, and on a single-duct VAV box the zone gets its outdoor air as a fraction of the supply air it is receiving. Throttle the box too far and the outdoor air delivered to the space falls below what the people in it require. The minimum airflow is set to keep ventilation met even when the cooling load is light.

The tension is that a high minimum, set to guarantee ventilation, is also the airflow you may have to reheat. Set the minimum higher than ventilation actually demands and you have built in reheat energy at every low-load hour, which is the simultaneous-heating-cooling penalty from the reheat section. The minimum primary airflow is commonly in the range of 20 to 40 percent of the cooling maximum, but the right value is a calculation, not a default, and it follows the space type and the 62.1 ventilation rate procedure.

Demand-controlled ventilation (DCV) lets the minimum breathe with occupancy. A CO2 sensor in the space lets the system raise outdoor air when the room fills up and pull it back when the room empties, so the box is not holding a high ventilation minimum for people who are not there. DCV is most worth it in spaces with swinging occupancy, like conference rooms and lecture halls. The 62.1 calculations and the energy code together drive whether and where it is required.

Supply air temperature reset

Supply air temperature reset raises the cold supply air setpoint at part load to save energy. The system normally makes air around 55°F because that is what the worst-case zone needs to hold its setpoint on a design day. On a mild day, no zone needs air that cold, so holding 55°F means the cooling plant works harder than it has to and the boxes throttle down to small flows that can dump and reheat. Resetting the supply temperature up, toward the low 60s, eases the plant and pushes the boxes more open.

The control logic mirrors static pressure reset. The DDC watches the box damper positions, and when most dampers are well throttled, indicating little cooling demand, it raises the supply temperature setpoint a step at a time. When dampers start opening up past a threshold, it lowers the setpoint to keep the zones satisfied. The supply temperature and the fan reset have to be balanced, because raising the supply temperature pushes boxes open and asks the fan for more air, which costs fan energy. Guideline 36 sequences this so the two resets do not work against each other.

The trade is plant energy against fan energy, and the balance depends on the building and the plant. Reset the supply temperature too aggressively and you save chiller energy while the fan ramps up and gives some of it back, plus you can lose dehumidification, since warmer supply air pulls less moisture out. In humid climates the supply temperature reset has to respect the latent load, or the building gets cool and clammy. The reset limits belong to the design and the controls sequence.

Controls and DDC

VAV lives or dies on its controls, far more than CAV does. A single-zone CAV unit needs a thermostat and a couple of actuators. A VAV system needs a direct digital control (DDC) network: a controller at the AHU running the fan, the static pressure reset, and the supply temperature reset, plus a controller at every box running its own zone loop and reporting its damper position back up. Those box controllers feed the resets, so the system is one connected control scheme, not a stack of independent thermostats.

ASHRAE Guideline 36 is the published, tested set of control sequences for this. It defines the box logic, including the dual-maximum reheat sequence, the trim-and-respond static pressure reset, and the supply temperature reset, written so they cooperate instead of fighting. Specifying Guideline 36 instead of a vendor's ad hoc sequence is one of the better moves on a VAV job, because the sequences have been vetted and the energy savings depend on getting that logic right.

Most VAV systems that run poorly are not broken hardware. They are control sequences that were never fully commissioned, resets that were disabled because someone got a comfort complaint and did not have time to tune, or box controllers reporting a damper position that does not match reality. The building automation system (BAS) ties it together and gives the operator the trends to see it, but only if the points are mapped and the sequences are actually running. Confirm the sequence against the design and the Guideline 36 logic in use.

Dual-duct, multizone, and single-zone VAV

Older buildings still run systems worth recognizing on a walk-through. Dual-duct CAV runs two full supply ducts, a hot deck and a cold deck, out to a mixing box at each zone, which blends them to hit the zone setpoint. Multizone CAV does the same blending, but inside the air handler, sending a separately blended duct to each zone. Both give per-zone control at constant volume, and both pay for it by conditioning air two ways and mixing it back, which is why they fell out of favor on energy.

Dual-duct VAV exists too, varying the flow in each deck as well as the mix, and it shows up in some labs and hospitals where both tight zone control and high turnover are needed. It is more complex to control and to commission, and the ventilation math is harder, since each deck's minimum has to be worked out separately for the outdoor-air calculation.

Single-zone VAV is the newer counterpart at the small end. A packaged rooftop unit with a VFD on its supply fan can serve one zone and vary both the airflow and the cooling stages to match the load, instead of cycling a constant-volume fan on and off. It captures much of the VAV fan saving on a single-zone job without any boxes, and energy codes have pushed it into a lot of packaged-unit applications that used to be plain constant volume.

When does CAV still make sense?

CAV still fits where constant airflow is the requirement, not a compromise. The clearest case is the single zone: one space, one load, one setpoint. A warehouse, a gymnasium, a single retail box, a server closet, an open space with one thermostat, all of these are served fine and cheaply by a constant-volume unit, and adding VAV boxes would buy nothing. Single-zone VAV with a VFD is the efficient version of that same single-zone job when the energy code or the operating hours justify the drive.

The other case is where the airflow itself has to stay constant for a process or a pressurization reason. Some laboratories need a fixed air change rate and constant directional airflow to keep contaminants moving the right way, and spaces with constant exhaust, like certain kitchens or processes, need constant makeup air to replace it. Critical pressurization between spaces is easier to hold when the volumes do not swing. In those rooms, varying the airflow is the thing you are trying not to do.

Process and equipment loads that run flat around the clock also lean toward constant volume, because there is no part-load diversity for VAV to harvest. If the load barely moves, the fan saving barely shows up, and the extra boxes and controls are cost without payback. The rule is straightforward: constant airflow as a feature points to CAV, varying load across many zones points to VAV.

Data centers: a constant-airflow world

Data centers are their own world, and they run constant high airflow on purpose. Computer room air conditioning and air handling units, CRAC and CRAH units, push large, steady volumes of air to hold the IT equipment within the temperature and humidity envelope it needs. The load is dense, runs around the clock, and does not have the daily occupancy swing that makes office VAV pay, so the design logic is different from a commercial office air distribution system.

That said, the data center world has its own version of varying the airflow. Many CRAH units now carry variable-speed fans (EC fans) and modulate them to hold a target supply or cold-aisle condition, which captures fan savings without the zone-by-zone box model of office VAV. The thermal targets follow the ASHRAE TC 9.9 thermal guidelines for data processing environments, which set the recommended and allowable ranges the cooling has to maintain.

The point for an HVAC engineer is not to apply office VAV thinking to a data hall, or data center airflow logic to an office. The same words, supply air and airflow and fan speed, mean different design problems. Size and control the data center to its own guidelines and its own constant, high-turnover load.

The duct design difference

VAV and CAV duct systems are sized and designed differently, and treating one like the other causes problems. A CAV duct carries one airflow, so it is sized once for that flow and balanced to it. A VAV duct sees airflow that changes constantly as boxes open and close, so it is usually a medium-pressure design upstream of the boxes, sized to deliver design flow to every box at the worst case while the static pressure control keeps the pressure where it needs to be. The friction rate and the static pressure the system is built around set how the fan behaves and how much reset it can give back.

The static pressure setpoint the fan holds is a direct consequence of the duct design. A tight, high-friction duct forces a higher pressure to push design air to the far boxes, which raises fan energy and limits how far reset can trim. A generously sized duct lets the fan hold a lower pressure and gives the reset room to work. This is why duct design and VAV energy are the same conversation, and the method for picking the friction rate and sizing the trunk is in the duct design friction rate guide.

Downstream of each box, the low-pressure runout to the diffusers is sized for the box's design flow like any low-pressure duct. The medium-pressure upstream system and the box selection are where VAV duct design departs from CAV, and where a duct sized by CAV habit will fight the controls instead of helping them.

Commissioning and balancing

Balancing a CAV system is close to set and forget. You set the airflow at the registers to design, confirm the fan is moving the total, and the system holds that one operating point because nothing about it varies. Test and balance (TAB) on a constant-volume unit is a finite job with one answer per outlet.

VAV is a different animal, because the system never sits at one operating point. Commissioning a VAV system means setting and verifying the minimum and maximum airflow at every box, calibrating each box's flow sensor against a measured hood reading so the controller's number matches the air actually moving, and then driving the control sequences through their range: the box reheat sequence, the static pressure reset, and the supply air temperature reset. You are commissioning behavior, not a single flow. The box-level work is detailed in the VAV box commissioning guide.

The mistake that shows up most is balancing a VAV system like a CAV system, setting flows once at one condition and walking away. A VAV box set right at full cooling can still be wrong at minimum, the reset that saves the energy may never have been tested, and the system gets occupied running on default setpoints nobody verified. The functional testing of the sequences is where VAV commissioning earns its cost, and it is the step most often shortchanged when the schedule gets tight.

The problems each system brings

VAV has failure modes that trace to running at low airflow. When a box throttles way down, the cold supply air can drop out of the diffuser instead of mixing across the ceiling, a problem called dumping, and the space stratifies with cold air pooling and the thermostat reading something the occupants do not feel. The fix is in the diffuser selection and the box minimum, picking diffusers that hold their throw at low flow and not setting the minimum below what the diffuser can distribute.

Reheat energy is the other VAV problem, and it is almost always a minimum airflow set higher than ventilation requires, so the box reheats air it should not be delivering at low load. The minimum-flow ventilation question cuts both ways: too high and you waste reheat, too low and you starve the space of outdoor air. Getting that number right is the heart of an efficient VAV system, and it is a calculation per zone, not a global default.

CAV's problems are the ones that drove the move to VAV: uneven comfort across a multi-zone unit, and energy wasted moving full airflow and, in dual-duct or multizone units, conditioning air two ways to mix it back. On a single-zone CAV unit none of that applies, which is the point. Each system's worst problems show up exactly where it is being used outside its strength, VAV pushed to very low flows and CAV stretched across many zones.

Retrofitting CAV to VAV

Converting an existing CAV system to VAV is a common energy retrofit, and the payback usually comes from the fan. The core moves are adding a VFD to the supply fan, adding VAV boxes at the zones, adding the DDC controls and the static pressure sensor, and rewriting the control sequence so the fan rides demand instead of running flat out. Where the old multi-zone CAV had comfort complaints, the retrofit fixes those at the same time it cuts energy.

The catch is the existing ductwork and the existing fan. A duct system designed for constant volume may not suit the medium-pressure VAV approach, and the static pressure the boxes need can exceed what the old design was built for, which limits the reset savings. The fan and motor have to suit a VFD, and the building has to take the added controls. A retrofit that drops boxes into an undersized duct and calls it VAV can end up noisy and starved at the far zones.

The smaller-scale version is putting a VFD on a single-zone constant-volume unit and converting it to single-zone VAV, which captures fan savings without any boxes. That is often the easiest first step on a packaged unit. The full multi-zone conversion is the bigger project, and whether it pays depends on the run hours, the energy rates, and how much the existing duct and fan can be reused.

Should I specify VAV or CAV?

Pick by the number of zones, the load diversity, and whether constant airflow is a requirement. VAV is the answer for multi-zone commercial buildings with varied loads and schedules: offices, schools, mixed-use floors, anything where spaces peak at different times and people expect independent comfort. The diversity across zones is what VAV harvests, and the energy codes are written assuming it for systems above a size threshold.

CAV is the answer for a single zone or where airflow has to stay constant. One space with one setpoint, a process that needs steady air change, a room held under constant pressurization, a space with constant exhaust needing constant makeup. On those jobs the boxes and controls of VAV buy nothing, and single-zone VAV with a VFD covers the case where you want the fan saving on a single-zone load.

The honest trade is first cost and complexity against energy and comfort. VAV costs more to build and far more to commission, and it needs an operator who understands the controls. CAV is cheaper and simpler and right where the load is one zone or constant. Match the system to the building's load behavior, not to a habit or a default, and let the energy code and the project requirements set the floor.

What to document

An air distribution system that nobody documented is one nobody can tune later. For VAV especially, the setpoints and sequences are the system, and they live in the controls where they are easy to change and easy to lose. The record is what lets the next operator or commissioning agent confirm the system still runs the way it was designed, and find the setting that got bumped when a comfort complaint came in.

Capture the system type and the design supply air temperature, the design and minimum airflow at each box, the static pressure setpoint and whether reset is enabled and within what limits, the supply temperature reset limits, the ventilation basis for the box minimums, and the control sequence reference, including whether Guideline 36 is in use. If resets were disabled or overridden, write down why, because a disabled reset is the most common reason a VAV system uses more energy than its design promised.

Common mistakes

• Specifying multi-zone CAV where the load diversity and comfort needs called for VAV.
• Running VAV without static pressure reset or supply air temperature reset, leaving the part-load savings on the table.
• Setting the box minimum airflow higher than the ventilation calculation requires, building in reheat energy at every low-load hour.
• Skipping demand-controlled ventilation in spaces with swinging occupancy, holding a high minimum for people who are not there.
• Setting a box minimum below what the diffuser can distribute, so cold air dumps and the space stratifies.
• Balancing a VAV system like a CAV system, setting flows once and never testing the control sequences.
• Dropping VAV boxes into ductwork sized for constant volume, so the far zones are starved and noisy.
• Treating data center or lab constant-volume logic and office VAV logic as interchangeable.

Field checklist

Standards and references

ASHRAE Standard 90.1 is the energy standard that shapes air distribution design, setting fan power limits, requiring variable-speed control on larger systems, and requiring controls like static pressure reset and supply air temperature reset above size and capacity thresholds. It is the reason VAV with reset is effectively the default for multi-zone commercial buildings of any size. The adopted edition and the local energy code amendments control, so confirm the version the jurisdiction enforces.

ASHRAE Standard 62.1 governs ventilation. The ventilation rate procedure sets the outdoor air each zone needs, and on a VAV box that requirement drives the minimum primary airflow and any demand-controlled ventilation. ASHRAE Guideline 36 provides the tested control sequences for VAV, including the dual-maximum reheat logic, the trim-and-respond static pressure reset, and the supply temperature reset. Specifying it gives the controls contractor a vetted sequence instead of an ad hoc one.

For duct construction and leakage, SMACNA gives the construction standards and leakage classes, and ACCA Manual references cover system design on the lighter-commercial end. Equipment selection and the box and AHU performance follow the manufacturer's data and listings, which govern the specific numbers. Cite the standard that controls the point, and let the project specification and the adopted code edition override any rule of thumb in this guide.

Units, terms, and conversions

Air distribution work mixes a few unit systems and a lot of shorthand, so the same idea reads differently across a drawing set, a controls submittal, and a manufacturer sheet.

Airflow is in cubic feet per minute (CFM) in US practice and liters per second (L/s) or cubic meters per hour in metric. Duct static pressure is in inches of water column (in. w.c., also written in. wg) or pascals (Pa); roughly 1 in. w.c. is about 249 Pa. Supply air temperature is in °F or °C, with the common VAV cold deck around 55°F, near 13°C. Fan speed control is by a VFD, sometimes called a variable speed drive or, on small EC-motor fans, an electronically commutated motor.

CAV / VAV

Constant air volume (fixed airflow, varied temperature) and variable air volume (fixed cold supply temperature, varied airflow)

VAV box

Terminal unit at each zone with a damper, controller, airflow sensor, and optional reheat coil

Supply air temperature (SAT)

Temperature of the air leaving the AHU, commonly around 55°F in a VAV system, subject to reset

Static pressure reset

Trim-and-respond logic that lowers the duct pressure setpoint to the minimum that still serves the hungriest zone

Minimum primary airflow

The lowest airflow a box will deliver, set for ventilation per ASHRAE 62.1 and the reheat floor

DCV

Demand-controlled ventilation, which adjusts outdoor air to occupancy, often by CO2 sensing

Guideline 36

ASHRAE's tested high-performance control sequences for VAV boxes and air handlers

Related tools

Calculators and readiness checks for this work

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