HVAC engineers are concerned with providing a comfortable and healthy indoor environment for building occupants. Comfort and indoor air quality (IAQ) are dependent on many factors, including temperature regulation, pollutant control from both internal and external sources, a supply of acceptable ventilation air, removal of unacceptable air, occupants' activities and preferences, and proper operation and maintenance of building systems.
Ventilation and infiltration are only a part of providing comfort. HVAC designers, occupants, and building owners must be aware of and address the many other factors as well. Reducing ventilation and infiltration to solve comfort problems and to reduce energy consumption can affect indoor air quality and may be against code. Such procedures should be approached with care and comply with the recently revised ASHRAE Standard 62, Ventilation for Acceptable Indoor Air Quality. This standard contains ventilation design and evaluation requirements for commercial and residential buildings. In the design of a new building or the analysis of an existing building, the version of Standard 62 that has been adopted by the local code authority must be determined. An existing building may be grandfathered under an older code, or it may be required to meet current code.
Outdoor ventilation air distributed through a building is often used to dilute and remove indoor air contaminants. Conditioning this outdoor air is often a significant portion of the total heating and cooling load. The amount of outdoor airflow into the building must be determined to properly size the HVAC equipment and evaluate the energy consumption.
Modern commercial and institutional buildings usually use ventilation air to slightly pressurize the building as this reduces or eliminates infiltration. Buildings without mechanical cooling and dehumidification require proper ventilation and infiltration airflows, as they are important in providing occupant comfort.
Fire concerns and the movement of smoke will affect the design of the airflow into buildings and between zones.
Ventilation and Infiltration
Air exchange of outdoor air with the air already in a building can be divided into two broad classifications: ventilation and infiltration.
Ventilation is the intentional introduction of air from the outside into a building; it is further subdivided into natural ventilation and forced ventilation
Natural ventilation is the airflow through open windows, doors, grilles, and other planned building envelope penetrations, and it is driven by natural and/or artificially produced pressure differentials.
Forced ventilation (mechanical ventilation) uses fans and intake and exhaust vents to provide the intentional airflow into and out of a building.
is also known as air leakage into a building and is where outdoor air flows into a building through the normal use of exterior doors and through cracks and other unintentional openings.
is the leakage of indoor air out of a building. Exfiltration and infiltration are driven by pressure differences, both natural and/or artificial
Air Exchange Rate
The air exchange rate compares the cubic feet per minute (cfm) of airflow to the interior volume of space in cubic feet and is expressed in units of time. When the time unit is hours, the air exchange rate is called air changes per hour (ACH). The air exchange rate may be defined for several different situations. For example, the air exchange rate for an entire building or zone served by an air-handling unit compares the amount of outside air brought into the building or zone to the total interior volume.
Outside Air Fraction
This is described by the ratio of the volumetric flow rate of outside air brought in by the air handler to the total supply airflow rate. When expressed as a percentage, the outside air fraction is called the percent outside air.
The design outside airflow rate for a building's ventilation system is found through evaluating the requirements of ASHRAE Standard 62. The supply airflow rate is that required to meet the thermal load. The outside air fraction and percent outside air then describe the degree of recirculation, where a low value indicates a high rate of recirculation, and a high value shows little recirculation. Conventionally, commercial and institutional buildings have approximately 10 to 40% outside air.
Air Change Effectiveness
Ventilation effectiveness is a description of an air distribution system's ability to remove internally generated pollutants from a building, zone, or space. Air change effectiveness is a description of an air distribution system's ability to deliver ventilation air to a building, zone, or space. The HVAC design engineer does not have knowledge or control of actual pollutant sources within buildings, so ASHRAE Standard 62 defines outdoor air requirements for typical, expected building uses. For most projects, the air change effectiveness is the most relevant to HVAC system design.
Cooling & Heating Ventilation Air Load
Outdoor air introduced into a building constitutes a large part of the total space-conditioning (heating, cooling, humidification, and dehumidification) load, which is one reason to limit the amount to the minimum required. This load typically represents 20 to 40% in a perimeter-dominated building. Ventilation air increases a building's sensible and latent heat load.
First, the incoming air must be heated or cooled from the outdoor air temperature to the indoor air temperature. The sensible heating or cooling is calculated by this formula:
q = 60*Q*d*Cp*TD
where: q = sensible heat load, Btu/h
60 = minutes per hour
Q = ventilation airflow rate, cfm
D = air density, lb/cu ft (about 0.075)
Cp = specific heat of air, Btu/lb-°F (about 0.24)
TD = indoor-outdoor temperature difference, °F
Second, incoming air changes the moisture content of the air in a building. This is particularly important in those locations where the summer outdoor air must be dehumidified. In the winter, when the relative humidity of the indoor air is below 30%, humidification may be needed. The latent load associated with these factors is shown in this formula:
q = 60*Q*Hfg*DWfg
where: q = latent heat load, Btu/h
60 = minutes per hour
Q = ventilation airflow rate, cfm
Hfg = latent heat of vapor at appropriate air temperature, Btu/lb (about 1,000)
DWfg= humidity ratio of indoor air minus humidity ratio of outdoor air, lb water/lb dry air
ASHRAE Standard 62 provides guidance on ventilation and indoor air quality in the form of two alternative procedures:
Ventilation Rate Procedure
Indoor air quality is assumed to be acceptable if (1) the concentrations of six pollutants in the incoming outdoor air meet the United States national ambient air quality standards, and (2) the outdoor air supply rates meet or exceed values (which vary depending on the type of space) provided in a table. The minimum outside air supply per person for any type of space is 15 cfm. This minimum rate will maintain an indoor CO2 concentration below 0.1% (1000 parts per million) and research which indicated that 15 cfm was required to satisfy the odor perceptions of 80% or more people.
Indoor Air Quality Procedure
In this procedure, any outside air supply rate is acceptable if (1) the indoor concentrations of nine pollutants are maintained be low specified values, and (2) the air is deemed acceptable via subjective evaluations of odor. If users of the IAQ Procedure control pollutant source strengths or use air cleaning or local exhaust ventilation, they may be able to reduce the outside air supply rates to below those specified in the ventilation rate procedure.
Links to Related Topics
Indoor Air Quality
Outside Air Control
Central Plant System
Central (or Built-Up) System
All-Air Central Systems
All-Air Central Reheat Systems
All-Air Central Dual Duct
All-Air Central Multizone
Air-Water Central Systems
All Water Central Systems
Two Pipe System
Three Pipe System
Four Pipe System