Ventilation Principles and Standards
Mechanical ventilation is essential in modern homes built to tight energy standards. ASHRAE Standard 62.2 specifies minimum ventilation rates for residential buildings, requiring 7.5 cubic feet per minute per bedroom plus 1 cfm per 100 square feet of floor area. This standard has been adopted by most building codes and serves as the minimum acceptable ventilation rate.
mechanical ventilation systems.
The three primary methods of providing ventilation are exhaust-only, supply-only, and balanced systems. Exhaust-only systems use fans to remove stale air from bathrooms, kitchens, and utility rooms, with fresh air entering through intentional and unintentional openings. Supply-only systems push filtered outdoor air into the home, pressurizing the building to reduce entry of soil gases and outdoor pollutants.
Heat and Energy Recovery Ventilators
Heat recovery ventilators transfer heat between exhausted indoor air and incoming fresh air, recovering 60 to 80 percent of the thermal energy that would otherwise be lost. Energy recovery ventilators additionally transfer moisture, maintaining indoor humidity levels during winter and reducing humidity load during summer. Both systems provide continuous, filtered fresh air while minimizing energy loss. blower door test procedures. weatherstripping techniques. cellulose insulation.
The efficiency of HRV and ERV systems is rated by their sensible recovery efficiency and total recovery efficiency. High-efficiency models achieve sensible recovery rates above 80 percent. The energy savings from heat recovery typically pay back the additional equipment cost within 3 to 7 years compared to exhaust-only ventilation, depending on climate.
Spot Ventilation Requirements
Bathroom exhaust fans must be vented to the outside, not into attics or crawlspaces. The minimum ventilation rate is 50 cfm for intermittent operation or 20 cfm for continuous operation. Kitchen range hoods should exhaust at least 100 cfm for intermittent operation with higher capacities needed for commercial-style cooking equipment. Ductless recirculating hoods are not considered ventilation and do not meet code requirements.
Ventilation ducts should be smooth metal or rigid semi-rigid ductwork rather than flexible duct, which restricts airflow. Duct runs should be as short as possible with minimal turns. The termination point must be at least 3 feet from any building opening and 10 feet from property lines to prevent re-entry of exhausted air.