**THREE STEPS TO
SIZING AIR DUCTS**

**1. Calculate the CFM
needed for each room**

**HOW:** Perform a whole house and
room-by-room load calculation
(Manual J). Then apply the following formula:

**Room CFM = (Room Load / Whole House Load) X Equipment CFM**

** *A
room-by-room load calculation must be performed in order to determine the CFM to
be supplied to each room otherwise the temperatures will be uneven throughout
the house**

**2. Calculate the
friction loss rate**

**HOW:** Determine the external
static pressure of the blower using the manufacturer's specs. Then, deduct the
pressure drops created by any components added to the air distribution system
(a/c coils, filters, grills, registers, dampers, etc). This is the **
available static pressure** for the duct system. Next, determine the **total
effective length**. The total effective length equals the **measured
length** from the farthest supply outlet through the equipment and to the farthest
return outlet **plus** the **equivalent lengths **of all turns and
fittings. To
get the **friction rate **apply the following formula:

**Friction Rate = (Available Static Pressure X 100) / Total Effective Length**

** NOTE: **
Although it is not an engineering acceptable practice, for most **typical **
duct systems, a default friction rate of .05-.08 may be used in lieu of
performing the above calculation.

**3. Select duct size
using friction chart or duct calculator**

**HOW:** **Using friction chart.**
For sizing the **supply and return trunks** select the duct size at the
intersecting lines of the** friction** **rate** and the** equipment CFM. **As
the trunks are reduced use the total CFM of the** remaining downstream branches**.
To size the **branch ducts**, select the duct size at the intersecting lines of the
**friction rate** and the** room CFM.**

**
Using a duct calculator. **For sizing either the trunks or branches
simply align the friction rate with the CFM and the duct size will be displayed

** NOTE: **Whether using
a friction chart or duct calculator, always check the **velocity** on the
friction chart or duct calculator**. **Generally, the velocity should be no
more than 700 FPM. If the velocity is too high, select one pipe size larger
until the velocity falls below 700 FPM.

**Before proceeding with your ductwork
project please take time to read DUCTWORK MYTH
BUSTER**

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