The Duct Area Calculation size of your home informs the size of your heating and cooling units—but it will also determine how large your duct sizes need to be. To accurately gauge the size of your ductwork, you will need to precisely measure the square footage of not just your house as a whole, but the size of each room, as well.
Basic Definitions
The following basic terminology is extensively used in this course.
• cfm: volume of air flow; cubic feet/minute
• fpm: velocity or speed of air flow; feet/minute
• sq.ft: duct size or cross-sectional area; square feet
Air volume in cfm can be calculated by multiplying the air velocity by the cross-sectional
area of the duct in square feet.
Since the size of your ductwork can increase or lower this measurement, you’ll have to find the necessary CFM for each room before you can get the right duct size for each space.
- Cubic Feet Per Minute = (HVAC Unit Tons x 400) / total square footage of home.
- Calculate for each individual room.
Gauge and Absolute Pressures
Gauge pressure is indicated on the gauge; absolute pressure is the total of the indicated
gauge pressure plus atmospheric pressure. The general equation for absolute pressure
is:
Gauge pressure + atmospheric pressure = absolute pressure
For example, if the gauge reads 10 psig then, using the above equation, the absolute
pressure would be 24. 7 psia:
10 psig + 14.7 psi = 24.7 psia
Ordinary heating, ventilating, and air conditioning duct systems read air pressures at 0.4
psi or less, often much less. 1 psi equals 27.7 inches of water gauge; a common duct
pressure of 0.25 inches water column is equal to (0.25 divided by 27.7 in-wc/psi) = 0.009
psi
Duct Pressure:
Duct system is pressurized by three pressures:
• Static pressure: It is the air pressure in the duct, which is used for fan selection.
• Velocity pressure: It is the pressure generated by the velocity and weight of the
air, which is used for measuring the flow (cfm) in a system.
• Total pressure: It is used to find velocity pressure. Static pressure plus velocity
pressure equals total pressure.
Pressure in the ductwork is measured in inches of water column (in-wc).
RECTANGULAR DUCT FORMULA
1 Rectangular. Duct Area 2x(A+B)x L/1000000
2 Taper Area (A+B+C+D)x L/1000000
3 Bend Area 2x(A+B)x((3.14x2x( Raduis +A/2)/4))/1000000
4 Side Branch Area (2x(A+B)xL/1000000)+(2x(A+B)x((3.14x2x(Raduis+A/2)/4))/1000000)
5 Twin Bend Area (2x(A+B)x((3.14x2x(Raduis+A/2)/4))/1000000)+(2x(A+B)x((3.14x2x(Raduis+A/2)/4))/1000000)
6 Shoe Collar Area 2x((A+100)+B)xL/1000000 7 Dummy Area (L+50) X (W+50)
ROUND DUCT RORMULA
OVEL DUCT FORMULA
| 1 | Oval Duct | Area | 2x(A-B)+(3.14xB)xLength/1000000 | |||
| 2 | Oval Reducer | Area | (A-B)+(C-D)+(3.14XB/2)+(3.14XD/2)X(length+100)/1000000 | |||
| 3 | Oval Bend | Area | 2x(A-B)+(3.14xB)x(3.14x2x((150+A/2)/4)+100)/1000000 | |||
| 4 | Oval Coupler | Area | 2x(A-B)+(3.14xB)xLength/1000000 | |||
| 5 | Oval Dummy | Area | ((((A-B)XB)+(3.14X(B/2)^2))+((2X(A-B)+(3.14XB))X50))/1000000) | |||
