April 2007
  
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Using Your HVAC System to Improve IAQ
Recognizing the Importance of Controlling Key Airflow Rates

David S. Dougan,
President
EBTRON, Inc.1
January 2007

The following is extracted from a paper that was published earlier this year by Mr. Dougan. The complete paper can be found at http://www.ebtron.com/Web_Pdfs/ImprovingIAQ.pdf

OVERVIEW

In the early 1990’s a methodology of rating indoor air was introduced which equated various contaminant sources (people, carpet, mold, etc.) by using the percentage of people that were dissatisfied with different indoor air samples. Thus, indoor air quality became quantifiable. Today the accepted units of indoor air quality are called decipols, with zero decipols being pure air. The task is to determine just how close to zero one can economically design.

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Most current codes, including the International Mechanical Code (IMC), reference the 2001 Ventilation Standard published by the American Society of Heating, Refrigeration, and Air Conditioning Engineers (ASHRAE) which, by definition, requires an indoor air quality of 1.4 decipols. This corresponds exactly to 20% of the occupants being dissatisfied. In practice, however, designs meeting the Standard will not meet the 1.4 decipol criteria during operation and occupant dissatisfaction will be much greater.

The updated ASHRAE Ventilation Standard, revised in 2004, attempts to separate the occupant and building components of the contaminants. In some cases it allows ventilation rates to fall below the level (15 cfm) shown to be required to dilute bioeffluents (body odor). However, this revised Standard does a better job addressing outside air quality, air distribution and building pressure. Unfortunately, the Standard can be difficult to apply and can result in increased energy consumption and equipment costs; something frowned upon in today’s economy. The 2004 document has not been incorporated into the IMC at this time and it is unlikely that it will be in its current state.

A major setback, of both the 2001 and 2004 Standards, is that they have no meaningful verification requirement for compliance in key areas. For example, the Standards do not require continuous verification of the dilution air flow rates specified and they do not offer a viable method to maintain proper, compartmentalized building pressure. A lack of meaningful compliance has allowed low dilution flows and poor pressurization which have been shown to be directly responsible for a majority of the building related sicknesses reported in the last decade.

Documentation of financial returns for improved indoor air quality is emerging. It is inevitable that indoor air quality will be written into property leases and employment agreements in the not so distant future.

The bottom line is that today’s designers and building owners carry the burden of design and operation compliance to maintain acceptable indoor air quality. Byproducts of the increase in occupant well-being are the reduction of liability, increase in building longevity and improved building performance.

IAQ BASICS

Contaminants are introduced into a reservoir (room) and dilution air removes these contaminants. Indoor air quality depends on the purity and flow rate of the dilution air and the contaminant generation rate. In buildings the dilution air is not fully mixed into the reservoir, so a ventilation efficiency factor must be used to determine the breathing zone air quality.

The fundamental indoor air quality equation, the Comfort Ventilation Equation, was developed by the late Professor P.O. Fanger. Fanger noted that the industry had no method for comparing the effect of various emissions. For example, what concentration of formaldehyde is equivalent to emissions from people? Fanger resolved the problem by exposing groups of people to a variety of indoor environments and recorded the percentage of people who were dissatisfied. In this way he was able to develop a system of measurement which rated the emission strengths (rates) of different contaminants on the same scale. These test groups were adapted people, people that had been in the space for some length of time, at least several minutes. People who are not adapted—those just entering a space—were more likely to find a space unacceptable because their senses have not tuned out offensive odors. The units of this common scale are olfs, with one olf equaling the emission of a sedentary person (with one bath daily). Fanger then defined decipols as the indoor air quality in a space with 1 olf emission rate and 10 liters/second of 0 decipol dilution air.

It is important to remember two facts. The current index of indoor air quality, i.e. decipols, is not based on a standard which correlates indoor air quality with health care costs or any other index of the harmful effects of dirty air. Instead the decipol index is based on the rather arbitrary standard of peoples’ perception of indoor air quality.

Secondly, with today’s technology, indoor air quality cannot be measured directly. While some contaminants and indicators can be measured relatively easily—such as volatile organic compounds (VOCs) and carbon dioxide—others are more difficult and expensive to measure, including ozone, particulates, and hydrocarbons (traffic exhaust), while still other contaminants, such as human bioeffluents, molds, yeasts, and pollen are currently beyond practical measurement. This leaves dilution air, which can be accurately measured, as a key factor in controlling and monitoring indoor air quality.

Mold and fungal growth can be minimized by controlling envelope and space moisture by assuring proper building pressurization and providing sufficient equipment capacity to remove the moisture of the dilution air passing through the air handling systems. Building pressure is most effectively controlled by maintaining airflow differentials into and out of individual pressure zones.

The objective of moisture management is to control the moisture content of exterior walls, attics and other perimeter cavities to prevent mold formation, corrosion and other structural damage such as efflorescence.

Walls, roofs, and overhanging plenums also form one or more pressure compartments, sometimes in combination with each other, allowing moisture to migrate between these spaces. The flow of moisture into and out of these spaces is due to: liquid water, either as rain or other leaks; moisture contained in leakage airflows; and molecular diffusion, the flow of moisture associated with vapor pressure differential.

PlantPROCORE Liquid flow is by far the most dangerous source of moisture, about 100 times more potent then airflow, and 1000 times greater than diffusion. Gravity is a strong driving force for liquid water and air pressure differential can augment or retard water flow. Besides the physical augmentation of liquid water flow, the moisture contained in moving air as water vapor is often overlooked.

Outside air will carry a significant quantity of water across the building envelope when the building pressure is negative. Assuming an internal wall surface temperature of 74° F, outside air with a dew point of 65° F or higher will result in conditions within the building envelope where the relative humidity is high enough for mold growth. These conditions are prevalent throughout much of the United States. Depending on geographic location, as much as 1,000 gallons of water can be transported per 1,000 cfm of outside air.

It is the architect’s responsibility to design a wall assembly that will prevent excessive moisture build up. Architects establish the location of the vapor barrier which divides the wall into two humidity chambers. Since walls need to perform over a long period, spanning many decades, it is inevitable that walls will experience excessive humidity. Consequently, architects should design walls to compensate for these incidents by giving walls the means to dry out on both sides of the vapor barrier.

An effective method for drying a wall is to pass dryer air through it. In the summer, that can be accomplished by maintaining a positive pressurization flow (positive pressure) across the building envelope. In the winter, the reverse is true.

Infiltration in the winter may reduce moisture content in the exterior walls but it decreases comfort by drafts and brings particulate into walls, a nutrient for mold. A logical approach during the winter is to maintain exterior walls net neutral.

Consideration should also be given to pressurization and humidity control during periods when buildings are not occupied. Mold does not go home at 5:00 PM.

From the above, it can be seen that accurate measurement and control of key airflow rates are essential in maintaining acceptable indoor air quality. Proper selection of materials, the addition of filtration and an accurate means of determining occupancy will not improve IAQ on properly designed systems but can significantly reduce energy consumption and equipment requirements for a given facility.

BUILDING PRESSURE

Buildings should be compartmentalized into unique pressure zones. Ventilation systems must simultaneously control the overall building envelope leakage airflow and the internal partition leakage airflows (the overall pressurization flow). Both the magnitude and the direction of the pressurization flow need to be controlled.

An idealized pressure compartment will have six equal area partitions (four walls, ceiling and floor) of like porosity suspended in still air of like temperature. In this idealized case, conservation of mass dictates that the leakage flow across each partition will be one sixth the difference between the ventilation in and out flows. Leakage flows through actual compartmental partitions will not be uniform (cfm/ft2) or even the same direction due to porosity differences and variations in surface pressure distribution over inside and outside partition faces. However, the net pressurization flow will always be equal to the difference between the air supplied and returned from the compartment, regardless of external forced infiltration/exfiltration (i.e. wind and stack effect).

Partitions that are too porous will prevent the compartment from being properly pressurized. Partitions that are too tight will create excessive pressure when doors are opened and closed as a piston effect. When the compartment height is too high, as a high rise building without effective floor to floor separations, ventilation systems do not have the capacity to pressurize the top of the compartment and bidirectional partition leakage occurs.

Under normal wind conditions there is no single point or even group of points that will provide a viable control variable signal. Consequently, building differential pressure sensors cannot be used to control leakage flow, even though labyrinth wind dampening probe chambers and various averaging of pressures in multiple building faces have been used for years. However, it may be necessary to monitor external differential pressure on every major building external partition to confirm proper direction of leakage flow when wind is not present. Under some circumstances, including after rain, the control system may temporarily increase leakage flow to reduce wall cavity moisture.

Partition leakage flow is ideally controlled by monitoring partition differential pressure indirectly with a “bleed” airflow sensor and resetting the total compartment differential flow, generally the supply and return airflow rates. In many cases this will also require reset of the outside air flow setpoint which, in turn, will also affect system relief/exhaust. Partition differential pressure positively indicates the direction and magnitude of partition leakage flow. Partition differential pressure is ideal for reset but should not be used for direct control because of its inherent instability. The differential between the total ventilation system flow into and out of a compartment (supply less return air flow) is the control variable that will maintain proper and stable compartment pressure.

Wind complications on exterior partition surfaces dictate that differential flow is always the surrogate control variable for controlling outside air leakage through exterior surfaces. Generally speaking, some degree of wind driven infiltration is inevitable. In some cases, resetting the differential flow setpoint by monitoring individual wall differential pressures using “bleed” airflow sensors can compensate for moderate changes in wind speed and direction as long as the maximum pressure of any exterior wall is not exceeded. When buildings have multiple compartments, the desired leakage flow between compartments is controlled by increasing the outside airflow differential setpoint in one compartment and reducing the outside air differential flow in other compartments by the same amount. This keeps the total building outside air ambient exchange constant. Interior partition “bleed” sensors can be used to confirm the direction and magnitude of the interior partition leakage flows.

DILUTION VENTILATION

Control Solutions, Inc Dilution ventilation (outside air) flow rates must meet the minimum requirements of ASHRAE Standard 62.1-2004 or building code, whichever is greater, to assure compliance. In cases where productivity and health are paramount, providing more outside air to the breathing zone than specified by ASHRAE Standard 62.1-2004 is desirable.

The scope of this document is not to specify minimum ventilation rates for acceptable IAQ; it is simply to provide a method to assure that the specified rates are maintained under all load conditions.

A properly designed dilution ventilation system must consider whether or not the outside airflow rates will be variable (due to free cooling, occupancy or pressure variations) or fixed. Since outside airflow rates are typically low, selection and placement of an outside airflow measuring station is critical. Damper sizing and quality are critical since improperly sized, poor quality control dampers will render your entire system useless. Energy can be conserved on VAV and variable occupancy systems by monitoring the occupancy and airflow rate to critical zones for reset of the outside airflow setpoint.

The control strategies used and supported by EBTRON and other companies can accommodate changes in the outside air setpoint. The sequential approach simplifies damper selection and results in better control with minimal system pressure loss, especially when oversized dampers are selected. We therefore also strongly recommend the use of high quality, opposed blade, airfoil dampers with smooth operating and tight sealing blades.

CONCLUSIONS

Buildings should be compartmentalized into pressure zones. Airflow rates must be controlled into the building and into and out of each pressure zone to maintain both pressure and dilution air requirements.

A significant financial benefit can be realized by improving indoor air quality. Standards and codes have attempted to address ventilation issues but have fallen short in meaningful methods to verify compliance. Today’s designers and building owners carry the burden of design and operation compliance to maintain acceptable indoor air quality.

Dilution air and building pressure control go hand-in-hand. Many attempts to conserve energy by reducing the outside air result in negatively pressurized compartments with high moisture envelopes, especially in humid climates. A sound strategy must assure that the minimum outside airflow rate at the air handling unit will a.) meet the ventilation requirements for IAQ and b.) meet the flow requirements for the individual pressure compartments being served.


1EBTRON recognizes the efforts of D.P.W. Solberg for his contribution on the development of this paper.

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