Design the Duct System

When installing ventilation equipment, it is important to use large enough duct to achieve the rated flow of the ventilation system that you have designed. If the duct system is inadequate or poorly designed, no fan will deliver its rated air flow. You may need to use larger duct than you have in the past to actually get desired airflows. Virtually all fans provide more airflow with less noise when the duct is up-sized an inch or two, when smooth duct is used, and when duct runs are as straight as possible.

The larger the duct diameter, the greater the airflow. Replace 3" duct with 4" or 6" duct.
Smooth, rigid duct delivers better airflow than metal flex duct. Metal flex duct performs better than plastic flex duct.
Whenever possible, avoid designing a bend or elbow within two feet of the fan to minimize static pressure and fan noise.
With metal duct, use wide-sweep elbows or bends to reduce the resistance to airflow.
A fan with a 4" duct connection will perform much better with an adapter and 6" duct. Change out the wall cap or roof jack to a larger one at the same time, if possible.
When designing ducting for the whole-building ventilation system, it s important to be conservative. After the fan and ducting are installed, the whole-building fan must deliver at least the airflow required by ASHRAE Standard 62.2 (Step 3), so do everything possible to ensure good air flow. Attempting to use an existing fan and ductwork may be difficult, since most older fans were rated at lower static pressure: 0.1 in. w.c. instead of 0.25 in. w.c. Also, many older fans were ducted with 3” or 4” ducting, which usually decreases fan performance. The following are ways to design for good airflow in a whole-building ventilation system:

Use the largest possible duct diameter, preferably in smooth rigid duct rather than flex duct.
Use the Prescriptive Duct Chart to estimate maximum duct lengths that will deliver the desired air flow.
Choose a fan that is certified
at 0.25 in. w.c. (not 0.1 in. w.c., which was the old standard for fan certification).
Avoid long duct runs and tight or multiple turns.
ASHRAE Standard 62.2 provides a table of maximum duct lengths to use to create no more than 0.25 in. w.c. of static pressure, which is the rating point for certified fans. Usually, using this table and good installation practices will provide the desired airflow. Table 5.3 shows the maximum combination of duct type, size, and fittings using the Equivalent Duct Length (EDL) method. Note that you must deduct 15 feet for each elbow in the duct system. One wall cap is assumed, so no deduction is made for the terminal device.