Design and Install Ventilation Ducts Properly

As with providing mechanical ventilation in general, the durability issue at stake relative to ventilation duct work is moisture. Elevated moisture levels and indoor humidity not only damage interior finishes, but can also lead to mold and rot of structural members. The best ventilation fans and controls simply cannot do their job of managing moisture if the ducts are improperly designed and installed (see examples in Figure 5–5).

• Insulate ductwork that runs through cold spaces or exterior wall cavities. In addition, slope ductwork down and away from the fan housing as much as possible so that any condensation will flow out towards the exterior.
• Use straight duct runs. Avoid sharp bends everywhere, but avoid all bends within 2’ of fan to minimize static pressure and fan noise.
• Adequately support ductwork but ensure that it is not pinched.
• Follow ASHRAE 62.2 guidance in sizing ductwork for local exhaust ventilation and whole-house ventilation according to the rated fan capacity and the length of the run, as shown in Table 5–4.
• Verify flow rates with flow hood measurements.