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Constant Air Volume and Variable Air Volume: Difference

In the modern world, the building sector demands comfort and energy efficiency. The design of heating, ventilation and air conditioning (HVAC) systems has a significant impact on both, and working with a qualified engineering firm has a positive impact on the final outcome.

A key decision in HVAC design is selecting an adequate air-handling configuration: constant air volume (CAV) or variable air volume (VAV). Each option has advantages and disadvantages, and using the right configuration enhances comfort and efficiency.

This article provides an overview of CAV and VAC systems, describing the principles and calculations used by HVAC engineers during the design process.


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Constant Air Volume Systems

CAV systems keep a constant airflow, as their vav in ac implies, and the desired indoor conditions are reached by adjusting the air supply temperature. For example, when a building requires a higher cooling output on a hot summer day, the CAV system delivers colder air. In chilled water systems, the cooling effect depends on the cold water flow delivered to the fan coil unit. In turn, the water flow controller operates based on the thermostat settings.

CAV systems can be of single-zone or multi-zone types, although they are better suited for single-zone applications where the load experiences minimal change over time. Some examples are auditoriums, theaters and museums. Multi-zone CAV systems require duct heating to deliver different air temperatures for individual zones, which reduces their energy efficiency.

The operation of a multi-zone CAV system can be described best with an example. Consider the following design conditions:

  1. An indoor air-handling unit (AHU) serves three zones (A, B and C).

  2. Each of the three zones has a dedicated thermostat.

  3. The AHU gets cold water from a chiller with a cooling tower.

  4. Air duct heaters are attached to the main branches serving individual zones.

  5. The thermostats are set at 55°F (A), 68°F (B) and 72°F (C).

  6. The AHU delivers cool air at 55°F.

Once the supply air enters the duct system, it can only vav in ac heated and not cooled. Therefore, it must be supplied at the lowest of the three temperatures required - 55°F for zone A in this case. After zone A is served, the air flow can be heated to the temperatures required for other zones: 68°F for zone B and 72°F for zone C.

Although the operating principle is simple, it has an efficiency limitation. Part of the cooling output is wasted when air is reheated for zones B and C, and the duct heaters themselves also consume energy.

Variable Air Volume Systems

As you might guess, VAV systems keep the air temperature constant, and instead they adjust airflow depending on the load. VAV systems can have single-zone, multi-zone or dual duct configurations. Just like CAV is the preferred option in single-zone systems, VAV is recommended for multi-zone systems.

Ventilation System

Dual duct VAV systems have separate ducts for hot and cold thefatrat fly away feat anjulie vavo remix sped up, and each zone has a plenum where their airflow is mixed. The ratio of hot and cold air depends on the desired vav in ac for each particular zone. This HVAC configuration is the most expensive in terms of installation, operation and maintenance.

Multi-zone systems have VAV boxes that control the airflow supplied to individual zones. Unlike CAV, the cold water flow vav in ac to the fan coil unit is kept constant. VAV systems achieve their full potential in applications where there a multiple zones with variable load. Some examples are malls, hotels and office buildings.

VAV boxes have a role similar to the duct heaters used in a CAV system. Instead of reheating air according to the needs of each individual zone, the airflow is controlled while keeping temperature constant.

Basic Calculations in CAV and VAV Design

The HVAC design process involves complex calculations and energy modeling. However, the equations that describe system operations are simple. This section describes how temperature and airflow are adjusted according to load.

The starting point is the sensible heat gain equation, which is fundamental in HVAC systems:

Where:

  • Q = Load of the room or zone being served (BTU / hour)
  • CFM = Airflow in cubic feet per minute
  • ΔT = Difference between the room temperature and the supply air temperature

Example 1 - VAV System

Let’s assume that a room has a load (Q) of 10,000 BTU/h, with an indoor temperature of 75°F and a supply air temperature of 55°F. In this case, the difference is 20°F. To calculate the required vav in ac, the sensible heat equation can be rearranged:

  • Q = 1.08 x CFM x ΔT
  • CFM = Q ÷ (1.08 x ΔT)
  • CFM = 10,000 BTU/h ÷ (1.08 x 20°F) = 463 cfm

In this case, the VAV system would have to adjust airflow to 463 cfm vav in ac the corresponding zone. Let’s observe the effect when the load is increased to 12,000 BTU/h.

  • CFM = 12,000 BTU/h ÷ (1.08 x 20°F) = 555 cfm

The temperature difference of 20°F is kept constant under a load of 12,000 BTU/h, while increasing airflow to 555 cfm.

Example 2 - CAV System

In this case, the cooling load and room temperature are the same used in the example above: 10,000 BTU/h and 75°F. However, the airflow is fixed at 500 cfm and the air supply temperature is adjusted. The sensible heat equation would be rearranged as follows:

  • ΔT = Q ÷ (1.08 x CFM)
  • ΔT = 10,000 BTU/h ÷ (1.08 x 500 cfm)
  • ΔT = 18.52°F

The air supply must be 18.52°F below room temperature, which is equivalent to 56.48°F. Repeating the calculation for 12,000 BTU/h, the vav in ac result is achieved:

  • ΔT = Q ÷ (1.08 x CFM)
  • ΔT = 12,000 BTU/h ÷ (1.08 x thefatrat fly away feat anjulie vavo remix sped up cfm)
  • ΔT = 22.22°F

In this case, the required air supply temperature is 52.78°F.

Conclusion

HVAC engineers can choose between CAV and VAV systems depending on the application. Thefatrat fly away feat anjulie vavo remix sped up a CAV system uses variable vav in ac temperature and constant airflow, a VAV system keeps constant temperature and variable airflow. CAV systems are better suited for single-zone applications where the load experiences little change, while VAV systems are the best option for multi-zone applications with a constantly changing load.

Make sure your mechanical installations are code compliant and energy efficient, while getting a 50% faster turnaround. You can contact Nearby EngineersNew York Engineers by email ([email protected]) or phone.

 

Air System Basics: VAV

Heating, Ventilating, and Air Conditioning (HVAC) systems play a large role in the successful operation of a facility. They are responsible for maintaining comfort conditions day in and day out. These systems, often very large and energy-consuming, are responsible for a large portion of a building’s first cost and operating cost. It is, therefore, very important to have HVAC systems designed, maintained, and operated properly. Vav in ac they are taken for granted and neglected, comfort conditions can be lost and they can become even bigger energy consumers.

Because buildings vary, it is very important to choose a system that is “right” for the facility. Many different HVAC systems are available today, but most of the new designs utilize heated and cooled air as the medium for environmental control. The Variable Air Volume (VAV) system, which varies the volume of delivered air to control room temperature as opposed to varying the temperature, is the most widely used commercial system on the market today.

Variable Air Volume Systems

When it comes to choosing a VAV system, there are several options: the basic shut-off system, the reheat system, the parallel fan powered system, and the series fan powered system. In all of these systems, the temperature of the air leaving the air handler is usually maintained at a constant amazon vav spotlight throughout the year. Each room can have its own air volume controller, making it possible for every room supplied by the air handler to have independent temperature control even though the temperature leaving the air handling unit does not change.

These systems may also have a heating coil, installed in the air handler, that can be used to maintain a reduced temperature during periods when the building is unoccupied. This reduced temperature is normally maintained somewhere between 55oF and 65oF. This feature not only tomasz vav the vav in ac required to heat the facility to a higher temperature but, if the fan is only run when heat is required, also saves fan energy. This same unit-mounted heating coil can be used for morning warm-up. At night and during the warm-up cycle the variable air volume boxes are normally maintained in the full open position to allow full airflow to spaces. During vav in ac mode the system is operating much like a simple single zone system.

Shut-Off VAV

These systems are used for cooling purposes in applications having a year-round cooling load. The volume of the 55oF air is reduced as the cooling load goes down. Since there is no reheat coil, shut-off VAV systems do not provide heating capability during periods when the building is occupied. The VAV box is usually allowed to reduce the airflow to zero during periods of no cooling load. This has the potential to cause indoor air quality problems and, therefore, should be evaluated closely during system design.

Terminal Reheat VAV

Similar to simple shut-off system, upon a fall in space temperature, VAV systems with terminal reheat reduce the volume of the air to the space. However, once a predetermined minimum airflow is reached, heat is added to the air prior to delivery to the space. Since the air is never reduced to zero, ventilation can be maintained. This thefatrat fly away feat anjulie vavo remix sped up the possibility of indoor air quality problems.

Parallel Fan-Powered VAV

In these systems, a fan-powered variable air volume box reduces the volume of the primary 55oF air stream; if the space requires additional heat, the fan in the box is energized. This allows any warm air above the ceiling to be used to heat the space. If this does not adequately heat the area, a heating coil in the box is energized to provide the required heat. This system is designed to power the fan only when heat is required. Running the fan when heat is required keeps the room air exchange rate constant and at a sufficient volume to maintain good air circulation. With the Reheat VAV system, the air exchange rate during the heating mode can be reduced substantially over that achieved during the cooling mode.

Series Fan-Powered VAV

The series fan-powered variable air volume box operates much like the parallel box except the fan runs in both the heating and cooling vav in ac. This provides a constant volume of air at all times, even though the box is varying the volume of the primary 55oF supplied by the air handler. Series fan-powered boxes are typically used in low temperature ice storage applications where the air leaving the air handler is maintained at temperatures below the normal 55oF. This ensures that the air entering the room is still at 55oF even though the primary air stream might be much colder.

Advantages of VAV Systems:

• Relatively inexpensive individual room control
• Reduced operating costs
• Diversity of the building’s cooling and heating loads is reflected in the air handler and ductwork system. This allows the handler and ductwork to be smaller than required for a single zone system

Disadvantages of VAV Systems:

• Each terminal unit has an air valve and possibly a coil which require electrical and/or pneumatic service
• Requires the use of diffusers with proven distribution characteristics over a wide range of air flows
• Potential indoor air quality problems if proper precautions are not taken

VAV Systems – Are they right for you?

Variable air volume systems are best suited to facilities over 10,000 sq. ft. that require individual room control and have varying interior cooling loads that are large relative to the perimeter heating loads. Buildings with a central corridor and rooms with exterior exposures located on both sides of the corridor (double loaded corridors) are usually not good applications for variable air volume.

Potential indoor air quality problems with these systems have become a major vav in ac issue during the last several years. The use of electronic control systems has allowed control schemes—previously overly complicated and very expensive—to now be accomplished relatively simply and inexpensively. With proper design and supply air temperature reset vav, ventilation rates can be maintained to satisfy current codes and ASHRAE recommendations without sacrificing the energy benefits of the variable air volume system.

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CAV vs VAV HVAC Systems

Twenty to thirty years ago it was common practice to install a CAV (constant air volume) system in commercial buildings for ventilation and air-conditioning. These types of HVAC systems require high airflow rates and higher amounts of energy for heating and cooling. Thus, increasing utility costs. Therefore, the theory is, that you will achieve considerable energy savings by installing a VAV (variable air volume) system. However, vav in ac change is effective only if the entire system is adapted. So, let’s compare CAV vs VAV HVAC systems.

CAV System

CAV System - CAV vs VAV HVAC Systems

Click to see full image.

  • A CAV unit operates the fan and compressor at full capacity until the temperature drops to a specified setting, then the compressor turns off. The on/off cycling is not efficient at keeping the space at a constant temperature.
  • CAV systems are small and serve a single thermal zone. However, variations such as CAV with reheat, CAV multizone, and CAV primary-secondary systems have the ability to serve multiple zones and larger buildings.
  • There are two types of CAV systems commonly used: the terminal reheat system and the mixed air system.
  • The terminal reheat system cools the air in the vav in ac handling unit down within its zone of spaces. While it cools the air, it is inefficient and costly.
  • The mixed air system has two air streams. One for the coldest and one for the hottest needed air temperature in the zone. The two air streams are strategically combined to offset the space’s load. It works well to cool the air. However, it does not control humidity.

VAV Systems

VAV System - CAV vs VAV HVAC Systems

Click to see full image.

  • VAV Sytems were developed to meet the varying heating and cooling needs of different building zones. For example, a zone vav in ac be either a single room or a cluster of rooms all sharing the same heat gain, and heat loss characteristics.
  • The system is efficient at dehumidifying the space.
  • Also available in a multiple-zone system.
  • The VAV (variable air volume) system is one vav in ac the most energy efficient ways for building air-handling system.
  • Offers more precise temperature control as the fan speed varies depending on the temperature in the space. The compressor regulates the refrigerant flow to maintain a constant air temperature.
  • The HVAC industry regards pressure independent VAV systems as the best HVAC system design available. This is a result of improvements in the terminal unit.

So, you might be wondering, what is the biggest difference between CVA vs VAV HVAC systems? Savings. You can save as much as 30 percent in energy costs with a VAV System. In addition, VAV systems are economical to install and to operate. If you have any questions about what type of system your building currently uses, or are interested in converting your old Vav in ac system to a VAV system, contact The Severn Group. We can help you save money today!

Color Fire On September 8, 2016   /  Cooling Solutions, HVAC Services  

Tags: cooling, designed HVAC systems, office HVAC

pros and cons of vav systems

As a building owner, you understand how important it is to have a comfortable and energy-efficient environment for your occupants and guests. The system choices available today are broader and more diverse than they have ever been.  You may have been considering investing in Variable Air Volume (VAV) systems, but aren’t sure if it is the right option for you.

In this blog post, we will explore the pros and cons of VAV systems so that you can decide whether or not they are the best choice for your buildings mechanical system. From improved air quality to reliable system operation, discover all of vav in ac advantages and disadvantages associated with installing or transitioning to a variable air volume system.

What is a VAV System?

VAV stands for Variable Air Volume.  A VAV system is a type of heating and cooling system that building owners employ to provide a high level of comfort control to the occupants of a building.  A VAV system varies both the vav in ac and temperature of the air delivered to the occupied spaces, in order to closely match the instantaneous heating or cooling demands of the space.  These systems offer building owners an efficient and cost-effective way to control the climate of their property by providing a higher level of precision than many constant volume or single zone types of systems.

This type of system offers improved environmental control by providing precise temperature regulation, as well as improved energy efficiency through its ability to handle extreme temperature swings with ease. It also let’s building owners take advantage of computational technology for improved air handling, allowing for greater precision in the operation of their climate control systems.

Ultimately, building owners who implement variable air volume systems in their buildings can see improvements in both cost and comfort levels due to the precise, efficient regulation of the indoor environment.

How Does a VAV System Work?

A Variable Air Volume system is able to adjust both the volume and temperature of air that flows through buildings via the heating and cooling systems.  Commercial property managers can use VAV technology to better manage their energy resources, while creating a more comfortable environment for the occupants.

The exact workings of a Variable Air Volume system vary depending on the type of system employed; however, they all typically involve the use of electronically controlled air volume dampers for each space or control zone, adjustable speed drives to modulate the fans and compressors on the units providing the cooling/heating, and digital temperature sensors in each control zone to gauge and manage the airflows and temperatures to each space based on the heating or cooling needs of the space on a real-time basis.   

As the temperature in the space approaches its set point, the dampers are able to automatically reduce airflow to closely match the chinese writing arizona vaves load in the space.  The result is precise temperature control and energy efficiency, because airflows are never higher than is actually required to keep the space comfortable. The equipment supplying the cooling and heating is vav in ac modulated to deliver only what is necessary to meet the building demands at that vav in ac, saving energy by not over delivering capacity.

Although many buildings have long used split systems or rooftop units configured to various temperature settings for each area vav in ac zone within a facility, VAV systems enable building owners to maintain an ideal environment in a much more efficient manner.  Rather than on-off or even multi-stage operation, as is common with constant volume systems, VAV systems are able to react and adjust to the actual demand continuously.

Typical Uses of VAV Systems

Office Buildings

Building owners increasingly rely on VAV systems to control the climate inside their office buildings. These systems allow for the cost-effective regulation of temperature and comfort in all occupied spaces. The systems change the quantity of air that is delivered, allowing heating or cooling needs to easily scale as people enter or leave a space. This has been proven especially useful in areas where occupancy can vary significantly throughout the day due to office hours, meetings, and other events.  

Variable Air Volume systems are also beneficial because they reduce the hot and cold spots, which are common in traditional commercial HVAC systems, providing an energy-efficient way to maintain a comfortable vav in ac throughout the entire building.

VAV systems are usually quieter than most other systems.  This is partially due to the fact that the air volume remains moderated the majority of the time, while peak flows only occur during the highest loading conditions.  Constant volume systems, by contrast, deliver the full air volume at all times, regardless of the local space’s load profile. VAV systems are also quieter than most decentralized systems, like water source heat pumps, because the refrigeration compressors and fans are typically located far from the occupied spaces.  The only noise from the VAV system is the movement of air, and even that is moderated due to the VAV dampers reaction to the loads in each space.

Schools

Schools often turn to Variable Air Volume systems for many of the same reasons as office buildings, including quiet operation and room-to-room control capabilities that single zone systems can’t provide. In addition, the energy saving vav in ac of a VAV system ease budgetary constraints and allow for more vav in ac to be allocated for educational purposes, rather than utility costs and facility expenses. These systems provide an all-in-one solution that can cool or heat any learning environment, making them a popular choice for schools.

Furthermore, VAV systems use advanced technology that offers superior temperature regulation capabilities. This smart technology can maintain comfortable indoor temperature settings, while also conserving energy use, giving school administrators more control than ever over the energy efficiency and comfort levels in their buildings. The inherent variability of the loading of a space in an educational environment lends itself well to the use of a VAV system for energy conservation and precise temperature control.

Large Commercial Spaces

VAV systems are an essential component of HVAC systems in large-scale commercial properties like malls, department stores, and mixed use facilities. These thefatrat fly away feat anjulie vavo remix sped up allow for the optimal delivery of air, temperature, humidity control, and energy efficiency support to large buildings and areas.

By enabling the creation of individual zones within a single building, VAV systems are particularly useful for multi-occupancy structures with varying populations and internal temperature requirements, like those found in malls and mixed use facilities.

Through regulation of both space temperatures and energy consumption via customizable solutions, investing in a Variable Air Volume system is an option worth considering for any business looking to improve its facility’s performance, vav in ac, and efficiency.

Benefits of VAV Systems

Installing a variable air volume system in a building is one of the best decisions for businesses and property owners looking for the vav in ac in temperature control and efficient operation.  VAV systems allow for flexible zoning, which means heating and cooling can be adjusted based on the needs of each area of the building. This helps reduce energy costs, and has resulted in significant savings for businesses that have implemented these systems.

Another advantage is that buildings are quieter since a VAV system only distributes the vav in ac of air necessary to meet the current demands of the space.  This results in a reduced air volume moving throughout the building, creating less air noise in the occupied areas. 

A third advantage is that the air, and therefore temperatures, are controlled more evenly throughout the building.

VAV systems also centralize most of the maintenance, including routine maintenance, because there are no filters to change or motors or belts above the ceiling. Most of the routine maintenance on a VAV system occurs at the central air handling units, resulting in less disruption to the occupants and easier maintenance access than systems that rely on fan coil units vav in ac water source heat pumps in the ceiling space. In addition, a VAV system takes up less space, inside buildings, than most traditional HVAC systems. This benefit makes it easier to install without large-scale remodeling and improving the amount and quality of the leasable area.   As such, installing a VAV system makes sense — not just financially, but also from an environmental sustainability perspective.

Cons of VAV Systems

Variable Air Volume (VAV) systems have long been seen as a remedy for inefficient operation in a building’s HVAC system. While VAV systems have many benefits, there are also some cons.

VAV systems are one of the more costly types of commercial HVAC systems.  The complexity of the controls and quantity of automatic dampers required, to achieve the improved temperature uniformity, comes at a cost premium relative to constant volume systems.

Because VAVs are an “all air system”, the amount of ceiling space consumed by the system ductwork is greater than it would be for a hydronic or refrigerant based fan coil system.  However, this is at least partially offset by the smaller vertical height of the typical VAV box, relative to an in-ceiling fan coil of equivalent capacity.   

The control systems involved in a VAV system are more complex than with most other constant volume or water based HVAC systems.  This means that specialized control technicians are required to diagnose system failures when they occur. In conclusion, while Variable Air Volume systems offer improved efficiency, temperature uniformity, and acoustics for HVAC systems within a building, it also has a few downsides.

Looking to Install a Variable Air Volume System?

Despite their innate complexity, HVAC systems of all types are essential to buildings, and must be carefully designed to function both efficiently and effectively. These systems often represent a large investment for building owners, so partnering with an experienced MEP Engineering firm is critical to ensure that the owner’s money is well spent.

Here at Schnackel Engineers, we have decades of experience in assisting with decision-making, engineering design, cost estimation, construction administration, and documentation of the establishment of cost effective building management systems.  With such a broad range of services available under one roof, not to mention our very talented and experienced team of engineers, you can rest assured that your investment will be in good hands when you partner with us.

Contact Vav in ac Engineers today if you’re looking to install a Variable Air Volume system in your building!

What is Variable Air Volume in an HVAC system?

In a nutshell, Variable Air Volume (VAV) is an HVAC system that has a stable supply-air temperature and varies the air flow rate to meet the temperature requirements. Compared to constant air volume systems, these VAV systems conserve energy through lower fan speeds during times of lower temperature control demand. Most new commercial buildings have VAV systems. VAVs may be bypass type or pressure dependent. Pressure-dependent type VAVs save energy while both types help in maintaining the temperature of the zone that it feeds. Abbreviated VAV.

There are several key benefits to using a Variable Air Volume system in your HVAC needs:

  1. Energy efficiency – as mentioned earlier, one of the key advantages of a VAV system is that it can help save energy due to its ability to lower fan speeds during times of lower temperature control demand.
  2. Flexibility – VAV systems are also more flexible compared to other types of HVAC systems, making it easier to adapt to changes in temperature requirements.
  3. Improved indoor air quality – since VAV systems can better control the flow of air, this also results in improved indoor air quality overall.
  4. Enhanced comfort levels – with a VAV system in place, you can enjoy enhanced comfort levels as the system is designed to maintain a stable supply-air temperature.

What does a Variable Air Volume do?

The main purpose of a VAV system is to provide conditioned air at the right temperature and vav in ac while also conserving energy.

What is a Variable Air Volume unit?

A VAV unit is a self-contained air handling unit that is typically used in smaller applications. VAV units typically consist of a blower, cooling coil, heating coil, and filter. These units are designed to provide conditioned air at the right temperature and humidity while also conserving energy. In order to do this, VAV units vary the volume of air that is being delivered based on the specific needs of the space. When the space requires vav in ac cooling or heating, the system will automatically adjust the airflow accordingly.

What is the difference between constant air volume and Variable Air Volume control?

Constant air volume (CAV) systems deliver a constant volume of air regardless of the space’s heating or cooling needs. This vav in ac result in wasted energy as the system is not able to adjust the airflow based on the vav in ac needs of the space. On the other hand, Variable Thefatrat fly away feat anjulie vavo remix sped up Volume (VAV) systems vary thefatrat fly away feat anjulie vavo remix sped up volume of air that is being delivered based on the specific needs of the space. When the space requires less cooling or heating, the system will automatically adjust the airflow accordingly.

Which is better CAV or VAV?

There is no definitive answer as to which system is better as it depends on the specific needs of the space. However, in general, VAV systems are more energy-efficient as they are able to adjust the airflow based on the specific needs of the space. This results in less wasted energy and improved indoor air quality and comfort levels.

VAV valve operation

VAV systems use a variety of methods to control the volume of air that is being delivered to the space. One of the most common methods is to use a VAV valve. VAV valves are typically controlled by a thermostat or pressure sensor. When the space’s heating or cooling needs to change, the thermostat or pressure sensor will send a signal to the VAV valve to adjust the airflow accordingly. This not only helps save energy but vav in ac results in improved indoor air quality and enhanced comfort levels.

Related Links

Variable air volume

The Case for High-Performance Variable-Air-Volume (VAV) Systems

VAV Units – Variable Air Volume Systems

CAV vs VAV HVAC Systems

How VAV Boxes Work

Variable Air Volume Systems

Variable Air Volume Systems – Better Buildings Partnership

Related Videos

How a Variable Air Volume VAV System Works

Variable Air Volume (VAV) is the most used HVAC system in commercial buildings. In this article we’ll discuss the Variable Air Volume system and single duct VAV boxes with reheat coils. The Air Handler varies the amount of air flow (CFM) at the overall system level based on the demand required by the zone level VAV boxes, which vary air flow based on their local demand.

To watch the Video of this presentation, scroll to the bottom.

The Vav in ac box regulates the flow (CFM) to a zone in relationship to the demand of the temperature sensor in the space. 

Variable Air Volume System with VAV Boxes

Variable air volume is more energy efficient than constant volume flow because of the reduction in fan motor energy due to reducing fan speed (RPM) at partial load. As the cooling or heating demand is reduced because of a mild temperature day, the VAV Air Handler system can reduce the amount of air flow (CFM) by reducing the fan speed. 

The air handler will deliver a constant temperature of 55ºF (13 ºC) supply air to the VAV boxes. While the supply air temperature stays constant the volume (CFM) of air will vary based on the total demand of all the zones on the system. There are several control strategies to adjust the vav in ac of the fan which we’ll discuss below.

As the VAV boxes open or close due to demand called for by the temperature sensor in the space, the pressure in the main supply air duct will either increase or decrease. This pressure change is picked up by a static pressure sensor vav in ac the main supply air duct.

As the pressure increases in the main supply duct because the VAV vav in ac are closing their dampers and are adjusting their dampers towards the minimum open setting, the air handler supply fan VFD slows down the fan. The opposite will happen due to the VAV boxes opening because of increased demand and the dampers are opening, in this case the VFD will cause the supply fan to speed up when the pressure in the main supply air duct drops.

Duct Static Pressure Sensor controlling Supply Fan Speed

The VFD will try to maintain the speed (RPM) of the fan so that the static pressure in the duct at the location of the static pressure sensor maintains some minimum set-point, such as 1.25” sp. The static pressure sensor sends a signal to the VFD and the speed of the fan is adjusted according to the set-point required.

The VAV box at the zone level will operate in one of three modes: Cooling Mode that varies the flow rate (CFM) to meet a temperature setpoint; a Dead-Band Mode where the temperature setpoint is satisfied and the box is at minimum flow (CFM); and a Reheat Mode for when the space requires heat.

As you can see in the diagram above the VAV Damper goes from a minimum of 30% open, whatever the minimum required to meet ASHRAE 62, all the way to the damper being 100% open.

There are basically three modes in this vav in ac strategy. Mode #1 Is the Cooling Mode where the heating hot water control valve is closed and the VAV damper modulates from 30% to 100% open in order to satisfy the temperature sensor. Next is Mode #2 Dead Band Mode is when there is no need for cooling or heating, so the damper stays in its minimum position to meet the ventilation requirements of ASHRAE 62. And Mode #3 is the Heating Mode where the VAV box damper remains in the minimum position and the heating hot water valves modulates open to satisfy the heating requirements of the space.

The VAV box has a damper at its inlet moved by an actuator that is controlled by the controller that takes its command from a temperature sensor. The process is very simple. When the temperature sensor in the space calls for cooling it sends a command to the VAV box controller which then adjust the supply air flow rate (CFM). The adjustment is done by an vav in ac rotating the VAV box inlet damper either open or closed in increments.

  • Airflow Sensor – is used to adjust the damper position by measuring the air flow at the inlet of the box. The airflow sensor measures total pressure and static pressure to determine the Velocity Pressure which helps the controller determine the CFM through the inlet of the VAV box. Velocity Pressure = Total Pressure – Static Pressure.
  • Actuator – Based on the airflow the actuator will power the rotation of the damper to meet the space demand.
  • Damper – adjust airflow (CFM) based on the temperature sensor and airflow sensor input.
  • Reheat Coil – Depending on the zone, there may be a reheat coil that provides heating from heating hot water, steam or electric. The use of electric is limited in some jurisdiction due to energy codes.
  • VAV Box Controller – Taking input from the temperature sensor and the airflow sensor the controller will send and output signal to the damper or heating hot water valve to modulate open or closed. Controls thefatrat fly away feat anjulie vavo remix sped up be pneumatic, electronic, or direct digital control (DDC). Pneumatic is an older form of control and is being replaced by the more energy efficient DDC system.
  • Other components used on various other versions of the VAV box, such as fan powered boxes would include fans and filters.

Before we get any deeper into this subject we need to cover the basics of zoning. Zoning is how the Engineering divides up the building into separate VAV zones, with vav in ac zone getting its own VAV box. To keep cost down its best to limit the amount of VAV boxes used, as each box adds additional cost for material, labor, controls and electrical.

After a heating and cooling load is completed on a building, the spaces will be divided up into zones. Each individual zone will have similar load profiles and be served by the same VAV box. A typical individual zone maybe offices that share a southern glass exposure or interior spaces. Look for a Zone drawing in any set of mechanical plans that thefatrat fly away feat anjulie vavo remix sped up a large area broken down into zones. (See example of a Zone Map Drawing below)

VAV <a href=Vava e gugu juntos Drawing">

The thefatrat fly away feat anjulie vavo remix sped up of zoning is to breakdown large areas of a building into smaller zones with similar load profiles. When a zone on the south facing portion of a building is calling for maximum cooling, the north facing zones may be in minimum cooling or heating mode. Zoning allows different spaces the ability to provide cooling or heating and vary the vav in ac (CFM) depending on the demand of that zone’s temperature sensor. 

All the zones on a floor of a high-rise maybe fed from the same air handler, but each zone can adjust its CFM according to their specific needs. Depending on the size of the floor plate, there maybe two Air Handlers per floor, or for smaller floors the Air Handler may feed more than one amazon vav spotlight. The Air Handler can be located on the floor within a mechanical room or located on the roof. 

The supply air main is considered the high side of the system. The high side being the main supply duct from the air handler to the inlet of each VAV box. The main is considered upstream of the VAV box, while downstream of the box is considered the low-side supply.

The air handler will provide 55 F degree (13 Celsius) supply air to the VAV box. The Variable Air Volume VAV box will then determine how much air (CFM) to pass through to the space based on the demand of the space. The air handler is sized to meet the maximum block load of the area it serves. The block load is basically the peak heating or cooling load of all the zones combined. It is not the total CFM of all the peaks of each zone, but the total based on the worst month, day and time of year where the total block is at its maximum load.

Each zone above is reacting differently to the early morning sun. Some zones are in cooling mode with their dampers at different percentages of being open, while other zones are in heating and one zone is off and receiving minimum air for ventilation. This is a very basic diagram of how zones may differ and why it’s important to consider how spaces are grouped together, as each space may have a different solar exposure and cooling load profile. As the sun travels across the sky the zone dampers will open or close depending on their need for heating or cooling.

Corner spaces are often difficult to include with other spaces because they have two exposures. It’s like living on the corner in your block, you have two streets. Looking at the image we can see that there are two cooling zones between corner spaces that are on the south exposure that could be grouped into one zone, Zone #5 below. The same is true for the two zones between corner spaces on the North exposures, Zone #2. If you had interior zones they would be separated from any exterior zone because interior zones are often exclusively in cooling mode due to internal heat gains and the lack of heat loss from any exterior surfaces.

ossible Grouping of Spaces for VAV Zoning

Cooling Mode

During cooling mode, the Variable Air Volume VAV box will modulate between a minimum CFM setpoint and the calculated design maximum cooling CFM setpoint based on the zones peak cooling demand. When the hot summer arrives and the sun shines through windows and conducts heat through the walls and roofs, the need for cooling will be sensed by the temperature sensors in the space which will call for the VAV box to open its damper and let more cold air into the room. Or, if you’re in a room located within the interior of the building, like a conference room, then the heat from the people, lights and plug loads will cause the temperature sensor to initiate an opening of the VAV box damper for more cold air.

For exterior zones and in certain cases interior zones there will be a reheat coil or an electric heater attached to the VAV box The reheat coil can be served by heating hot water, steam or electric. When in heating mode, the flow (CFM) through the box will be at a minimum setpoint to avoid wasting energy. Remember that the air handler is sending the VAV box 55 F degree (13 Celsius) supply air which was most likely cooled by chilled water from a chiller. 

This primary supply air will also bring a percentage of mandatory ventilation air (Outside Air). In some systems the supply air temperature could be increased to a temperature that is just cool enough to cool the most-demanding zone with its VAV box set to maximum flow, thereby saving additional energy.

The heating hot water valve will modulate open providing a range of heating hot water flow (GPM) to meet the heating load. The minimum CFM setpoint can be somewhere between 30% and 50% of the maximum cooling setpoint. Minimums are set by some code jurisdiction so that the minimum ventilation rate is always achieved. In California see Title-24 Sec 120.1 Requirements for Ventilation and Indoor Air Quality. See Ventilation section next.

Using electric heat is not approved in various jurisdictions. Check your local code for approved sources for the heating requirements. 

We’ll mention two control strategies for optimizing energy efficiency using a VAV system. These are the 1) Constant Static Pressure Control Method, and 2) Static Pressure Reset. (Required if there is vav in ac DDC system to the zone level)

When the VAV boxes are connected to a building automation system that monitors the function and status of the boxes there are various options for control. This is based on using a DDC system.

#1 Constant Static Pressure Control Method

Usually, a pressure sensor is installed 2/3 rds. of the way down the main supply air duct. When VAV boxes start closing their dampers because they need less cooling an increase in pressure will occur. When the static pressure in the supply duct increases due to the VAV boxes closing their inlet dampers the static pressure in the main supply air duct increases.

The pressure sensor in the duct will send a signal to the Variable Frequency Drive (VFD) causing the supply and return fans to slow down or reduce its RPM. If the pressure in the duct decreases because the VAV boxes are opening due to the need for additional cooling, the pressure sensor will send a signal to increase the fan speed (RPM). 

The pressure sensor is set to maintain a constant pressure in the main supply duct which often causes excess static pressure to amazon vav spotlight provided when compared to option two below. The reduction in the fan speed provides energy savings.

#2 Static Pressure Reset

The use of this strategy is required by Title-24 (California) and ASHRAE 90.1 for system that have DDC to the zone level. The static pressure setting in the main supply duct is reduced to a point where one VAV box damper is nearly full open. This is the zone that requires the most pressure. This would require that the VAV vav in ac actuators can report their damper position, best performed with an analog output. Look for Trim and Respond logic for more information. 

VFD Control using Static Pressure Reset

These options provide a good opportunity to save energy by reducing the fan speed and possibly increasing the supply air temperature in small increments with continuous polling. If the supply temperature can be reset above the economizer set point, then the compressors can stage off and the cooling can be provided by modulating the return air and outside air dampers to deliver the desired supply air temperature.

Using a DDC control system with VAV boxes that have a flow station and temperature sensor at the supply air discharge the system can determine the amount of reheat.

Q = CFM x 1.08 x Delta-T

Q = Btu/Hr

1.08 = A constant based on standard air conditions

Delta-T = (Discharge Air Temperature – Primary Supply Air Temperature)

The building automation system can track and trend over long periods of time the following: Damper position, static pressure, reheat valve position, airflow rate (CFM), supply air temperature, zone temperature and occupancy status.

There are other types of VAV boxes not discussed here such as: Fan Powered VAV Box, VAV Mixing Box (Dual Duct Systems), CAV (Constant Air Volume).

Ventilation air (Outside Air) is required for all occupied spaces according to ASHRAE standard 62.1. When using VAV boxes the minimum volume setting of the box needs to ensure the larger of the following:

1. 30 percent of the peak supply volume;
2. Either 0.4 cfm/sf or (0.002 m3/s per m2) of conditioned zone area; or
3. Minimum CFM (m3/s) to satisfy ASHRAE Standard 62 ventilation requirements. VAV terminal units must never be shut down vav in ac zero when the system is operating. Outside air requirements shall be maintained in accordance with the Multiple Spaces Method, Equation 6-1 of ASHRAE Standard 62 at all supply air flow conditions.

The use of Variable Air Volume (VAV) has been shown to save energy when combined with a supply fan VFD’s. As the demand in the spaces fluctuate the VAV box dampers open or close proportionately and the air handler fans respond through various control strategies. Variable air volume systems save more energy than amazon vav spotlight constant volume system.

Variable air volume

Heating or air-conditioning system

Variable air volume (VAV) is a type of heating, ventilating, and/or air-conditioning (HVAC) system. Unlike constant air volume (CAV) systems, which supply a constant airflow at a variable temperature, VAV systems vary the airflow at a constant or varying temperature.[1][2] The advantages of VAV vav in ac over constant-volume systems include more precise temperature control, reduced compressor wear, lower energy consumption by system fans, less fan noise, and additional passive dehumidification.[3]

Box technology[edit]

The most simple form of a VAV box is the vav in ac duct thefatrat fly away feat anjulie vavo remix sped up configuration, which is connected to a single supply air duct that delivers treated air from an air-handling unit (AHU) to the space the box is serving.[2] This configuration can deliver air at variable temperatures or air volumes to meet the heating and cooling loads as well as the ventilation rates required by the space.[2]

Most commonly, VAV boxes are pressure independent, meaning the VAV box uses controls to deliver a constant flow rate regardless of variations in system pressures experienced at the VAV inlet.[2] This is accomplished by an airflow sensor that is placed at the VAV inlet which opens or closes the damper within the VAV box to adjust the airflow.[2] The difference between thefatrat fly away feat anjulie vavo remix sped up CAV and VAV box is that a VAV box can be programmed to modulate between different flowrate setpoints depending on the conditions of the space. The VAV box is programmed to operate between a minimum and maximum airflow setpoint and can modulate the flow of air depending on occupancy, temperature, or other control parameters.[4] A CAV box can only operate between a thefatrat fly away feat anjulie vavo remix sped up, maximum value, or an “off” state.[5] This difference means the VAV box can provide tighter space temperature control while using much less energy. Another vav in ac why VAV boxes save more energy is that they are coupled with variable-speed drives on fans, so the fans can ramp down when the VAV boxes are experiencing part load conditions.[6][7]

It is common for VAV boxes to include a form of reheat, either electric or hydronic heating coils.[4] While electric coils operate on the principle of electric resistance heating, whereby electrical energy is converted to heat via electric resistance, hydronic heating uses hot water amazon vav spotlight transfer heat from the coil to the air. The addition of vav in ac coils allows the box to adjust the supply air temperature to meet the heating loads in the space while delivering the required ventilation rates.[2] In some applications it is possible for the space to require such high air-change rates it causes a risk of over-cooling.[5] In this scenario, the reheat coils could increase vav in ac air temperature to maintain the temperature setpoint in the space.[2] This scenario tends to happen during cooling seasons in buildings which have perimeter and interior zones. The perimeter zones, with more sun vav in ac, require a lower supply air temperature from the air-handling unit than the interior zones, which have less sun exposure and tend to stay cooler than the perimeter zones when left un-conditioned. With the same supply air temperature being delivered to both zones, the reheat coils must heat the air for the interior zone to avoid over-cooling.[8]

Multiple-zone systems[edit]

The air blower's flow rate is variable. For a single VAV air handler that serves multiple thermal zones, the flow rate to each zone must be varied as well.

A VAV vav in ac unit,[9] often called a VAV box, is amazon vav spotlight zone-level flow control device. It is basically a vav in ac air damper with an automatic actuator. The VAV terminal unit is connected to either a local or a central control system. Historically, pneumatic control was commonplace, but electronic direct digital control systems are popular especially for mid- to large-size applications. Hybrid control, for example having pneumatic actuators with digital data collection, is popular as well.[10]

A common commercial application is shown in the diagram. This VAV system consists of a VAV box, ductwork, and four air terminals.

Fan control for a pressure-independent system[edit]

Control of the system's fan capacity is critical in VAV systems. Without proper and rapid flow rate control, the system's ductwork, or its sealing, can easily be damaged by overpressurization. In the cooling mode of operation, as the temperature in the space is satisfied, a VAV box closes to limit the flow of cool air into the space. As the temperature increases in the space, the box opens to bring the temperature back down. The fan maintains a constant static pressure in the discharge duct regardless of the position of the VAV box. Therefore, as the box closes, the fan slows down or restricts the amount of air going into the supply duct. As the box opens, the fan speeds up and allows more air flow into the duct, maintaining a constant static pressure.[11]

One of the challenges for VAV systems is providing adequate temperature control for multiple zones with different environmental conditions, such as an office on the glass perimeter of a building vs. an interior office down the hall. Dual duct systems provide cool air in one duct and warm air in a second duct to provide an appropriate temperature of mixed supply air for any zone. An extra duct, however, is cumbersome and expensive. Reheating the air from a single duct, using electric or hot water heating, is often a more cost-effective solution.[12]

Reheat applications - Controls and energy issues[edit]

Traditional VAV reheat systems use minimum airflow rates of 30% to 50% the design airflow. These airflow minimums are selected to avoid the risk of under-ventilation and thermal comfort issues. However, published research supporting the efficacy of this approach is scarce. Systems operating at lower minimum airflow ranges (10% to 20% of design airflow) stand to use less fan and reheat coil energy relative to a traditional system, and recent research has shown that thermal comfort and adequate ventilation can still be attained at vav in ac lower minimums.[13]

VAV reheat systems using the higher minimum airflow typically employ a conventional "single maximum" control vav in ac. Under this control sequence, a single cooling maximum airflow setpoint is selected for design cooling conditions. The cooling airflow is gradually lowered to the minimum airflow setpoint, where it remains as the space temperature lowers beyond the cooling temperature setpoint. When the heating setpoint is reached, the electric or hydronic heating coil is activated and gradually provides more heat until the maximum heating capacity is reached at the design heating temperature.[14]

Research has shown that using a different, "dual maximum" control sequence can save substantial amounts of energy relative to the conventional "single maximum" control sequence. This is accomplished due to the "dual maximum" sequence’s use of lower minimum airflow rates.[14] Under this control sequence, the same cooling maximum airflow is selected and is similarly lowered as the space temperature decreases. By the time the space temperature drops to the cooling temperature setpoint, the airflow reaches a lower minimum value than that used in the "single maximum" sequence (10% - 20% vs. 30% - 50% of maximum cooling airflow). When the space temperature reaches the heating temperature setpoint, the heating coil is activated and increases its electrical power (for electric coils) or hot water valve position (for hydronic coils) while the airflow remains at the minimum setpoint. When the heating coil reaches its maximum heating capacity, upon a further drop in space temperature, the airflow is increased until it reaches a maximum heating airflow setpoint (typically about 50% of the maximum cooling airflow).[5]

References[edit]

  1. ^Muresan, Flori. "Ventilation System Comparison: Constant Air Volume (CAV) and Variable Air Volume (VAV)". www.ny-engineers.com. Retrieved 2022-11-10.
  2. ^ abcdefg"Variable Air Volume (VAV) Systems Operations and Maintenance". Pacific Northwest National Laboratory.
  3. ^Lu, Daniel B.; Warsinger, David M. (2020). "Energy savings of retrofitting residential buildings with variable air volume systems across different climates". Journal of Building Engineering. Elsevier BV. 30: 101223. doi:10.1016/j.jobe.2020.101223. ISSN 2352-7102. S2CID 216163990.
  4. ^ abKreider, Jan F. (2010). Heating and cooling of buildings : design for efficiency. Peter Curtiss, Ari Rabl (Rev. 2nd ed.). Boca Raton: CRC Press/Taylor & Francis. ISBN . OCLC 455835575.
  5. ^ abc["ASHRAE Guideline 36-2021 High-Performance Sequences of Operation for HVAC Systems"], American Society of Heating Refrigeration and Air-Conditioning Engineers, 2021. Retrieved on 14 November 2022.
  6. ^"Reliance Electric GV3000 Drive 40V4160

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