Constant Air Volume vs Variable Air Volume HVAC

Constant Air Volume (CAV) systems are a type of HVAC system that provides a consistent airflow throughout the building, regardless of the heating or cooling load.

CAV systems use a single-speed fan to circulate air, which can be less efficient than variable air volume systems.

In a CAV system, the airflow is maintained at a constant rate, even when the building is not fully occupied. This can lead to wasted energy when the building is empty.

One of the benefits of CAV systems is their simplicity and reliability.

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A Constant Air Volume system, or CAV for short, is a type of HVAC air distribution system used in buildings where the volume of air supplied is kept constant, but the temperature of the air is varied.

CAV systems are an older design method, but they're still found in existing buildings.

They're less common in newer buildings because Variable Air Volume systems, or VAV, are now the preferred choice of installation.

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CAV systems are easier, quicker, and cheaper to install, making them a good option for smaller buildings.

However, the running cost of the system will potentially be far higher than a VAV system, because CAV is less energy efficient.

In a CAV system, the return grille is connected to the main return duct, which runs the length of the building and returns to the Air Handling Unit, or AHU.

A separate return fan pulls the air in through the grille and brings it back to the AHU.

The used air can then be rejected from the building, although some designs of AHU allow a portion of this air to be recycled back into the supply.

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Constant air volume systems come in three main types: Single Duct System, Terminal Reheat System, and Mixed Air System.

A Single Duct System is a CAV application consisting of an HVAC unit, a main supply duct, and a controlling device, such as a thermostat. This system is effective for single-zone applications like warehouses, restaurants, and residential settings.

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It's primarily used in open spaces and dedicated single-zone applications. However, it's not recommended for multi-zone applications, as it cannot deliver proper space comfort for specific rooms.

The Terminal Reheat System, on the other hand, is a more suitable alternative for multi-zone applications. It's not mentioned what it consists of, but it's mentioned as an alternative to the Single Duct System.

There's also a less common type of CAV system known as the Dual Duct CAV System. It has two ducts running along and supplying the rooms, one carrying cold air and the other carrying warm air.

Here are the three main types of CAV systems:

Constant air volume systems operate under the same basic principle of maintaining a constant stream of air while changing the discharge air temperature.

The mechanical and control components vary for each of these systems, as well as their sequence of operation. This is what sets them apart from one another.

The different types of constant air volume systems have distinct components, but they all work together to achieve the same goal.

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Dual Duct systems are not as common as they used to be, but you can still find them in older buildings.

These systems consist of two ducts running along and supplying the rooms, one carrying cold air and the other carrying warm air.

The air streams are then mixed by dampers to suit the individual room, and the air is collected in the main return duct and sent back to the AHU.

Dual Duct systems do provide improved thermal control compared to single supply duct CAV systems, but they lack control of humidity.

They're also not energy efficient, with high frictional losses in the ductwork that the fan has to overcome.

You can make Dual Duct systems more energy efficient by enabling a temperature reset, which monitors demand and reduces the hot air stream temperature to the lowest acceptable temperature.

This way, you're minimizing heating and cooling energy demand and operating costs.

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Constant air volume systems operate under a basic principle: maintaining a constant stream of air while changing the discharge air temperature.

The types of constant air volume systems vary in their mechanical and control components, as well as their sequence of operation.

In a mixed air system, room temperature is controlled by individual mixed air boxes serving each room independently. This allows for more precise temperature control and flexibility in meeting the desired setpoint.

The damper operation in a mixed air system does not vary the duct pressure of the system, as the resulting air mixture will always maintain a constant volume airflow. For instance, the damper can allow 100% cold air and 0% hot air, and in another instance, the mixture can be 70% hot air and 30% cold air.

The dedicated cooling supply duct in a mixed air system shall provide constant cold air at all times, with the unit controller monitoring and controlling the cooling duct discharge air temperature to maintain 55 °F. This value can be reset higher or lower based on the space load.

The dedicated heating supply duct shall provide constant hot air at all times, with the unit controller monitoring and controlling the heating duct discharge air temperature to maintain 75 °F. This value can be reset higher or lower based on the space load.

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A key benefit of mixed air systems is their effectiveness in controlling room temperature in multi-zone applications. However, this effectiveness comes with the trade-off of not having humidity control for all space load scenarios.

The mixed air system is still an inefficient application, as it has to provide two thermically opposite streams of air at all times and lacks humidity control for all load conditions. This HVAC application is rarely seen nowadays, as variable air volume systems are becoming the standard for multi-zone applications.

Here's a comparison of the two thermally opposite streams of air in a mixed air system:

In a Constant Air Volume (CAV) system, the airflow is fixed, but the supply air temperature is adjusted to meet the varying cooling loads of different zones. This approach is based on the sensible heat equation, Q = 1.08 x CFM x ΔT, where Q is the load of the room or zone, CFM is the airflow in cubic feet per minute, and ΔT is the difference between the room temperature and the supply air temperature.

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For instance, if the load is 10,000 BTU/h, with an indoor temperature of 75°F and a supply air temperature of 55°F, the difference is 20°F. To calculate the required airflow, the sensible heat equation can be rearranged to CFM = Q ÷ (1.08 x ΔT), resulting in 463 cfm.

In a CAV system, the airflow is fixed at 500 cfm, and the air supply temperature is adjusted to meet the cooling load. The sensible heat equation is rearranged to ΔT = Q ÷ (1.08 x CFM), where ΔT is the difference between the room temperature and the supply air temperature.

For example, if the cooling load is 10,000 BTU/h, the air supply must be 18.52°F below room temperature, which is equivalent to 56.48°F. This approach ensures that the supply air temperature is adjusted to meet the varying cooling loads of different zones.

Here's a summary of the key points:

A CAV system offers a range of benefits, including a more even temperature throughout the building. This is particularly noticeable in areas with varying occupancy rates, where the system can help maintain a consistent temperature.

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One of the key advantages of a CAV system is its ease of design and installation. This is due to the constant air flow rate, which makes it simpler to plan and set up the system.

A more even temperature throughout the building is just one of the perks of a CAV system. This can lead to a more comfortable and productive environment, especially in areas with high occupancy rates.

Constant air volume systems can be quite complex, and understanding their differences is key to selecting the right one for a specific application.

The implementation cost of a constant air volume system varies depending on the type, with the mixed air system being the most expensive.

Mixed air systems require individual mixing air boxes for each room and two supply air ducts, which increases the cost due to the need for a larger blower fan wheel and motor.

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The implementation cost of a system can vary significantly depending on the type of system chosen. The mixed air system is definitely the more costly option.

This is largely due to the need for individual mixing air boxes for each room. The additional complexity of the system requires a larger blower fan wheel and motor to provide air to both ducts simultaneously.

A larger blower fan wheel and motor drive up the overall cost of the system. This increased cost can be a significant factor in the decision-making process.

The costs associated with the mixed air system can be substantial. It's essential to carefully consider the implementation cost when choosing a system.

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Constant air volume systems can be a cost-effective option for certain applications, such as large commercial buildings where consistent air flow is required.

Understanding the different types of constant air volume systems is key to selecting the right one for a specific project. Constant air volume systems are more suitable for applications where consistent air flow is required, like large commercial buildings.

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Cost is a significant factor in choosing a constant air volume system, with the costs associated with implementation varying depending on the type of system. The costs associated with implementation can provide a complete picture to engineers to properly select a type of constant air volume system that is more cost-effective for the underlying application.

In some cases, the cost of a constant air volume system can be offset by the energy savings achieved through its use.

Variable Air Volume (VAV) HVAC systems are designed to adjust airflow according to the load of the room or zone being served. This means that the airflow can be increased or decreased as needed to maintain a consistent temperature.

In a VAV system, the air flow rate is varied, which can lead to significant energy savings compared to Constant Air Volume (CAV) systems. For example, if a room has a load of 10,000 BTU/h, the required airflow would be 463 cfm, but if the load increases to 12,000 BTU/h, the airflow would need to be increased to 555 cfm.

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The VAV system would adjust the airflow accordingly, maintaining a constant temperature difference between the room and the supply air. This is achieved through the sensible heat equation: Q = 1.08 x CFM x ΔT.

The temperature difference is kept constant, while the airflow is adjusted to meet the changing load. For instance, if the load remains the same at 10,000 BTU/h, but the airflow is fixed at 500 cfm, the air supply temperature would need to be adjusted to 56.48°F to maintain a 20°F temperature difference.

Here's a comparison of the airflow rates for different loads in a VAV system:

This table illustrates how the VAV system adjusts airflow to meet the changing load, while maintaining a consistent temperature difference.