Install Whole-House Mechanical Ventilation

As energy codes and above-code programs drive homes to become increasingly tighter, it’s critical to install some type of “intentional” whole-house mechanical ventilation. Not only is the turnover of indoor air beneficial to help remove indoor contaminants and allergens, but it also helps maintain desirable indoor humidity levels in some climates.

In designing, installing, and commissioning a whole-house ventilation system, follow these key provisions drawn from the ASHRAE 62.2 standard:

• Select a fan based on its cfm—as rated for 0.25” w.c. static pressure—to assure adequate airflow and allow for somewhat longer duct runs. Common practice is to pick a fan based on its cfm at a lower pressure resistance like 0.1” w.c. If the duct layout and other components offer more resistance than this level, the fan doesn’t move enough air.
• Make sure duct diameter is at least 4”—preferably 6”. Again, sizing the system for more flow and not less provides some flexibility. If the installed system moves too much air, it can be throttled down to a low flow relatively easily (but the reverse isn’t true). ASHRAE 62.2 recognizes that supply, exhaust, or balanced systems can all serve as the whole-house ventilation system.
• Size, install, and commission a system with the capacity to deliver the ventilation rates shown below in Table 5–2 (ASHRAE Standard 62.2 Table 4.1a). In reality, builders are well served to educate homeowners on what the system does, how it works, and why it is important. The home’s “fresh air system” is just like the air-conditioning—the homeowner may not use it all of the time, but he/she should know what it does and when/how to use it. Homeowner know-how is especially critical in more severe climates with extreme temperature or humidity levels.

• For intermittent operation of the Whole-House Mechanical Ventilation system, maintain a maximum duty cycle of four hours, meaning that the fan operates at least 1 hour in every 4-hour segment. For intermittently operating systems, the flow rate (from Table 5–2) must be multiplied by the appropriate factor from Table 5–3 below to account for periods of fan off time.

The preceding recommendations are admittedly only a sampling of good whole-house mechanical ventilation system design, and readers are encouraged to review the Additional Resources links at the conclusion of this section for more in-depth guidance.