How to Design a VAV System

A VAV system enables a single RTU to supply air to different zones with different cooling demands. The RTU provides air at a constant temperature. But the airflow will vary depending on the air pressure in the supply air duct. VAV terminal boxes placed downstream of RTU in the corresponding zones will control the zone temperature by modulating the airflow based on the input from a zone temperature sensor.

A VAV system comprises the following:-

• Rooftop unit with an RTU controller controlling supply air temperature and supply duct static pressure.
• A VAV terminal box with direct digital control(DDC).

SELECTING ROOF-TOP UNIT (RTU)

RTU should have external static pressure (ESP) of atleast 2 in. wg. This ensures that each VAV terminal box receives sufficient airflow at the right air pressure to deliver the maximum design airflow to the zone. The supply air temperature/leaving air temperature should be constant at 55°F. Entering air temperature for cooling should be mix-air temperature for systems with outside air. Entering air temperature for heating should be mix air temperature of outside air and return air when the supply airflow is 30-50% of the design flow rate. The blowers/fans should be controlled by a variable frequency Drive (VFD). RTU should be equipped with a heating system for tempering supply air. Reheat systems in VAV terminals will not eliminate the heating system in the RTU. The heating systems in RTU can be:-

1. Heatpumps with strip heat (Available for limited capacities only)
2. Gas furnace.
3. Electric heating coils.

RTU with economizers should include a proper relief method to prevent over-pressurization of the building.

The relief method can be:

1. Barometric Relief Damper (Economical & Energy efficient) : Barometric relief dampers shall be used for HVAC systems with low return air pressure drop.(<=0.1 in. wg.)  
2. Power Exhaust fans (Lower initial and running cost than return fan systems) : Power exhaust fans shall be specified for RTU if the return air pressure drop is up to 0.8 in. wg.
3. Return air fans (Costly and least efficient) : Return air fans are required for systems with high return air pressure drop. (> 1 in. wg.)

SELECTING VAV TERMINAL BOX

Factors considered for VAV box selection:

1. Static pressure drop across the terminal box.
2. The ability of the terminal box to measure and control the desired minimum and maximum airflow rates.
3. Cost of the VAV box with its controllers and installation.
4. Noise Generation
5. Space constraints

Pressure-independent VAV terminal box:

It has two control loops.

• Control space temperature. Output is an airflow range between the minimum and maximum airflow rates of the VAV terminal box.
• Modulates the damper position to maintain the airflow at the setpoint output received from the first control loop.

Pressure-dependent VAV Terminal Box:

• It does not have a flow-measuring provision.
• It only has a single room temperature control loop to control the room temperature.
• When the room temperature rises, the damper will modulate from minimum position to maximum position to deliver more air supplied from the AHU to space.
• The occupants may experience discomfort if the duct static pressure is not balanced correctly, which can result in fluctuating airflow into the room.

DIFFERENT TYPES OF VAV TERMINAL BOXES

Single Duct Terminal Units

• Unit contains a damper, flow sensor, actuator, and an optional reheat coil.
• The damper is modulated according to the cooling demand of the zone received from the zone thermostat.
• To maintain the set point for the zone, the air from the air handling unit is reheated by the reheat coil in the unit during the heating process.

Parallel Fan Powered Terminal Units

• Mode of operation of the unit during cooling is the same as that of the single duct terminal units.
• During the heating operation, the damper closes to a minimum position to provide the minimum ventilation rate to the zone. Along with this the fan and heating coil also turns on.
•  Fan draws air from the plenum space into the unit to make up the required airflow to maintain the optimum zone temperature.

Series Fan-Powered Terminal Units

• Unlike parallel fan-powered units, the fan is located in the primary air stream of the unit.
• The fan in the series fan-powered unit shall operate simultaneously with AHU. 

Selecting a better system style.

Two common approaches of VAV system design can be tabulated as follows:

How airflows for a VAV box is calculated?

Minimum Airflow

According to 2018 VECC and ASHRAE 90.1-2019, the minimum airflow of the box will be the largest of the following:

• 0.4 cfm/sq.ft of floor area.
• 30% of the VAV’s maximum airflow.
• Minimum ventilation requirement calculated as per 2018 VMC.

The minimum flow of fan-powered VAV terminal boxes can be as low as the ventilation requirement of the zone since the heating requirement of the zone can be handled by plenum air induction.

Maximum Airflow

The maximum airflow of a terminal box will be the design airflow rate. By the rule of thumb, the maximum airflow of a VAV box is 75-80% of its nominal airflow.

Initially, The peak airflow requirement for the zone shall be determined from heat load calculation. Then the VAV terminal box can be selected so that the specified peak airflow rate is 80% of its nominal airflow.

Total Pressure Drop Across Terminal Box.

For optimum life cycle costs, VAV terminal box with reheat coils shall be sized with a total pressure drop ranging from 0.6 to 0.7 in. wg.

VAV Reheat Selection Procedure

Reheat coils are provided for variable air volume (VAV) terminal units to control the temperature of the air supplied to the space they serve. VAV systems are designed to vary the air volume to a space based on the heating or cooling needs, but the volume control can result in temperature fluctuations. Reheat coils help to maintain a consistent air temperature by reheating the air before it enters the space, thereby compensating for the cooling effect of the volume reduction. This way, the VAV system can maintain a consistent temperature in the space while still providing energy efficient heating and cooling. Reheating the cool air provided by the air handler using the reheat coils can be an inefficient use of energy. By the use of fan-powered units, the heating demand will be reduced by introducing plenum air induction. However, using fan-powered units also requires energy for the  operation of the fans. Therefore, for moderate environmental conditions, taking into account both the initial cost of fan-powered units and the operating costs throughout the year, single duct terminal units with reheat coils are the most cost-effective solution. Despite the fact that heating cold air may appear to be a waste of energy, it does have some advantages:

1. Able to provide comfort with a great diversity of loads.
2. Supplement baseboard perimeter heat.
3. Maintains minimum required ventilation rates.
4. Enhanced Humidity control.

Types of reheats

1. Hot water heat

2. Electric heat

Sizing A Reheat Coil in the Vav Box

Supply air temperature of a VAV box during heating operation shall range from 85 °F to 100 °F.

For calculation, the supply air temperature shall be selected as 90 °F. The supply air from a VAV rooftop unit shall be considered as 55°F. The reheat coil shall be sized to heat the minimum airflow of the VAV box.

The required reheat capacity shall be:

𝑄 = 1. 08 × 𝑉𝑚𝑖𝑛 × (90 − 55)

Q : Reheat capacity in BTU/h

Vmin : Minimum airflow of VAV box in CFM.

For a hot water reheat coil, the water flow rate through the coil can be determined by the following

equation

Water flow rate (GPM) = Q / (500 * ΔT)

Q : Required Reheat Capacity

ΔT : Temperature difference between inlet and outlet in °F

Inlet water temperature shall be determined from the specifications of hot water system.

Outlet water temperature = Desired air temperature + (ΔT x 1.08)

In this equation, ΔT is the desired temperature rise of air.

Ductwork Design

1. Medium-pressure ducts shall be used to supply air from the rooftop unit to the VAV terminal boxes.
2. Medium-pressure ducts are used in systems with fan static pressure between 3 to 6 in. wg.
3. Air velocities in medium-pressure ductwork are less than or equal to 2000 fpm for ducts above occupied spaces and less than or equal to 2200 fpm in mechanical rooms/shafts.
4. The ducts downstream of VAV terminals can be low-pressure ductwork.
5. Air velocities in low-pressure ductwork are less than or equal to 1500 fpm.
6. Diffusers for systems with VAVs shall be selected considering the operating range of the VAV terminal box.
7. For example, if air from the VAV terminal with a maximum flow of 2800 cfm and a minimum flow of 840 cfm is distributed through 8 diffusers, then each diffuser will have a minimum airflow of 105 cfm and a maximum flow of 350 cfm. The diffuser shall be selected considering this operating range. The figure shows an optimum selection for this condition.[Basis of Design - Make: Price industries, Model: SMCD - Modular Core Directional Diffuser.]

8. Diffusers with better horizontal air patterns even at lower air volumes can be considered suitable diffusers for a VAV system.

Controls

The control sequence of a VAV system shall be briefed as follows. In modern VAV systems, Direct

Digital Controls [DDC] shall be specified for the VAV systems. Old-age VAV systems may have pneumatic or electrical control instead of DDC. This should keep in mind while working on renovation projects. The VAV system receives a signal from a temperature sensor in the space that indicates the current temperature.

1. The VAV system compares the current temperature to the setpoint temperature, which is the desired temperature for the space.

2. If the current temperature is above the setpoint temperature, the VAV system opens the damper to increase the airflow into the space, which can cool the space down.

3. If the current temperature is below the setpoint temperature, the VAV system closes the damper to decrease the airflow into the space, which can heat the space up.

4. The VAV system also receives a signal from an airflow sensor in the space that indicates the current airflow demand.

5. If the airflow demand is low, the VAV system reduces the airflow to the space by closing the damper partially.

6. If the airflow demand is high, the VAV system increases the airflow to the space by opening the damper more.

7. In fan-powered VAV systems the fan speed is also modulated with the damper control.

8. The fan speed of air handler is adjusted based on the combined airflow demands of all the VAV boxes in the system.

References

• 2018 Virginia Energy Conservation Code, Section C407.5, Table C407.5.1(3), Section C403.6.2 to C403.6.8.
• ANSI/ASHRAE/IES Standard - 90.1-2019, Section 11.
• 2018 Virginia Mechanical Code, Section 403.3.
• ASHRAE Journal - Sizing VAV Boxes - Steven T Taylor, P.E., Fellow ASHRAE, and Jeff Stein, P.EMember ASHRAE.
• Specifying VAV Boxes by Jeff Stein, P.E, Taylor Engineering, Alameda, California.
• ASHRAE Journal - Return Fans in VAV Systems By Steven T Taylor, P.E., Fellow ASHRAE.
• YORK Application Guide - Return Fan and Exhaust Fan Economizers and Building Pressurization