Selection of a single duct VAV terminal air units can involve three elements:
1. Air Valve selection
2. Heating coil selection (if required)
1. Air Valve Selection
The wide open static requirement pressure versus airflow curve is found in the performance data section of the catalog. To select an air valve, locate the required design cooling airflow for your terminal unit type and find its vertical intersection, with the smallest air valve size that has a pressure drop equal to or lower than the maximum wide open static pressure requirement.
Cooling Only Terminal Air Unit:
Design cooling airflow: 1700 cfm
Maximum wide open air pressure drop: 0.25 in. wg
Minimum cooling airflow: 300 cfm
From the manufacturer performance data charts, select a valve size which has a wide open static pressure drop of 0.16 in. wg. For example assume size 17.
Check the minimum and maximum cfm desired with the minimum and maximum cfm allowed in the manufacturer table in the general data section. The maximum setting of 1700 cfm is within the acceptable range. The desired minimum setting of 300 cfm is acceptable for the cooling only box desired. Note that if an electric reheat box was selected; the minimum cfm would be dependent upon the kW of the electric heater. (See Electric Heat Unit Selection.)
2. Heating Coil Selection (If required)
First, determine the amount of heat required to meet space and downstream duct heat losses from a load calculation.
Hot Water Heat:
Select a hot water coil sufficient to meet the design heat loss.
Hot Water Unit Heat, for the selected TAU Size (See air Valve Selection)
Design heating airflow: 850 cfm
Hot water flow: 1.0 GPM
Winter room design Temperature: T = 68 F
Design Heat Loss MBh: Q = 25
Select hot water coil from the coil performance table in the Performance Data section of the manufacturer catalog.
A Standard Capacity coil is sufficient to meet design conditions. From the Hot Water Coil Capacity Data of the Performance Data Section, a standard capacity coil for a Size 17 air valve will operate at the above conditions as follows:
Coil Capacity: 25.7 MBh
Water pressure drop: 0.33 ft WPD
Air pressure drop (APD) of the hot water coil is included in the chart preceding the hot water coil performance data section.
APD = 0.042 in. wg
Determine the kW required to meet zone design heat loss.
kW = MBh / 3.414
MBh = Design Heat Loss
Select nearest available kW with voltage and steps desired from the electric heater kW guideline table in the Performance Data section of the manufacturer catalog.
Electric Unit Heat, Size 17 (See Air Valve Selection)
Design heating airflow: 850 cfm
Minimum unit airflow: 300 cfm
Voltage: 277/60/1 VAC
Winter room design temperature: T = 68 F
Design Heat Loss: Q = 25 MBh
kW = Q / 3.414
kW = 25 / 3.414
kW = 7.3
Select 7.5 kW from the electric heat table in the voltage and stages required. The table shows the minimum cfm allowable for the kW selected. At 7.5 kW, the minimum allowable airflow is 510 cfm. The minimum cfm must be increased to 510 to assure adequate airflow over the 7.5 kW electric coil. The static pressure requirement is shown as 0.17 in. wg. for this example, with a design cooling flow of 1700 cfm.
Check Leaving Air Temperature
LAT = Q + T
1.085 x CFM
Decide if leaving air temperature of 95.8 F is satisfactory for your application.
LAT = 3414 x 7.5 + 68 = 95.8
LAT = 1.085 x 850
The acoustical data found in the Performance Data section of the VAV catalog is used to make a determination of the amount of noise the terminal unit will generate. Locate the table for the VAV terminal unit of interest. Sound power data and an equivalent NC level for an ARI 885-90 transfer function is listed.
Cooling Only Terminal Unit, Size 17 (See air Valve Selection)
Cooling Airflow Cfm = 1700
Maximum inlet static pressure: 1.5 in. wg.
Downstream static pressure requirement: 0.30 in. wg.
Maximum NC Criteria = NC-35
Maximum Inlet Static Pressure is the maximum static at the inlet to the box.
Downstream Static Pressure requirement is the static loss downstream of the VAV box, which includes everything from the discharge of the box to the room.
The catalog assumes 0.5 inches of downstream static pressure for all but the minimum inlet static pressure valves. Since we have a downstream static pressure loss of 0.30 inches, the static across the box will be more than the catalog’s assumption using 0.5 inches downstream. Look into the catalog’s acoustic table at 1.5 + (0.5-0.3) or 1.7 inches.