Although ventilation systems consume less energy than heating or air conditioning equipment, they affect how much air must be heated or cooled first. Therefore, any upgrade that optimizes airflow also leads to heating and cooling savings. Ventilation systems can be classified into constant air volume (CAV) and variable air volume (VAV) systems, and this article will compare the two types.
- CAV systems provide constant airflow at varying temperatures.
- VAV systems provide variable airflow at a constant temperature.
As with any engineering decision, there are tradeoffs when choosing between CAV and VAV ventilation. In general, CAV systems are cheaper and simpler to design and install, while VAV systems offer superior performance and energy savings for a higher initial cost. In most cases, VAV ventilation is the best option because the long-term energy savings far outweigh the additional cost of the system. However, there are applications where the ventilation load varies little, and CAV is recommended because the additional features of VAV are rarely used.
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Ventilation upgrades can offer an excellent return on investment in New York City. Consider that ventilation equipment typically runs on electricity and kilowatt-hour prices in New York are among the highest in the country. Also consider that efficient ventilation helps offset two of the highest energy costs in buildings: space heating and air conditioning. If you have a poor ventilation system, consider upgrading it before any improvements to your heating and cooling equipment – the savings you can make with a high-efficiency refrigerator, boiler or heat pump are limited if the ventilation system is still inefficient. . You can also end up with oversized equipment if the ventilation system is not upgraded first.
Constant Air Volume Ventilation (CAV)
CAV systems are suitable for applications where the ventilation load is constant over long periods. Warehouses, call centers and production facilities that operate 24/7 are good examples of where CAV can be deployed effectively – these are facilities where the number of occupants and ventilation requirements show almost no variation. . CAV ventilation can also be used in concert halls or other event venues that are used sporadically but with predictable occupancy – the system always operates at rated airflow, but only when the facility is in use.
CAV ventilation systems can be divided into three subtypes:
- Single duct systems are the simplest type of CAV ventilation. A single set of ducts distributes conditioned air throughout internal spaces, using common heating and cooling equipment. Therefore, this type of installation can provide heating or cooling, but not both simultaneously.
- Reheat systems also use centralized mechanical equipment, but have reheat coils further downstream in the ducts to serve specific zones. This configuration can provide different air temperatures to separate zones if necessary.
- Mixed air systems have two sets of ducts, one for space heating and one for cooling. Each zone has a mixing box where both air currents meet, and the proportions of hot and cold air are adjusted according to the needs of each zone.
Regardless of the specific subtype, all CAV ventilation systems suffer from the same limitation: the airflow volume is fixed and there is no way to reduce it when the full design airflow is not required. The only viable control method is to operate the fans intermittently with ON-OFF switches, but this is not energy efficient and causes uncomfortable temperature variations. Additionally, CAV systems generally have poor humidity control
Despite the shortcomings of CAV ventilation systems, there are viable applications for them. In general, if ventilation requirements are constant over time, CAV systems can be deployed. VAV ventilation systems cannot save much energy if there is no chance to reduce airflow.
Variable Air Volume Ventilation (VAV)
VAV systems offer superior performance in any application where ventilation equipment is subject to frequent part-load conditions. This describes the vast majority of commercial spaces, where the occupant load is random and constantly changing.
VAV systems should not be confused with demand-controlled ventilation (DCV). Although related, the concepts are not equivalent: DCV consists of automatically adjusting ventilation in response to occupancy, while a VAV system allows variable airflow, but control can be manual. Demand-controlled ventilation can also be used in CAV systems if airflow is controlled based on occupancy, even though the ventilation equipment always operates at 100% capacity when active.
In addition to energy efficiency, a VAV system provides superior control over temperature and humidity. Equipment also lasts longer because it is not subject to frequent replacement, such as equipment that uses ON-OFF controls in CAV ventilation systems.
VAV systems rely on two main elements to operate:
- VAV boxes, which adjust airflow to individual zones using dampers.
- Fan speed controls. Variable frequency drives (VFD) are typically used for fan motors above 1 HP, while electronically commutated motors (ECM) offer integrated speed control in fractional horsepower applications.
VAV boxes open or close the air damper automatically in response to the air temperature in the area they serve. They are useful when the VAV system serves multiple zones with different HVAC requirements. However, in single-zone systems, a VAV box wastes energy by restricting airflow; In a single-zone system, fan speed controls can provide variable airflow without the need to use a VAV box.
Operating conditions are different from multi-zone systems, where the combination of fan speed controls and VAV boxes achieves best performance.
- The VAV box for the zone with the greatest need for air flow is fully opened to avoid wasted energy such as loss of pressure in the air damper.
- Other VAV boxes are partially opened as required by the zones they serve.
- Fan speed is controlled so that total airflow meets the combined demand of all zones (VAV box fully open plus all other boxes partially open).
In buildings where ventilation load is variable, VAV systems typically offer energy savings of over 30% compared to CAV systems. The best recommendation is to obtain professional assistance: if you work with qualified design engineers, they will be able to determine the ventilation system configuration that works best in your building, while ensuring that it is properly designed.
