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June 18, 2000
High performance building envelopes require controlled ventilation systems, sealed combustion appliances and allow the use of innovative air distribution systems. The optimum controlled ventilation system in all climate zones is a supply ventilation system that allows for filtration of outside supply air and the pressurization of building enclosures to exclude environmental contaminants. The high performance aspects of the building envelopes allow for the simplification of duct distribution systems. Advanced space conditioning involves the integration of a building’s heating, ventilating and air conditioning (HVAC) system with the building enclosure or building envelope.
Heating, ventilating, and air conditioning (HVAC) systems involve both thermal comfort (the heating and air conditioning components) and dilution of interior pollutants (the ventilating component). The HVAC system includes all heating, cooling, and ventilation equipment serving a building including their associated distribution systems.
The five principle functions of a HVAC system are as follows:
- maintain thermal comfort under operating conditions
- provide fresh air to the "head space" of occupants in sufficient quantity and quality to dilute pollutants generated by occupants, furnishings and the structure (outdoor air supply and ventilation effectiveness)
- facilitate source control of pollutants by providing adequate interzonal and interstitial pressure control (system operation, maintenance, commissioning/balancing)
- facilitate source control of pollutants by providing "capture at source" exhaust ventilation at pollutant generation locations and preventing reintrainment of this exhaust ventilation (local exhaust)
- facilitate source control of pollutants by providing air cleaning (filtration of particles, gases and dehumidification of airborne moisture)
The ability of a HVAC system to provide these functions is directly dependent on the characteristics of the building envelope and the climate.
The indoor environment involves the interrelationship of comfort factors such as temperature and relative humidity, physical stressors such as noise, lighting and psycho-social factors (personal relationships, peer pressures, work stress, etc.) as well as chemical, particulate and biological concentrations.
The ultimate objective of a successful environmental system is to provide for the comfort and welfare of the occupants and prevent an accumulation of unpleasant and/or harmful pollutants.
Occupant comfort complaints constitute a significant portion of indoor environmental complaints. If occupants are uncomfortable, they will complain. Thermal comfort must be maintained within a building in order to provide for an acceptable indoor environment.
Many variables interact to determine whether occupants are comfortable. The American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) Standard 55-1992: Thermal Environmental Conditions for Human Occupancy describe the temperature and humidity ranges that are comfortable for most people under various activities. Some of the variables discussed in Standard 55-1992 are:
- uniformity of temperature (temperature stratification, convection)
- radiant heat transfer (cold or warm surfaces such as windows)
- relative humidity
- activity level, age and physiology of occupants
- clothing levels
There is considerable debate among researchers, building scientists, engineers and health professionals concerning recommended levels of relative humidity. Interior relative humidities which are too low are uncomfortable. Interior relative humidities which are too high are unhealthy. Consensus is emerging that the critical relative humidity for biological activity to occur on building envelope surfaces is 70 percent. Where relative humidities above 70 percent occur at surfaces, mold growth, dust mite growth, decay, corrosion, etc. can occur. Therefore, conditions should be maintained within a building such that the critical 70 (or higher) percent relative humidities at building envelope surfaces do not occur. Due to climate differences, interior conditions which must be maintained to avoid the critical surface relative humidities vary from region to region and time of year. They also vary based on the thermal resistance of the building envelope.
In general relative humidities should be kept as low as comfort levels and HVAC equipment will allow. People are rather insensitive to relative humidity variations between 25 percent and 60 percent. However, relative humidity variations within this range do affect perception of temperature. In other words, interior relative humidities should be maintained as low as possible, and interior temperatures adjusted within the parameters specified in Standard 55-1992 to maintain thermal comfort. Relative humidities below 20 percent are uncomfortable to most people and should be avoided. Relative humidities below 50 percent during air conditioning periods in hot, humid climates may not be economical/practical to achieve. During heating periods interior relative humidities should range in the 25 percent to 35 percent range. During air conditioning periods interior relative humidities should range in the 50 to 60 percent range (if equipment allows).
Outside Air Supply
One principle focus of a healthy indoor environment is on concentrations of pollutants. In simple terms, the greater the concentration, the greater the risk. Pollutant concentrations are determined by two factors (Figure 1):
- source strength; and
- rate of removal
HVAC systems impact both the source strength of pollutants and the rate of removal of pollutants. Source strength is determined by the rate of pollutant entry into a conditioned space from the exterior, the rate of pollutant generation within the conditioned space, and the rate of pollutant off-gassing from the building products comprising the building assemblies, components, elements, sub-systems and furnishings (Figure 1).
Removal or dilution is typically determined by air change, air cleaning and storage. Air change can be considered a combination of natural ventilation (infiltration/exfiltration) and mechanical (controlled) ventilation. Air cleaning usually occurs through the filtration of particulates (and some gases) and by the removal of moisture (dehumidification). Storage of pollutants involves furnishings and surface coverings absorbing pollutants and/or becoming receptors for particulates (carpets) (Figure 1).
In general, if the rate of entry, generation and off-gassing of pollutants exceeds the rate of removal, concentrations will rise. In general, if the rate of removal exceeds the rate of entry, generation and off-gassing, concentrations will fall. The key is to maintain pollutant concentrations at levels sufficiently low to avoid problems.
One simplistic approach is to provide mechanical ventilation to control pollutant concentrations. The greater the source strength, the greater the mechanical ventilation required. The problem with this approach is that this ventilation air must come from the exterior. This exterior air must be heated or cooled and humidified or dehumidified depending on climate. In some locations this air must also be cleaned (filtered) prior to use within a conditioned space. The conditioning of this air consumes energy and increases operating cost.
The greater the rate of dilution by mechanical ventilation and infiltration/exfiltration, the greater the operating cost and capital cost for equipment to provide this dilution and conditioning. Furthermore, dilution is very inefficient. Any powerful pollutant source will overpower even large ventilation rates. Dilution is not the solution to indoor air pollution. At least not by itself. In hot, humid climates, dilution can become the problem. The greater the rate of dilution in these climates, the greater the rate of entry of moisture. Moisture is a major indoor air pollutant. Mold and other biological agents can quickly multiply out of control. In heating climates, high dilution rates during the heating season can lead to excessive loss of interior
moisture and in order to compensate, humidification may be necessary.
Another simplistic approach is to provide source control to control pollutant concentrations. If pollutants are prevented from entering conditioned spaces, are not generated within conditioned spaces, and are not off-gassed from building materials and furnishings, then dilution by mechanical ventilation and infiltration/exfiltration is not required. The problem with this approach is that one of the major pollutant sources in a building enclosure happens to be people.
People can be considered as evaporatively cooled, unvented combustion appliances. They burn a hydrocarbon fuel (food) and generate three principle by-products of combustion, carbon dioxide, moisture and odors depending on diet and activity. When deodorants, perfumes, clothes, and hair sprays are added, people become thermal cracking towers and analogies can be made to petroleum distillation. It has been said that if people were not allowed into buildings we wouldn’t have problems. It is also obvious that source control for people is not practical.
Combining source control and removal is the preferred approach. Source control and air cleaning can be provided for all the pollutant sources except for people. Ventilation is then provided for the people. This can be rephrased as: “You ventilate for people not for buildings”. Ultimately, this provides the most cost effective and energy efficient approach. Since the higher the ventilation rate, the higher the operating and energy costs, a great incentive exists to reduce ventilation rates to save operating costs and energy. However, ventilation rates should not be lowered beyond those required by people to control the pollutants generated by the people themselves (carbon dioxide, moisture and odors). All other pollutant sources can be controlled by source control and air cleaning. There is a great economic incentive to provide source control and air cleaning for all other sources, since ventilation (the only other alternative) to control these other sources increases operating costs and the capital costs for installation and maintenance of systems.
Building Envelope Leakage
Relying on random leakage openings and the effects of wind and stack to provide air change through infiltration/exfiltration does not ensure constant dilution or dilution when required. Controlled mechanical ventilation is a requirement in all buildings, regardless of tightness, for health reasons.
Maintaining an acceptable indoor environment by natural means such as wind, is unacceptable by today’s standards. In the past, energy was inexpensive and natural means, such as wind, was relied upon to bring fresh air into buildings. Building envelopes were leaky and indoor moisture and other indoor pollutants were able to move outdoors (in heating climates) through the leakage openings around windows and doors without causing serious problems. In cooling climates, air conditioning was not common and extensive natural ventilation was required for minimal comfort. The introduction of air conditioning in cooling climates resulted in a significant reduction in natural ventilation.
Building enclosures have become significantly tighter over the past several decades and mechanical cooling and heating is common in all climate zones. At the same time, the introduction of hundreds of thousands of new chemical compounds, materials and products occurred to satisfy the growing consumer demand for goods and furnishings. Interior pollutant sources have increased while the dilution of these pollutants through air change by natural ventilation has decreased.
Ventilation is the process of removing and supply air by natural (infiltration/exfiltration) or mechanical (intake, exhaust) means to and from any indoor space. Controlled ventilation is defined here as a combination of mechanically exhausting indoor air and mechanically or passively supplying outside air to maintain adequate indoor air quality.
Natural ventilation, or infiltration/exfiltration driven by the stack effect and wind through random openings or through deliberate, discrete openings such as operable windows, doors, ductwork or holes is not adequate to remove pollutants and odors from an enclosure on a continuous basis due to the lack of the consistency of the stack and wind driving forces. However, this inconsistency does add significantly to the annual space conditioning (heating and cooling) energy consumption. Natural ventilation reduces the ability of occupants to control ventilation rates and seldom provides ventilation effectively to the critical “breathing zone” or head space.
Natural ventilation may be adequate at a certain instant in time, but may not be adequate at another instant in time when there is no wind, or when there is a change in wind direction, or when an insufficient interior to exterior temperature differential exists. In typical building envelopes, the normally random distribution of leakage openings influences the exfiltration/infiltration process which is responsible for the rate of natural ventilation. This random or accidental distribution of leakage openings resulting in accidental leakage does not provide assurance that adequate air change can take place.
The random distribution of leakage openings in a typical building causes the instantaneous infiltration/exfiltration rate in the building to vary substantially due to the influences of wind pressures, stack pressures and pressures induced by air consuming devices. Thus, a building or areas of a building can have adequate natural air change at one point in time and not have adequate natural air change at a subsequent point in time. The variation can be so substantial that the infiltration/exfiltration rate may be on the order of several hundred cubic feet per minute during a wind gust, and moments later zero if the wind suddenly dies down and the majority of the randomly distributed leakage openings accidentally and temporarily happen to fall along the neutral pressure plane. Furthermore, there is seldom provision under natural ventilation for air change to be “effective” in that the air quality in the breathing zone improves. Controlled ventilation involves the provision of a controlled driving force to remove and supply ventilation air through deliberate, discrete openings. This driving force can be provided on a continuous basis or, as necessary, based on the level of pollutants and odors within an enclosure.
The principles behind acceptable indoor air quality and providing controlled ventilation to achieve it are very basic.
- ventilate for occupants; and
- provide source control for everything else.
This is the single most energy efficient and cost effective approach. Occupants produce carbon dioxide, odors and moisture. Ventilate for the carbon dioxide, odors and moisture generated by occupants. . .
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