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The Residential Ventilation Standard

Max Sherman is the Chair of ASHRAE Combustion Safety's Standard Project Committee 62.2, which is reviewing public comments on the ventilation standard's first draft. This article describes the general outline of the draft's contents.

In August 2000, the American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE) released the first public review draft of its proposed residential ventilation standard entitled "Ventilation and Acceptable Indoor Air Quality in Low-Rise Residential Buildings." The Standard Project Committee (SPC 62.2P) responsible for developing this draft is now reviewing the results of that public review and will likely make changes to it, but the fundamental principles are unlikely to change.

The standard is an attempt by the Society to address concerns over indoor air quality in dwellings and to set minimum standards that would allow energy-efficiency measures to be evaluated. and source control. In addition to code-intended requirements, the standard also contains guidance information for the designer or user of the standard.

ASHRAE has long been in the business of ventilation, but most of the focus of that effort has been in the area of commercial and institutional buildings. Residential ventilation was traditionally not a major concern because it was felt that between operable windows and envelope leakage, people were getting enough air. In the quarter of a century since the first oil shock, houses have gotten much more energy-efficient. At the same time, the kinds of materials and functions in houses were changing in character in response to people's needs. People were also becoming more environmentally conscious not only about the resources they were consuming but about the environment in which they lived.

All of these factors contributed to an increasing level of public concern about residential indoor air quality and ventilation. Where once there was an easy feeling about the residential indoor environment, there is now a desire to define levels of acceptability and performance. Many institutions both public and private, have interests in indoor air quality, but ASHRAE, as the professional society that has had ventilation as part of its mission for over 100 years, was the logical place to develop a consensus standard. That standard has just finished its first public review.

ASHRAE Standard 62.2P defines the roles of and minimum requirements for mechanical and natural ventilation systems and the building envelope intended to provide acceptable indoor air quality. It applies to spaces intended for human occupancy within single-family houses and low-rise multi-family structures and generally excludes institutional buildings.

The standard appears to be principally about ventilation, but the purpose of ventilation is to provide acceptable indoor air quality. The most effective strategy for keeping exposure to undesirable pollutants low is to keep them from being released to the general indoor environment in the first place. Such "source control" measures actually make up the bulk of the pages in the standard. Local ventilation is intended to exhaust pollutants from specific rooms before they enter the general environment. Whole-house ventilation is intended to bring fresh air into the general environment to dilute the pollutants that cannot be effectively controlled at the source.

Overview of the Standard

In developing this standard the committee recognized that there were many different kinds of houses, many different climates, and many different styles of constructions. To accommodate these difference, the major requirements were designed with several alternate paths to allow users flexibility. Some requirements are performance-based, with specific prescriptive alternatives. The standard recognizes that there are several different ways to achieve a specified ventilation rate and allows both mechanical and natural methods.

There are three primary sets of requirements in the standard and a host of secondary ones. The three primary sets involve whole-house ventilation, local exhaust, and source control. Whole-house ventilation is intended to dilute the unavoidable contaminant emissions from people, materials, and background processes. Local exhaust is intended to remove contaminants from those specific rooms (e.g., kitchens) in which sources are expected to be produced by design. Other source-control measures are included to handle those sources that can reasonably be anticipated.

The secondary requirements focus on properties of specific items that are needed to achieve the main objectives of the standard. Examples of this include sound and flow ratings for fans and labeling requirements. Some of the secondary requirements as well as the guidance in the appendices help keep the design of the building as a system from failing because ventilation systems were installed. For example, ventilation systems that push moist air into the building envelope can lead to material damage unless the envelope is moisture-tolerant.

Required ventilation for different size houses.

Required ventilation for different size houses.

Whole House Rates

The committee decided to make the target ventilation rate comprise a sum of the ventilation necessary to dilute background sources and sources attributable to occupancy. To find the total amount of outside air needed, one needs to add 2 cfm/100 sq. ft. (10 l/s/100 sq. m.) to the 15 cfm/person (7.5 l/s/person). Thus the air change rate requirement will vary by the size of the house and the occupancy. The figure shows the required air change per hour (ACH) for typical houses.

Ventilation System Requirements

The ventilation system, whether it be natural or mechanical, has to meet some basic requirements:

Capacity and Distribution. Because activities in the normal use of a house (cleaning, smoking, parties, painting, etc.) will produce pollutants in excess of what is handled by the basic rates, the standard requires that each room have either a window or a local exhaust system. The requirement would usually be met by the code-required amount of window area. There is no explicit requirement, however, for air distribution.

Filtration. Air handlers are required to have particulate filters having a minimum efficiency of 60% for 3-micron particles. Although this level of filtration has some direct benefit to the occupants, its main benefit is in keeping the HVAC and distribution system from becoming a contaminant source. In hot, humid climates dirt build-up combined with moisture can lead to microbiological growth.

Sound Ratings. In most cases noisy fans will not be used. Occupants are more likely to disable them than to run them. The standard requires that whole-house fans be very quiet (1 sone) and that bathroom and kitchen fans be reasonably quiet (1.5 sones) at their rated flows.

Flow Rating. To make sure that the fan actually delivers the amount of air intended, the standard requires either that the air flow rate be measured in the field or that certain prescriptive requirements be met. These prescriptive requirements are on the size and length of ducting as well as the manufacturer's ratings.

Source Control

While many of the potential sources of pollution are beyond the control of a standard such as 62.2P, various measures can reasonably be taken to reduce pollutant sources at the design stage and thus reduce the need for excessive ventilation. Indeed, for some sources, ventilation may make them worse and not better. This section summarizes some of the source control measures in the standard.

Outdoor Air. The outdoor air can be a source of pollution. The ventilation rates in the standard assume that the outdoor air is relatively clean and able, therefore, to improve indoor air quality by diluting indoor pollutants. When outdoor air quality excursions are foreseeable (e.g., excessive ozone) the standard requires that the occupants be able to reduce whole-house ventilation rates.

Ventilation Inlets. Even if the outdoor air is of good quality, pollution in the building's microclimate can make the air that enters in through windows or other intakes of low quality. The standard requires there be adequate separation between inlets and exhausts or other known sources of pollution.

Garages. Attached workspaces or garages can be a source of significant pollution. Carbon monoxide is of particular importance when combustion (e.g., from cars) is taking place. The standard requires than any air-handling equipment placed in these spaces be sealed to prevent entrainment of these contaminants.

Clothes Dryers. 62.2P requires that clothes dryers be vented directly to the outdoors both to minimize moisture and laundry pollutions. Clothes dryers are treated as exhaust fans for the purposes of combustion safety and ventilation.

Moisture Migration. If moisture is forced into building cavities or the building envelope and allowed to condense, molds and other microbiological contamination can become a threat to indoor air quality and material serviceability. The standard forbids the use of ventilation methods (e.g., supply ventilation in very cold climates) that would contribute to that effect unless the building envelope has been designed to accept it.

Combustion Safety

Keeping combustion appliances from becoming indoor pollutant sources is a concern of the standard. Vented combustion appliances can become a problem if there is any significant backdrafting. 62.2P is not a standard about combustion safety, but indoor combustion sources can be a significant source of pollution and the requirements of 62.2P could have adverse impacts on those sources. The standard considers the impact that envelope tightness or ventilation systems could have on the operation of a combustion appliance.

To minimize the potential for backdrafting, the standard requires that naturally aspirated combustion appliances in the conditioned space pass a specific backdrafting test if the total of the largest two exhaust appliances exceed about 1 air change per hour of ventilation (not counting any summer cooling fans). Many new houses would be exempt from these considerations either because all their vented combustion appliances are outside the pressure boundary or are sealed combustion or because their two biggest exhaust appliances fall below the limit.

CO Alarm. The draft standard requires that a carbon monoxide alarm be installed. Ideally no carbon monoxide should be generated or allowed to come into the occupied space, but the requirements to assure this would be prohibitively costly (e.g., such control measures would include prohibiting air handlers in garages). Installing a CO alarm in the space provides more flexibility for builders.

— Max Sherman

For more information, contact:

  • Max Sherman
  • (510) 486-4022; fax (510) 486-6658

Max Sherman is Chair of ASHRAE's Standard Project Committee (SPC) 62.2

Download the draft ASHRAE residential standard.

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