Healthy Indoor Air Quality with Mechanical Ventilation
While it is important to seal up buildings to prevent drafts, water damage, heat loss and high energy bills, ventilation is key to maintaining the health and safety of a building. Most of our exposure to environmental pollutants occurs from breathing indoor air, which contains up to 100 times more pollutants than outdoor air. Mechanical ventilation replaces stale air from inside a home with fresh air from outside. To download a printable guide (pdf) of this information, see: Mechanical Ventilation Guide.
Ventilation can limit exposure to unhealthy indoor pollutants by removing:
- Cooking Odors
- Chemicals & VOC’s
- Carbon Dioxide Fumes from Gas Combustion Appliances
New ventilation requirements became effective on January 1, 2010 under the 2008 Update to the California Building Energy Efficiency Standards (Title 24, Part 6) designed to improve the quality of indoor air. These regulation require any newly constructed single-family homes, low-rise residential units and all residential units with additions over 1,000 square feet to meet ASHRAE 62.2 mechanical ventilation standards.
Meeting the New Standards:
The new standards require local exhaust ventilation for kitchens and bathrooms and specify airflow requirements for whole-building ventilation. In order to meet the requirements, each house must determine its minimum ventilation rate and install the requisite fans. The required minimum ventilation rate is determined by the size of a home, the capacity of the fan which is measured in cubic feet per minute (cfm), and whether fans are run continuously or intermittently. Opening windows alone does not meet ventilation requirements.
To calculate how much mechanical ventilation your home needs, use the tables and formulas in the Mechanical Ventilation Calculations Guide.
Ventilation Types: Whole-House Versus Local
Whole-House Continuous Ventilation: For smaller homes, bath and kitchen fans can meet all the whole house ventilation requirements. Larger homes will need a combination of local exhaust and whole house ventilation or existing forced air system can be modified to meet the requirements. A continuous system must operate independently of the occupant.
Local Ventilation: In addition to meeting the whole-building ventilation requirements, ASHRAE 62.2 requires that each kitchen and bathroom have a local ventilation exhaust system installed to discharge odors and humidity to the out-side. Local ventilation exhaust systems may operate intermittently or by a sensor unless they are being used to meet the whole-building ventilation system, at which point they will need to operate continuously. Intermittent fans intended to meet the whole house ventilation requirement are programmed to run off a timer, at calculated intervals.
- Kitchen Exhaust: Kitchen fans must either be a ceiling exhaust fan or a kitchen range hood. For the kitchen fan to meet the whole-house ventilation requirements, it needs be run continuously and achieve a minimum of 5 air changes per hour for the kitchen area. For local ventilation requirements, the intermittent (as needed) kitchen fan must be at least 100 cfm.
- Bathroom Exhaust: Bathroom fans are required in all baths even if there is an operable bathroom window. Bathroom fans can include sensors or humidistats that turn on automatically when a certain amount of moisture is present. The minimum fan size required for a bathroom is 25 cfm, if run continuously or 50 cfm if it is operated by a switch. Continuous bath fans must be sound rated at 1 sone. Intermittent fans must be sound rated at no more the 3 sones unless they are rated for more than 400cfm.
Applicable projects can compute minimum ventilation requirement using ASHRAE 62.2 using the reference table or formula Section 10-103 (or see Mechanical Ventilation Calculation Guide). The type of system (local intermittent or whole-house continuous), make, model and location as well as the calculation must be included on building plans. Final inspections will confirm that ventilation requirements were completed as documented on plans, as well as verify other requirements, including: Labeling of ventilation system controls & providing owner’s manual of ventilation system with installation and operation instructions.
Positive Versus Negative Pressure: Know Where Your Air is Coming From
Negative pressure: Negative pressure systems mechanically push air out of a home and rely on replacement air to pas-sively infiltrate through cracks in the building envelope. These systems are the least expensive and easiest method to meet the ventilation standards. They require extensive sealing measures to ensure that the replacement air is not being pulled from undesirable and unhealthy locations such as crawlspaces, garages or attics. Examples: kitchen exhaust fan, bathroom exhaust fan.
Positive pressure: Positive pressure systems mechanically pull air from the outside into the home and rely on exhausted air to passively leak out of the building. Because most positive pressure systems do not heat or dehumidify incoming air prior to it entering the house, these systems can result in higher heating and cooling costs. Examples: central heating outside air intake, outside air input to heating system.
Balanced: Balanced systems, such as a heat recovery ventilator, ensure that the air coming into the house is approximately equal to the amount being exhausted. Although balanced systems are generally more expensive to install, they are designed to provide the best indoor air quality with the most energy savings as they warm the incoming air with the heated air that is exiting the building. Note: For positive pressure and balanced systems, all intake air is required to have a screened inlet away from any possible source of contamination such as other exhaust ducts and be filtered to at least MERV 6. Examples: heat recovery ventilation system.
Return to The Office of Energy and Sustainable Development homepage