Standard Air Conditioning Duct Sizes

The following duct sizes are based on a fraction drop of 10 inches per 100 feet of lineal duct.

Standard air conditioning duct sizes.

Optimal airflow based on cubic feet per minute as well as velocity of air provided by other components of the hvac system determine the correct ductwork sizing for each scenario. Duct system is pressurized by three pressures. 1 psi equals 27 7 inches of water gauge. This equal friction method of duct sizing should be adequate for normal residential furnace heating and air conditioning applications.

Duct sizing how to size ductwork. Also you want to make sure that the return ducts are the same size as the supply ducts. Rectangular rectangular or square ducting comes in standard sizes ranging from 3 by 7 inches to 35 by 40 inches. So if you have a two ton system you need ductwork that will carry 800 cfm.

If your btu requirement is between sizes simply go with the bigger unit. For example the manual j calculation may say you need 25 000 btus for cooling. Larger volumes or higher static pressures should be dealt with on an individual job basis. One six inch round duct is good for about 120 cfm a six inch duct carries about 150 cfm and eight inch carries about 200 cfm of cool air.

Design cfm 4 cfm 6 cfm 8 cfm 10 cfm 12 60 6x4 60 4x6 90 4x8 120 4x10 150 4x12 90 8x4 110 6x6 160 6x8 215 6x10 270 6x12 120 10x4 160 8x6 230 8x8 310 8x10 400 8x12 150 12x4 215 10x6 310 10x8 430 10x10 550 10x12 180 14x4 270 12x6 400 12x8 550 12x10 680 12x12 210 16x4 320 14x6 490 14x8 670 14x10 800 14x12 240 18x4 375 16x6 580 16x8 800 16x10 950 16x12 270 20x4 430 18x6 670 18x8 930 18x10 1100. But your hvac dealer only has air conditioners that handle 24 000 or 30 000 btus. 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. If you don t have one you can find them in the following links.

To size the ducts you re going to need a duct sizing chart. 4 ton unit x 144 square inches 576 squared. You can obtain these from ductwork manufacturers or from industry bodies such as cibse and ashrae. 4 tons x 144 square inches return air duct in square inches.

These charts hold a lot of information. Ordinary heating ventilating and air conditioning duct systems read air pressures at 0 4 psi or less often much less. Step 4 calculate the square root of the total. Hvac systems heating ventilation and air conditioning systems design and dimensions.

Diameter of duct x 3 14 return air duct in square inches. Size of air conditioner example. Ventilation systems for ventilation and air handling air change rates ducts and pressure drops charts and diagrams and more. Air duct components and minor dynamic loss coefficients minor loss pressure or head loss coefficients for air duct distribution.