Although ventilation systems consume less energy than space heating or air conditioning equipment, they affect the amount of air that must be heated or cooled in the first place. 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 both types.
- CAV systems supply a constant airflow at variable temperature.
- VAV systems supply a variable airflow at constant temperature.
Like in any engineering decision, there are trade-offs when choosing between CAV and VAV ventilation. In general, CAV systems are less expensive and simpler to design and install, while VAV systems offer superior performance and energy savings for a higher upfront cost. In most cases, VAV ventilation the best option because long-term energy savings far outweigh the additional system cost. However, there are applications where the ventilation load shows little variation, and CAV is recommended because the added 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 normally runs with electricity, and kilowatt-hour prices in NYC are among the highest in the country. Also consider that efficient ventilation helps offset two of the highest energy expenses in buildings: space heating and air conditioning. If you have a deficient ventilation system, consider upgrading it before any improvements to heating and cooling equipment - the savings you can get from a high-efficiency chiller, 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 (CAV) Ventilation
CAV systems are well-suited for applications where the ventilation load is constant for large periods of time. Warehouses, call centers and manufacturing 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 a predictable occupancy - the system always operates at rated airflow, but only when the facility is being used.
CAV ventilation systems can be divided into three subtypes:
- Single-duct systems are the most simple type of CAV ventilation. A single set of ducts distributes conditioned air throughout indoor spaces, using common heating and cooling equipment. Therefore, this type of installation can provide either 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 for separate zones if required.
- Mixed-air systems have two sets of ducts, one for space heating and the other for cooling. Each zone has a mixing box where both airstreams meet, and the proportions of warm and cool air are adjusted according to the requirements 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 fans intermittently with ON-OFF switches, but this is not energy efficient and causes uncomfortable temperature variations. In addition, CAV systems in general 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 are no chances for them to reduce airflow.
Variable Air Volume (VAV) Ventilation
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 occupant load is random and constantly changing.
VAV systems should not be confused with demand-controlled ventilation (DCV). While related, the concepts are not equivalent: DCV consists on adjusting ventilation automatically in response to occupancy, while a VAV system allows variable airflow but control can be manual. In fact, demand-controlled ventilation can also be used in CAV systems if airflow is controlled based on occupancy, even if 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 switching, like equipment using ON-OFF controls in CAV ventilation systems.
VAV systems depend on two main elements to operate:
- VAV boxes, which adjust airflow for individual zones using dampers.
- Fan speed controls. Variable-frequency drives (VFD) are normally used for fan motors above 1 hp, while electronically commutated motors (ECM) offer built-in speed control in fractional horsepower applications.
VAV boxes open or close the air damper automatically in response to the air temperature of the zone 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 with no 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 the best performance.
- The VAV box for the zone with the highest airflow requirement is fully open to avoid wasting energy as pressure loss across the air damper.
- Other VAV boxes are opened partially as required by the zones they serve.
- Fan speed is controlled so that total airflow meets the combined demand of all zones (the fully open VAV box plus all other partially-open boxes).
In buildings where ventilation load is variable, VAV systems typically offer energy savings above 30% compared with CAV systems. The best recommendation is to get professional assistance: if you work with qualified design engineers, they can determine the ventilation system configuration that works best in your building, while ensuring it is properly designed.
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