Most people don't know that ASHRAE 62.2-2007 Ventilation and Acceptable Indoor Air Quality in Low-Rise Residential Buildings is the U.S. national minimum ventilation standard. It is a consensus document developed by the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE). It applies to all single-family houses and multifamily dwellings three stories or less in height. Th e latest version was published in 2007 and is the basis for virtually all of the ventilation requirements of the U.S. green building programs.
Th e U.S. International Building Codes still use the higher ventilation requirements from the 1989 version of ASHRAE 62, but several states have adopted 62.2 or are considering it. Basically, ASHRAE 62.2 requires low-level, continuous ventilation in a home using a whole-building fan or other ventilation system. Intermittent whole-building ventilation can be used as an alternative, but the airflow rates need to be considerably higher. This is primarily because the contents of a home release a wide variety of chemicals into the air continuously, whether the home is occupied or unoccupied. Th ose gases build up if the ventilation system is not running continuously. Also, since this is a national standard, it has been written to apply to all homes under all conditions. There are a few exceptions for very temperate climates.
Providing continuous, low-level ventilation is the most eff ective solution in almost every case. The standard does allow for other design approaches if they have been approved by a “licensed design professional.” Note that the standard uses the term “whole-building ventilation” fan, not to be confused with the terms “whole-house fan” or “whole-house comfort ventilator.” The latter are terms used to describe a very large fan used to quickly cool a house with lots of outdoor air, typically at night.
In a whole-building ventilation system, the stale indoor air must be exhausted directly to the outside. Dumping exhaust air into an attic or other internal space can have dire consequences when warm, humid air hits a surface cold enough that condensation forms, increasing the likelihood that mold growth and structural damage may occur.
Continuous ventilation rates for whole-building ventilation can be determined by calculation. To calculate the required fl ow, count 1 cfm (cubic foot per minute) per 100 square feet of floor area to account for the building. Next, to account for the occupants, multiply the number of bedrooms plus one (which assumes two occupants in the master bedroom) by 7.5 cfm (cubic feet per minute). Then add those results together. For example, a 1,500-square-foot house with three bedrooms would require 45 cfm (15 cfm for the building plus 30 cfm for the four occupants).
The standard allows a wide variety of ventilation systems to be used from supply or exhaust fans to heat recovery ventilators (HRV) or energy recovery ventilators (ERV). Standard bath fans can be used to meet the continuous requirements if they move the required amount of air and are quiet – 1.0 sone or less based on HVI-Certifi ed performance data.
Additionally, bathroom fans should be able to operate at 50 cfm intermittently or 20 cfm continuously. A kitchen fan should be able to move 100 cfm intermittently or provide 5 air changes per hour continuously based on the volume of the kitchen. When used to meet the intermittent ventilation requirements, both kitchen range hoods and bathroom fans must be HVI-Certifi ed at no more than 3.0 sones.
The standard contains many other requirements such as transfer of air between rooms, combustion air supply, sealing of attached garages, air filtration and other indoor air quality considerations.
HVI recommends you become familiar with this important standard. A read-only version is available at ASHRAE.
When considering the ventilation requirements for any home, it is important to remember that ASHRAE 62.2 outlines the minimum ventilation requirements. While the minimum rate may be enough to meet basic needs, it does not provide for homes with additional occupants, active children, pets and the needs of those with impaired respiratory health. HVI recommends that in addition to the minimum ventilation requirements, the continuous ventilation system should have at least 100 percent additional capacity to allow the occupants to select the fresh-air delivery rate that best suits their needs.
For Canada the most recent version of the National Building Code (NBC) is the 2005 edition, and although there are some provincial variations, the ventilation provisions (Section 9.32 of the NBC) are being consistently applied across the country. Th e 2005 NBC ventilation requirements have many similarities to the Ontario Building Code requirements that were adopted as early as 1993. Th e following information is useful to HVAC contractors, builders, building offi cials and homeowners to interpret and implement the NBC 2005 requirements.
In all cases, quali fied designers can choose to design ventilation systems using the Canadian Standards Authority (CSA) F326 Residential Mechanical Ventilation Standard. The Section 9 requirements were designed to be consistent with CSA F326 but are eff ectively a simpli fied, prescriptive version of it. Designers have more fl exibility if they use F326 directly.
There are three key elements to remember:
- Th e required ventilation capacity is determined by projected occupancy of the house. Bedroom counts are used to determine occupancy.
- Ventilation requirements are tied to the type of fuel-fired heating and hot water equipment installed in the house. The key issue is the potential for backdrafting or spillage from combustion equipment due to negative pressure ventilation strategies.
- The 2005 NBC has defi ned performance requirements for ventilation equipment and installation to ensure better operational performance.
Ventilation Capacity
Ventilation capacities are determined by bedroom counts. Th ere are both minimum and maximum ventilation amounts to avoid oversizing of systems. The capacities were set to try to minimize the variety and number of fans needed.
Examples:
- A fan with a capacity of 24 L/s (50 cfm) at 62 Pa would be suitable for use in houses with 1–3 bedrooms.
- A fan with a capacity of 30 L/s (63 cfm) at 62 Pa would be suitable for use in houses with 3–5 bedrooms.
Note: These are installed capacities – the responsibility of HVAC contractors and builders is to ensure that the installed fans provide this ventilation rate. Timers or other controls can be used to allow homeowners to operate the fans at lower rates. Th e ventilation system must have clearly marked control switches located in an occupied zone to allow homeowners to turn them on or off .
Ventilation and the Potential for Combustion Spillage
Ventilation options are impacted most by the potential for spillage of combustion products.
Other NBC Performance Requirements
- Fans and HRVs used as the principal ventilation system must be rated at higher static pressures 62 Pa (0.25 in. wg). Th ese better refl ect actual installed conditions.
- Fans used as the principal ventilator must meet speci fic sound ratings that are typically quieter than the old code. Fortunately, the HVI-Certi fied ratings are now specifically referenced in the code as an alternate to the seldom-used Canadian Standards Association (CSA) C260 Standard.
- Additional or supplemental ventilation requirements are much simpler to understand.
o Each bathroom needs at least a 25 L/s fan or must be vented as part of the principal ventilation system.
o The kitchen must have at least a 50 L/s fan or the principal ventilation fan can be used for the kitchen (only if the fan is not connected to any other location).
o All supplemental fans must have a manual switch in the room where they are located – this means bathrooms served by a central exhaust fan or HRV need a manual
switch for the central system in each bathroom.
To Summarize: Manufacturers’ Directory Section 9 of the NBC represents a prescriptive path for ventilation systems. Designers can always use the more comprehensive and flexible CSA F326 Ventilation Standard.
