HVAC Duct Calc Buddy

Duct Calc helps you choose optimal duct sizes for HVAC duct systems in residential, commercial and industrial buildings. Duct Calc takes several input parameters and evaluates several duct size combinations based on inputs and friction loss equations recommended in the ASHRAE Handbook of Fundamentals 2009 to come up with a list of acceptable ducts. Other duct sizing applications typically only show one duct and it's characteristics meeting specific input dimensions. Duct Calc goes one step further and displays several suitable combinations and their specific loss, velocity. Ducts close to the boundaries are indicated in yellow, those further outside the limits are red. It's easy to adjust the range by pressing up or down arrows as appropriate to expand or contract the list of suitable ducts.

Features

  • English or SI Metric Units
  • Min and Max Friction Loss
  • Round, rectangular, and Oval Ducts
  • Multiple Duct Materials Galvanized, Flex, etc.
  • Configure Even or Odd Sizing
  • Min and Max Velocity constraints
  • Duct sizes from 5" (12.5cm) to 100" (254cm)
  • Equations from ASHRAE Handbook of Fundamentals 2009

At all times a range of acceptable ducts is calculated and displayed real time in tabular fashion sorted by duct size.

If you are using English Imperial and your selection is "even" you will see even sizes, 6, 8, 10, etc. Odd displays odd sizes. The Round, Rect, Oval button selects the shape. Sizes from 5 inches ( cm) to 100-inches (254 cm) are evaluated.

The Setting tab has a few options to narrow your choices. Minimum and maximum velocities establish a range visually representing ducts suitable for your application. Pressure drops resulting in a duct velocity outside the range result in a yellow icon to the left of the duct description. A red icon indicates the resulting velocity exceeds the range by 50% or more.

With respect to height and width you may also limit the aspect ratios the application will take into consideration with the "Aspect Ratio" parameter. Any aspect ratio must fall under or be less than this number. i.e. if the aspect ratio is "2" a duct size of 12 x 20 is okay but 10 x 20 is an aspect ratio of 2 which is not less than 2 so it will not be evaluated.

Duct Calc Tab Detail

Top View

The top of the view contains a series of toggle buttons and three input fields. The table in the lower part of the view contains ducts that meet the selection criteria in the top section. Each toggle button has more than one potential value. Tapping the button changes the value and causes the ducts to be reevaluated and redisplayed. If you tap inside any of the three text fields for airflow, minimum friction, and maximum friction a number keyboard pops up for you to enter a new value. Tapping "done" dismisses the keyboard and recalculates the values.

Round, Rectangle, Oval: Indicates the shape of the duct. For rectangular ducts the equivalent hydraulic diameter is calcuated using the Huebscher equation.

Material: Select the duct material. This is used primarily for the roughness calculation. The roughness of each material is displayed in the table row.

Even, Odd: When the display is imperial this toggles the display of duct sizes to even (6, 8, 10, etc.) or odd (5, 7, 9, etc.) numbering. Metric increments are 2.5 cm. When set to "Even" increments are 5, 10, 15, etc. "Odd" for Metric is not as useful and defaults to 37.5, 42.5, etc.

Min: The minimum acceptable loss for the duct in inches of water per 100 feet (in/100') or for Metric Pascals per 30 meters (Pa/30m). Increment or decrement the value by tapping on the up or down arrows. The increment amount comes from the Settings tab in Imperial units and converted to an equivalent Metric amount when displayed. The minimum friction loss per 100 feet of duct is 0.01 in/100' ( Pa/30m).

Max: The maximum acceptable loss for the duct in inches of water per 100 feet (in/100') or for Metric Pascals per 30 meters (Pa/30m). Increment or decrement the value by tapping on the up or down arrows. The increment amount comes from the Settings tab in Imperial units and converted to an equivalent Metric amount when displayed. The maximum friction loss per 100 feet of duct is 4 in/100' ( Pa/30m).

Airflow: The airflow in the duct in cubic feet per minute (CFM) or for Metric in Liters per Second (L/s). Increment or decrement the value by tapping on the up or down arrows. The increment amount comes from the Settings tab in Imperial units and converted to an equivalent Metric amount when displayed.

Table View Fields

Section Heading: The section heading displays the number of duct size combinations that meet the criteria in the top view fields. The display is limited to 100 combinations. If more than 100 are found the section header will indicate that there are more than 100 combinations. In this case narrow your acceptable minimum and maximum loss values in the top view.

Round Ducts: Acceptable round duct sizes are shown in inches (cm). Duct sizes increment by 2 inches (5 cm) starting at 5 inches (12.5 cm) in Odd sizes or 6 inches (15cm) in Even sizes. The maximum duct size is 100 inches (254cm). While the friction range displayed in the top view is establishing a tolerance range for the loss the value shown here for loss is calculated based on this particular duct's dimensions and CFM (L/s) displayed in the top view. The loss is displayed in in/100' (Pa/30m). Velocity is shown in ft/min (m/s).

Rectangular Ducts: Acceptable rectangular duct sizes are shown in inches (cm). Duct sizes increment by 2 inches (5 cm) starting at 5 inches (12.5 cm) in Odd sizes or 6 inches (15cm) in Even sizes. The maximum duct size is 100 inches (254cm). The aspect ratio of the ducts displayed are controlled by the Aspect Ratio field on the Setting tab. Only aspect ratios that are less than what is entered are considered. i.e. if the aspect ratio is 2 a 10 x 20 duct will not be evaluated. While the friction range displayed in the top view is establishing a tolerance range for the loss the value shown here for loss is calculated based on this particular duct's dimensions and CFM (L/s) displayed in the top view. The loss is displayed in in/100' (Pa/30m). Velocity is shown in ft/min (m/s).

Oval Ducts: Acceptable Oval duct sizes are shown in inches (cm). Duct sizes increment by 2 inches (5 cm) starting at 5 inches (12.5 cm) in Odd sizes or 6 inches (15cm) in Even sizes. The maximum duct size is 100 inches (254cm). The aspect ratio of the ducts displayed are controlled by the Aspect Ratio field on the Setting tab. Only aspect ratios that are less than what is entered are considered. i.e. if the aspect ratio is 2 a 10x20 duct will not be evaluated. While the friction range displayed in the top view is establishing a tolerance range for the loss the value shown here for loss is calculated based on this particular duct's dimensions and CFM (L/s) displayed in the top view. The loss is displayed in in/100' (Pa/30m). Velocity is shown in ft/min (m/s).

Settings Tab

Airflow Increment: The amount that the airflow value will increment or decrement with each tap of the arrow. Enter a value in CFM. If the display is Metric the CFM value will automatically be converted to the equivalent Liters per Second value.

Friction Imncrement: The amount that the friction value will increment or decrement with each tap of the arrow. Enter a value in in/100'. If the display is Metric the in/100' value will automatically be converted to the equivalent Pa/30m value.

Aspect Ratio: For round and oval ducts this is the maximum duct aspect ratio that will be considered. The calculated aspect ratio must be less than this number.

Min and Max Velocity: Guidelines for selecting appropriate ducts. Enter a value in ft/min. If the display is metric the value is automatically converted to the equivalent m/s value. If the velocity returned is greater than this number it is indicated in yellow. If the velocity is 150 percent out of the range it is indicated with red.

Methodology:

To determine appropriate duct sizes the pressure drop for a 100' segment of the round, rectangular, or oval duct candidate the steps are as follows:
  1. Roughness factor - based on material
  2. Duct size characteristics - determined by calc and application settings
  3. Viscosity - standard air (ASHRE Fund. 2009 p21.6 (21)
  4. Velocity as flow over duct cross sectional area
  5. Equivalent hydraulic Diameter (Huebscher)
  6. Reynolds number using standard air, hydraulic diameter, and velocity
  7. Friction Factor from explicit form of Colebrook equation (Altshul-Tsal ASHRAE Fund. 2009,p.21.6)
  8. Resulting pressure drop (Darcy)

Roughness Table

  • PVC 0.0001
  • Aluminum 0.0001
  • Galv Steel 0.00025
  • Spiral Steel 0.0003
  • Smooth Liner 0.003
  • Fiber Liner 0.015
  • Flex Duct 0.007