Step 6. Duct Design

Design the Duct System

Title 24 Requirement
ASHRAE 62.2-2007 & ASHRAE 62.2-2010

When designing ducting for local exhaust ventilation, use the Prescriptive Duct Chart. Bath fans and kitchen range hoods can be sized prescriptively. As long as you meet the local exhaust ventilation requirements (Step 2) by installing the proper size fan and ducting, the local exhaust requirement is met.

Title 24 Requirement
ASHRAE 62.2-2007

When the air handler or return duct is located in the garage, the ductwork for the HVAC system must be sealed to have air leakage of no more than 6% of the air handler airflow when measured at 0.1 in. w.c. (25 Pa, whether or not the air handler is used for ventilation.

ASHRAE 62.2-2010

When the air handler or return duct is located outside the pressure boundary, the ductwork for the HVAC system must be sealed to have air leakage of no more than 6% of the air handler airflow when measured at 0.1 in. w.c. (25 Pa), whether or not the air handler is used for ventilation.

Best Practice

If you have flow testing equipment, it’s a good idea to measure the airflow of all fans, even if the code doesn’t require it. Flow testing allows you to identify and solve problems with airflow rates and installation.

Seal all ductwork for HVAC equipment, no matter where it is located.

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.

ASHRAE Std. 62.2 Table 5.3: Prescriptive Duct Sizing Chart
Duct Diameter (inches) Maximum Length in Feet
Fan Rating in cfm Certified @ 0.25 in. w.c.
Flex Duct (cfm) Smooth Duct (cfm)
50 80 100 125 50 80 100 125

n/a = not allowed: No use of this duct diameter will meet ASHRAE Standard 62.2 requirements.

This table assumes no elbows. Deduct 15 feet (5 m) of allowable duct length for each elbow.

3 n/a n/a n/a n/a 5 n/a n/a n/a
4 70 3 n/a n/a 105 35 5 n/a
5 no limit 70 35 20 no limit 135 85 55
6 no limit no limit 135 95 no limit no limit no limit 145
7 and above no limit no limit no limit no limit no limit no limit no limit no limit

Ductwork Can Distribute Pollutants Throughout the House

Sometimes HVAC equipment or ducting is located in places where the air is polluted. If connections and ductwork are not well sealed, harmful pollutants may be pulled into leaky ducts and delivered throughout the house. Make sure all heating, cooling, and ventilation ducting is well sealed, especially in areas such as:

  • a garage where cars idle or toxic chemicals are stored
  • an attic with loose-fill insulation, fumes from asphalt shingles, or rodent presence
  • a damp crawlspace
  • a basement hobby area

Locate Outdoor Air Intakes Carefully

If outdoor air intakes are located near pollutant sources, the ventilation air brought into a home can make indoor air quality problems worse. Locate outdoor air intakes at least 10 ft from pollutant sources such as:

  • barbecues
  • trash cans
  • dog runs
  • busy streets
  • combustion appliance vents or clothes dryer vents
  • exhaust vents from fans
  • places where a car idles