VAV System Design Considerations | nailor.com

A mechanical engineer must consider several variables and equipment types when designing a VAV system.  This includes the load on the space, the static pressure in the ductwork, the types of terminal units, and the occupancies in the space.  The engineer must also consider how the terminal units are going to be controlled.  These decisions must weigh the initial cost with the long-term energy efficiency. Using a theoretical office building, we will walk through the general design process and the decisions the engineer must work through to arrive at a final design.

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Design Process

When an architect is designing a building, for purposes of this exercise an office building, they’ll start with a core and shell.  The information they’ll provide the engineer will include:

• Size of building
• Orientation of the building
• General Construction
  • Window Glazing
  • Insulation values
  • Ceiling heights and type
• Building Use
• Sizes of mechanical rooms

Using this information, with the help of a load calculating software, the engineer will determine how much heating and cooling will be required to maintain the comfort of the building.  This load will be communicated in a total amount of BTUs and then the amount of air at a certain temperature will be needed to achieve these BTUs.  

Now that the engineer knows the overall demand for the space, she will use this as well as the size of the mechanical room to select the air handling unit for the space. For many VAV systems, the air handling unit will contain a cooling coil and a fan.  The cooling coil will have to provide the amount of cooling with the use of chilled water. The selection software from the air handling unit will provide the engineer with the right size of coil and amount of chilled water flow necessary to cool the building.  On a side note, the engineer will combine all the selections for the building to determine the total amount of chilled water required for the project to size and select the chiller for the entire project.

Once the AHU has been selected, the engineer will design the air distribution system for delivering the cooling to the space. This starts with the branch ductwork.  To design this ductwork, the engineer will have to consider the ceiling heights and static pressure created by sizing the duct.  In general, for the same air flow, larger ductwork will create a smaller pressure drop.  Higher pressure drops will increase the size and amount of fan energy to deliver the airflow.

Across the floor, varied occupancies and occupants will have different needs and expectations of comfort.  If we designed the air handling unit, added branch ductwork, and then tapped directly into this ductwork to serve the entire floor all in the same way some spaces would be over cooled, and others would be undercooled.  In some cases, one side of the building will need cooling while the other is heating. To ensure each area has independent control over their comfort, the floor must be broken up into spaces with similar demand.  During the phase of calculating the load, the engineer will break the core up into sections (as shown below).  The floor will contain interior and exterior zones.  When the engineer starts to design the air distribution, each one of these sections will be served by a terminal unit.

Using the loads from each one of these zones, terminal units will be selected along with the ductwork from the terminal unit needed to serve the space.  Resulting in a design like what is shown below.

Terminal Unit Selections

Generally, the interior spaces will be served by single duct terminal units and the exterior spaces will be served by fan powered terminal units. 

Single Duct Terminal Units

Single Duct Terminal Units are controlled by a zone thermostat that the occupant sets to their comfort level. Depending on the temperature of the space relative to the set point of the thermostat, the terminal unit will allow less or more air from the air handling unit into the space.  In some cases, the terminal unit will have supplemental heating for when the space is below the desired set point. 

There are two types of air-handling systems: constant volume (CV), and variable air volume (VAV). In a CV system, a constant amount of air flows through the system whenever it is on. A VAV system changes the amount of airflow in response to heating and cooling load changes. The latter allows commercial buildings to better regulate the interior temperature throughout different zones/spaces. A VAV system is typically found in mixed-use office buildings for improved energy efficiency and occupant comfort.

In VAV systems, the air volume of the main trunk system will remain constant, but the volume or flow rate through the supplied or branch air will vary. As a result, the VAV system allows different zones to operate at different temperatures from the same air-handling unit (AHU) and ventilation system.

The benefit of a VAV system is the ability to have a balanced condition in the building, or equalized temperatures across the entire building. An example is a solar gain on one side of the building, creating a considerably warmer area than another space that’s away from the sunny side of the building, as in the illustration below.

Components

The VAV terminal box is installed on the primary supply ductwork. A pressure sensor is also installed so that the static pressure in the supply duct can be monitored and controlled. This will control the fan speed and help with saving energy.

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NOTE: The air-handling unit (AHU) includes the air filters, cooling coils, heating source, and supply fans.

Depending on the type of the VAV box, the following describes key components for this system:

• airflow sensor. It measures the flow of air and adjusts the damper position.
• damper. It opens and closes to modulate the airflow based on the airflow sensor and temperature needs.
• fan. This is for systems that need to supplement ducted airflow (series fans) or supplement/displace reheat needs (parallel fans).
• filter. This is for fan-powered boxes to filter the air from the plenum or other return-air sources.
• reheat coil. This is an electric or hydronic coil installed to warm the air leaving the box.
• system controls. These may be pneumatic, electronic, or direct digital. An airflow sensor in the box measures airflow. The box controller modulates the damper and heating control using the airflow and zone temperature inputs to satisfy the zone requirements.
• zone temperature control. This is the primary control point for any VAV system. Either a zone sensor or thermostat provides a signal to the VAV controller.

Operation

The central AHU, rooftop unit (RTU), or package unit provides airflow through a duct system. The airflow passes through a VAV box before being ducted into the desired zone/space. Within the box is one of the four types of VAV systems that opens or closes dampers, and, in some cases, modulates the airflow through mixing boxes powered by VAV fans. The airflow may be static and constant from the source or thermostatically controlled, and it fluctuates as loads in various zones of the building change. If the system is designed only to provide cool air, the temperature from the AHU will be constant, typically around 55º F.

A thermostat will activate the damper in the VAV box and control the airflow through it. The temperature will either be too hot, too cold, or just right. If the temperature is too cold, the damper might close. If the temperature is too hot, it might open wider. If the temperature is just right, it might close partially or remain in the same position.

Additionally, if the temperature is too cold, an electric reheat system may be installed in the VAV. This electric strip heat will activate and heat the air as it passes through the VAV, or it may be a heat exchanger with a radiator with hot water tubing instead. A central boiler system would provide this hot water in a central mechanical room. Both of these methods would provide warmer air to the desired zone being served by that particular VAV box.

The exception would be an AHU that distributes air volume solely through the ducts. A reheat element or heat exchanger is attached to hot- and cold-water sources at the VAV. The hot source may be a boiler, and the cold source may be a chiller system. The two-pipe system would be able to provide hot and cold water through two different heat exchangers. This type of system allows for the greatest opportunity to control any temperature variations.

Inspection

Commercial property inspectors should operate VAV systems unless the system is shut down or in the off position.

Consider the following during an inspection:

• locate individual zones and thermostats;
• locate VAV boxes;
• activate thermostats to open or close VAV boxes and observe their operation; and
• adjust the thermostat to active the opposite mode (if present).

The International Standards of Practice for Inspecting Commercial Properties (ComSOP) does not require the inspector to move drop ceiling tiles, but the movement of the tiles might be the only way to expose the VAV system. If the tiles are not moved, the inspector can still assess the system using infrared thermometers and cameras at registers, grilles, and vents, in addition to operating the thermostats. In this case, the inspector cannot view the system but can identify whether the system is operating properly by recording the temperature. The inspector should note in the report the method of inspection.

Conclusion

VAV systems are a common method of providing comfort throughout an office space. These systems are part of a commercial inspection, so understanding how they function is key. Testing their operation using non-invasive methods will allow the inspector to inform the client about what type of system is installed, and whether it is functioning properly.

Article Written By: Rob Claus, CMI≤sup>®

Additional Resources for Commercial Property Inspectors:

Are you interested in learning more about VAV Control System? Contact us today to secure an expert consultation!

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