ASHRAE Standard 62 Ventilation for Acceptable Indoor Air Quality
ANSI/ASHRAE Standard 62, Ventilation for Acceptable Indoor Air Quality1 was published in 1971, 1981 and again fully revised in 1989. The complete revisions made it easy to reference in Building Codes. Designers could refer to the edi- tion stipulated, and there was no question about the reference. The policy was changed for this standard in 1997, to align with the ANSI “continuous maintenance” process. Under continuous maintenance, the Standard is updated a bit at a time and is not required to be a consistent, whole document. The information in this section is based on the 2004 printed edition.
Standard 62.1-2004 applies to “all indoor or enclosed spaces that people may occupy” with the provision that additional requirements may be necessary for laboratory, industrial, and other spaces. As noted at the beginning of this chapter in the introduction, residential ventilation is specifically covered in Standard 62.2-2004 Ventilation and Acceptable Indoor Air Quality in Low-Rise Residential Buildings. You should also note that many local authorities have more demanding and specific requirements for residential ventilation than the ASHRAE standards. For industrial occupancies, refer to Industrial Ventilation, published by the American Conference of Governmental Industrial Hygienists.
“The purpose of this standard is to specify minimum ventilation rates and indoor air quality that will be acceptable to human occupants and are intended to minimize the potential for adverse health effects.”
Note that this is a minimum standard, that it is aimed at providing “accept- able indoor air quality” which is defined as:
“air in which there are no known contaminants at harmful concentrations as determined by cognizant authorities and with which a substantial majority (80% or more) of the people exposed do not express dissatisfaction.”
The Standard defines two types of requirements to maintain indoor air quality: requirements to limit contamination; and requirements to provide ventilation to dilute and remove contaminants. The requirements to limit contamination also include several system and building design requirements to minimize moisture problems that typically lead to mold problems including:
Requirements for filtering
Separation distance between outside air inlets and contaminated exhausts
Rules about recirculation of air between zones that have different contamination levels
Requirements for maintenance and operation Requirements for design and maintenance documentation
Standard 62.1-2004 requires that “Air from smoking areas shall not be recirculated or transferred to no-smoking areas.” Also smoking areas “shall have more ventilation and/or air cleaning than comparable no-smoking areas.” However no specific recommendations are included for smoking areas.
There are two approaches to providing ventilation for the occupants to breathe and to dilute the inevitable pollutants:
e “The Indoor Air Quality Procedure” Acceptable air quality is achieved within the space by controlling known and specifiable contaminants to acceptable limits. The application of the Indoor Air Quality Procedure allows the use of particulate and gaseous filters to assist in maintaining acceptable indoor air quality. The complexity of the procedure is beyond this course and will not be discussed.
e “The Ventilation Rate Procedure” Acceptable air quality is achieved by providing ventilation air of the specified quality and quantity.
The Ventilation Rate Procedure is based on providing an adequate supply of acceptable outdoor air to dilute and remove contaminants in the space to provide acceptable IAQ. Acceptable outdoor air must have pollution levels within national standards.
The basic required outside air for ventilation is based on a rate, cfm, per person, plus a rate per square foot, cfm/ft2. This basic requirement is then adjusted to allow for the ventilation effectiveness in each space and the
effectiveness of the system. Let us briefly go through those steps. An excerpt of the base ventilation data from Table 6–1 in Standard 62.1-2004 is shown in Figure 4.5.
Look at the first occupancy category, the hotel bedroom. The requirement is here is for 5 cfm per person and 0.06 cfm/ft2. Based on the default occupancy density of 10 persons per 1000 ft2 the combined outdoor rate per 1000 ft2 is
10 people · 5 cfm/person + 1000 ft2 · 0.06 cfm/ft2 = 50 cfm + 60 cfm = 110 cfm
The default combined outdoor air rate is thus 110 cfm for 10 people occupying 1000 ft2. Divided by the default population of 10 persons we get 11 cfm/person for the base requirement per person.
Now look at the last hotel category, multi-purpose assembly. The rate per person, 5 cfm, and rate per ft2, 0.06 cfm, are the same. What is different is the default occupancy density of 120 persons/1000 ft2. With the much higher occupancy density the ventilation for the space is much less significant and therefore the combined outdoor air rate per person is halved to 5.5 cfm, shown rounded up to 6 cfm in the table.
These default outdoor air rates must then be adjusted to allow for the proportion of ventilation air that actually circulates through the breathing zone. If we suppose that only 90% of the outdoor air enters the breathing zone, and the other 10% circulates above the breathing zone and is exhausted, then only the 90% of outside air is being used effectively. Therefore, the proportion of air that actually circulates into the breathing zone is called zone air distribution effectiveness. In the example, the zone air distribution effectiveness would be 0.9. The breathing zone is defined as between 3 and 72 inches from the floor and 24 inches from walls or air- conditioning equipment.
Let us consider a space with the ventilation air being provided from a ceil- ing outlet. Standard 62.1-2004 gives the zone air distribution effectiveness for cool air supplied at ceiling level as “1.” To obtain the corrected ventilation rate, we divide the base rate by the zone air distribution effectiveness. In this case, default outdoor air rate divided by a zone air distribution effectiveness of “1” means the default rate is unchanged.
Now let us suppose that the same system is used for heating in the winter. In this example, the maximum design supply temperature is 95°F and space design temperature is 72°F. The supply air temperature is
95°F 72°F = 23°F
above the temperature of the space. According to Standard 62.1-2004, “For warm air over 15°F above space temperature supplied at ceiling level and ceiling return, the zone air distribution effectiveness is 0.8.” In this example, with the default rate divided by 0.8, we obtain the corrected required ventilation, 1/0.8 = 1.25. This means that the outside air requirement has increased by 25%, compared to the cooling-only situation. If this system runs all year, then the ventilation should be designed for the higher winter requirement.
Thus far, we have used the Table 6-2 rates to obtain base ventilation rates and then corrected those to recognize zone air distribution effectiveness within the space. Now we must consider the effectiveness of the system.
If the system supplies just one zone or 100% outside air to several zones, the calculated rate is used. However, if the system serves multiple zones with a mixture of outside air and recirculated return air, we may have to make a system adjustment to allow for differing proportions of outside air going to different zones.
For example, an office building might require 15% outside air to the offices, but 25% to the one conference room. If the system provides only 15%, then the conference room will be under-ventilated. However, 25% for the conference room will provide much more than the required ventilation
to the rest of the offices. Standard 62.1-2004 includes a simple calculation to obtain a rate between 15% and 25% that provides adequate outside air for all the zones.
Further adjustments can be made to allow for variable occupancy and for short interruptions in system operation. Just one example of this type of adjustment can occur in churches with high ceilings. If the services are of limited duration, say under an hour and a half, and the volume of the zone is large per person, then the outside air ventilation rate can be based on an aver- age population over a calculated period. This may substantially reduce the required flow of outside air.
This discussion has all been based on Standard 62.1-2004. In many jurisdictions, earlier versions of the standard will remain the legal requirement for many years. If this is the case in your jurisdiction, it is important to know that previous versions of the Standard generally calculated the required ventilation based on cfm-per-person and took no separate account of the size of the zone. The simpler requirement facilitated a simple method of adjusting ventilation rates to meet actual occupancy needs in densely occupied spaces. The following section describes how carbon dioxide can be used to determine ventilation requirements in these situations.
4.5.1 The Use of Carbon Dioxide to Control Ventilation Rate
All versions of the Standard allow for reduced ventilation when the population density is known to be lower. For example, the ventilation for a movie theatre must be sized for full occupancy, although the theatre may often be less than half-full. In these “less-than-full” times it would save energy if we could reduce the ventilation rate to match the actual population. In the versions of Standard 62 that preceded 2004, the ventilation rates were based on cfm/person. As a result, the ventilation could be adjusted based on the number of people present.
Conveniently for the purposes of measurement, people inhale air that contains oxygen and exhale a little less oxygen and some carbon dioxide. The amount of carbon dioxide, CO2, that is exhaled is proportional to a person’s activity: more CO2 is exhaled the more active the person. This exhaled CO2 can be measured and used to assess the number of people present.
In our movie theatre, the people (assume adults) are all seated and the metabolic rate is about 1.2 met. At 1.2 met, the average CO2 exhaled by adults is 0.011 cfm. At the same time as the people are exhaling CO2, the ventilation air is bringing in outside air with a low level of CO2, as diagrammed in Figure 4.6.
This process can be expressed in the formula:
This is about 700 parts per million of CO2 in the exhaust air Note that this calculation is based on the ventilation for one person and the CO2 produced by one person. The result is the same, regardless of how many people are in the space, since everything is proportional.
The outside CO2 is typically in the range of 350 to 400 parts per million, ppm, so the incoming CO2 level is raised by the CO2 from the occupants:
for the same ventilation rate.
In our theatre, we can install a CO2 sensor to measure the CO2 level, and connect it to a controller to open the outside air dampers to maintain the CO2 level at no higher than 1000 ppm. In this way the outside air provided matches the requirements of the people present. If the outside CO2 concentration is above 300 ppm, then our controller, set at 1000 ppm, will cause over-ventilation rather than under-ventilation.
In this process CO2 is used as a surrogate indicator for the number of people present.
The use of CO2 control works really well in a densely populated space served by a dedicated system. It works poorly in a building with a very variable and low population.
This calculation assumes a perfect world. As we all know, this is a false assumption. The main assumptions are:
Perfect mixing. Mixing is usually quite good but some ventilation air may not reach the occupied space.
Steady state. It will take from 15 minutes to several hours for the CO2 con- centration to become really steady. The length of time depends on the volume of space per person. In densely populated spaces, steady state can be reached quite quickly, but in low population density areas, it can take hours.
An even distribution of people in the space. If people are clumped together then the level will be higher in their area and lower in the less densely occupied parts of the space.
This simple use of carbon dioxide as a surrogate cannot be used under the requirements of Standard 62.1-2004, due to the cfm/ft2 ventilation requirement for the space. More sophisticated methods are possible for use under the requirements of Standard 62.1-2004, but they are beyond the scope of this course.
The Next Step
Having introduced the ideas of: Air-conditioning zones in Chapter 2; Thermal comfort in Chapter 3; and Indoor air quality and ventilation rates in this Chapter, we will go on in Chapter 5 to consider why air conditioning zones are required, how to choose zones and how they can be controlled.