All-air HVAC systems control space temperature by delivering conditioned air. There are two basic ways to vary the cooling or heating delivered to a space: change the temperature of the air while keeping the volume fixed, or keep the temperature roughly fixed and change the volume. The first is a constant volume (CV) system; the second is a variable air volume (VAV) system.
Both remain common in Canadian commercial buildings. Many older buildings have CV systems, often with reheat, while VAV has been the dominant approach for offices and institutional buildings for decades. Understanding the differences matters when designing new buildings, upgrading old ones or troubleshooting comfort and energy problems.
How constant volume systems work
A CV system delivers a fixed airflow to each space. Temperature control comes from changing the supply air temperature.
Single-zone constant volume
A single unit serves one zone. The thermostat cycles or modulates heating and cooling to vary supply temperature. This is simple and effective for spaces such as gyms, auditoriums, small retail stores and single rooms, where one thermostat represents the whole space.
Multi-zone constant volume with reheat
To serve multiple zones, a CV system supplies cold air to all zones, and each zone has a reheat coil that warms the air as needed. This gives good zone control but is very energy-intensive: the system cools all the air and then reheats much of it. Energy codes, including NECB, restrict simultaneous heating and cooling, which effectively limits new CV reheat systems except in specific applications.
Multizone and dual duct
Older multizone units mix hot and cold deck air at the unit for each zone. Dual duct systems run separate hot and cold ducts to mixing boxes at each zone. Both provide good zone control but mix heated and cooled air, wasting energy. Many have been converted to VAV over time.
How VAV systems work
A VAV system supplies air at a relatively constant temperature, typically around 12 to 14 °C (54 to 57 °F) in cooling, and varies the airflow to each zone using a VAV terminal unit (VAV box) with a damper controlled by the zone thermostat.
- As zone cooling load drops, the box damper closes, reducing airflow.
- The supply fan slows down, controlled by a duct static pressure sensor.
- When a zone needs heat, the box reduces to a minimum airflow and a reheat coil or perimeter heating provides the heat.
Because fan power drops sharply as airflow falls, VAV systems use much less fan energy at part load than CV systems. They also reduce reheat, since airflow is lowered before reheat is added. See VAV box types and selection.
Comparison
| Factor | Constant volume | Variable air volume |
|---|---|---|
| Temperature control method | Varies supply temperature | Varies airflow at near-constant supply temperature |
| Fan energy at part load | High; fan runs at full speed | Low; fan speed tracks load |
| Reheat energy (multi-zone) | High | Lower, though minimum airflows still drive reheat |
| Zoning flexibility | Limited without reheat | Good; one terminal per zone |
| First cost | Lower for single zone | Higher due to terminals, controls and sensors |
| Controls complexity | Simple | More complex; requires good commissioning |
| Humidity control | Good with continuous airflow and proper coil control | Can suffer at low airflow without proper design |
| Ventilation delivery | Constant | Varies with zone airflow; requires attention |
| Noise | Constant | Varies; boxes must be selected for low noise |
Ventilation in VAV systems
Because outdoor air is part of the supply air, reducing supply airflow can reduce ventilation to zones. Designers address this by setting appropriate zone minimum airflows, measuring and controlling outdoor air at the AHU, and using the multiple-zone system calculations in ASHRAE Standard 62.1. Demand-controlled ventilation and dedicated outdoor air systems are alternatives. See ASHRAE Standard 62.1 ventilation rate procedure.
VAV minimum airflows and reheat
Historically, VAV boxes were often set to minimum airflows of 30 to 50% of design. High minimums waste fan and reheat energy and can overcool spaces. Research and ASHRAE Guideline 36 support lower minimums, set to the larger of the ventilation requirement and the lowest airflow the box controller can reliably measure. Lowering minimums in existing buildings, after verifying ventilation and controllability, is one of the most cost-effective retro-commissioning measures.
Fan control and static pressure reset
VAV supply fans are controlled to maintain a duct static pressure setpoint. A fixed setpoint chosen for design conditions wastes energy most of the time. Static pressure reset, where the setpoint is lowered until at least one box is nearly fully open, saves significant fan energy. See static pressure reset with trim and respond.
When constant volume still makes sense
- Single large spaces with uniform loads, such as gyms, arenas, auditoriums and warehouses.
- Spaces that require constant airflow for pressurization or contamination control, such as some laboratories, operating rooms and cleanrooms (though many now use variable airflow with careful controls).
- Small buildings where the cost and complexity of VAV is not justified.
- Systems where constant air changes are required by code or standards, such as some healthcare spaces under CSA Z317.2.
Even in these cases, two-speed or variable-speed fans and demand-controlled ventilation can reduce energy use when spaces are unoccupied or lightly occupied.
Converting CV to VAV
Converting an existing CV reheat, multizone or dual duct system to VAV is a common energy retrofit. Typical steps:
- Survey existing zones, ductwork and terminal units.
- Calculate zone loads and new design and minimum airflows.
- Replace or retrofit terminals with VAV boxes and controls.
- Add a variable frequency drive to the supply fan, and to the return fan if present.
- Add duct static pressure sensing and outdoor air flow measurement.
- Implement sequences for static pressure reset, supply air temperature reset and ventilation control.
- Rebalance the system and commission the controls.
Check that existing ductwork and diffusers perform well at reduced airflow. Some diffusers dump cold air at low velocity, causing drafts.
Frequently asked questions
Is VAV always more efficient than constant volume?
For multi-zone buildings with varying loads, generally yes. For single-zone spaces, a single-zone VAV unit with a variable-speed fan provides similar benefits. Poorly commissioned VAV systems with high minimums and fixed setpoints can lose much of the advantage.
Why are some VAV zones too cold?
Common causes include box minimum airflows that are too high, failed reheat valves, sensors in the wrong location and supply air temperatures that are too low for the load. Each can be checked through trend data and functional testing.
Can a rooftop unit be VAV?
Yes. Many packaged rooftop units are available with variable-speed supply fans and staged or variable-capacity compressors for VAV operation.
Key takeaways
- CV systems vary supply temperature; VAV systems vary airflow.
- VAV saves fan and reheat energy at part load and offers flexible zoning.
- CV remains appropriate for single large zones and some constant-airflow applications.
- Ventilation, minimum airflows and static pressure control must be designed carefully in VAV.
- Converting CV reheat or dual duct systems to VAV is a proven energy retrofit.
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