The pressure drop shown in the table is for perfectly straight flex duct
, which has a roughness factor of about 2.1 relative to hard sheet metal duct.
Branch-to-diffuser distance is the total length of the flex duct
from the branch in the post VAV rigid duct to the diffuser.
The unit is easy to install with 4-inch round flex duct
and has a high performance rating of 70 CFM air movement for bathrooms up to 65 square feet.
The supply and return ductwork should not be flex duct
(Photo 8), as it is extremely difficult to seal--go with rigid duct (Photo 9).
A common method of testing duct system leakage of forced-air systems is to fabricate a seal to cover a return grille and then use a flange to attach a calibrated fan/flowmeter device directly or via a 4.7 cm (10 in.) diameter flex duct
. This method can also be used to measure the flow at a return grille.
Using flex duct
or hard elbows can increase the sound produced by an outlet (on the average of a noise criteria [NC] of 5) compared to manufacturer's catalog data obtained per the above.
(ASHRAE 2007) If a horizontal duct is attached directly to a ceiling diffuser, either by bending flex duct
or a rigid elbow, the room discharge flow velocity can be more than four times higher from one side to the other (Landsberger 2008).
Type of all rigid rigid all flex duct
elbow, flex duct
INLET DISCHARGE APPLICATION LOCATION LOCATION 1 Return Air Return Air Lined 5 ft Boot Plenum Plenum (Figure 3) 2 Return Air Return Air Flex Duct
Both Sides Plenum Plenum 3 Return Air Return Air Single Elbow Plenum Plenum (No Turning Vanes) 4 Return Air Return Air Double Elbow Both Plenum Plenum Sides (No Turning Vanes) 5 Ceiling Return Air Flex Duct
to Grille Plenum Perforated Face Grille 6 Ceiling Ceiling Flex Duct
to Grille Grille Perforated Face Grilles 7 Return Air Ceiling Toilet Makeup.
Round metal data from the 2005 ASHRAE Handbook--Fundamentals (ASHRAE 2005), measured round metal duct data and flex duct
data for 12" (305 mm), 14" (356 mm) and 16" (406 mm) flex ducts
are shown in Figure 2.