Real-Time IAQ monitoring for VAV systems
May 1, 2008
In fact, the author asserts, we are already late in guiding and teaching good design incorporating real-time IAQ monitoring for VAV systems. Beyond examining Standard 62.1’s suggested shortfall and a corresponding solution, we also review a new filtration strategy that offers sustainable value both through its more selective operation and possible tax deductions.
Today we spend more time than ever indoors in commercial multiuse buildings, and as a result, sustainable IAQ is a greater concern to occupant health, well-being, and comfort, particularly after 9/11. Emissions from indoor contamination sources - including everything from furnishings to combustion processes and mold growth - are no longer the dominant factors in IAQ. Outdoor contaminants, particularly in densely populated urban areas, are now also of major concern.
HVAC practitioners rely on ASHRAE Standard 62.1 when assessing applicable criteria for client HVAC system ventilation, filtration, and air-cleaning needs based upon local microclimate. The standard is also a reference for design state conditions when selecting among alternative HVAC system selections. However, the design state conditions listed in Standard 62.1 presently ignore how to deal with real-time VAV system part-load conditions, as will be illustrated.
In addition, we will review findings from an Illinois Institute of Technical Research Institute (IITRI) study that examined bypass filtration/air cleaning strategy. These findings demonstrate the use of a value-added IAQ post-treatment method for minimizing occupant exposure to harmful, time-dependent indoor air concentration excursions, while simultaneously reducing ventilation air energy usage when incorporated with VAV (and to a lesser degree, with constant volume HVAC systems).
As a practical matter sole reliance on latest available updates of ASHRAE Standard 62.1 or its proposed code compliance measures cannot relieve HVAC system practitioners of their inherent responsibility to select HVAC systems on a sustainable IAQ basis consistent with foreseeable building occupant health and safety needs. Ensuring long-term occupant safety and health also requires a methodology for estimating real-time airborne concentration levels under normal, potential post 9/11 emergency or accidental harmful indoor and/or outdoor contaminants that may impact the health of occupants in both new and remodeled building occupancies.
IAQ And Value-Added Sustainability IssuesIn many of our recent urban projects, we employed a peer-reviewed strategy to simulate off-normal airborne contaminant excursions in real-time. This strategy has yet to be incorporated in either the 2004 or the latest version (2007) of ASHRAE Standard 62.1 to evaluate foreseeable occupant exposure to potential harmful indoor air contaminant excursions that are odorless and not otherwise detected.
The above-noted Standard 62.1 limitations should concern HVAC system practitioners who are mandated under ANSI/ASHRAE Standard 62.1-2004 with specifying “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” and are, “intended for regulatory application to new buildings, additions to existing buildings and those changes to existing buildings that are identified in the body of the standard.”
Ensuring long-term occupant safety, under the above referenced ASHRAE Board of Directors-approved "Purpose" for Standard 62.1-2004 and its more recent 2007 release, still remains the HVAC practitioner’s responsibility to minimize the potential for adverse health effects. Yet the IAQ Procedure described in either of the above-referenced ASHRAE 62.1 Standards does not include a method for estimating real-time airborne concentration levels to assist HVAC system practitioners in confirming those expectations.
Examples of some potentially harmful contaminant excursions likely with some VAV systems are given along with methods of estimating the occupant dosage that may have occurred, are illustrated employing available information taken, for example, from other Standard.62-2004 sections.
While IAQ compliance issues may not be directly linked to value-added benefits, indirectly, they are (Wheeler, 2001). Regarding sustainability, energy savings do play a vital part. Convergence, however, does take place with the selection of the type of HVAC system to be employed. Interestingly enough, the Energy Policy Act of 2005 (EPAct) stipulates tax deductions for buildings that exhibit a 50% energy improvement over ASHRAE 90.1-2001. EPAct offers a tax deduction of $1.80/sq ft. The details, however, can be complicated.
For example, the tax deduction can be compartmentalized for HVAC, lighting, and the building envelope. Additionally, a 16.67% improvement over ASHRAE 90.1-2001 results in any of those areas provided the building technologies involved would be eligible for a $0.60/sq ft tax deduction.
VAV systems are generally more energy efficient and lower in first cost than comparable constant volume (CV) systems and are and have been a major choice of HVAC systems for both new and existing buildings and additions to existing buildings (Persily, A, 2001). This is as a result of an approximate 30% annual fan energy savings, in addition to a less costly air distribution system, principally via reduced ductwork.
Yet from an IAQ standpoint, VAV systems (Meckler, M., 1992) present greater challenges when controlling contaminant concentrations. Since supply, related return, and exhaust airflows diminish with reduced part-load conditions, complications frequently arise with regard to both contaminant and humidity control.
Occupant IAQ Issue: VAV System Operation At Part LoadWhile there are a number of publicly available IAQ simulation programs funded directly by U.S. governmental agencies and various university researchers, they serve to validate academic research or monitor individual or common zoned building space(s). Most, if not all, are not capable of estimating the time-varying concentration responses of a variety of commercially utilized VAV air distribution systems. Later VAV systems (Meckler, M., 1993), comprising 75% of the configurations listed under Table D-1 of Standard 62.1, are subject to actual time-varying heating and cooling loads imposed upon building air handling and refrigeration equipment. Also, space contaminant concentration relationships as given under Table D-1 in their present form are not as useful as needed by HVAC practitioners to ensure their designs will maintain satisfactory IAQ at all part-load VAV conditions.
There is no reference in Standard 62.1 giving guidance for unplanned, yet, foreseeable situations where odorless, high, and unhealthy contaminant emissions can enter HVAC air distribution systems from indoor or outdoor sources, such as toxic spills, adjacent building and outdoor fires, harmful outdoor and indoor contaminants (Cobb, N, 1991, Goldstein, IF, 1989), wind-driven soil-bearing micotoxins, spores and pollens, acts of terror, etc. Additionally, building occupants and visitors can be exposed to high concentration levels of contaminants in their breathing air beyond acceptable short- or long-term thresholds established by cognizant medical authorities, which result in unhealthful dosages whose health effects may have short term, long term or possibly irreversible consequences, e.g., asthmatics.
The stated HVAC system relationships listed in Table D-1 of Standard 62.1-2004 were not intended to inform the society’s HVAC practitioners employing any of the listed VAV systems listed in the table on how to estimate time-varying space/air contaminant concentrations under foreseeable part-load conditions (Cummings, J.B., 2001). This can be particularly critical in exterior occupancies with highly variable daily and seasonal conditions (Dillard, W, 2001). For example, HVAC practitioners need to compute contaminant concentrations under all foreseeable time-varying supply, return, and proportional outdoor air rates.
Thus, Table D-1 and Appendix A need to be modified to permit synchronizing time-varying contaminant concentration(s) with time-varying VAV supply and related return, exhaust, and outdoor airflows (Meckler, M., 1995), while maintaining pre-determined space zone comfort, relative humidity levels, and satisfactory ventilation conditions.
Relating Occupant Physical Activity And DosageContaminant dosage ingested by building occupants during a typical office workday are a function of cumulative contaminant levels over time and occupant activity dictating breathing rates. It is currently treated minimally under Appendix C in terms of Figure C.2 and in its present form may not adequately address the need to “minimize the potential for adverse health effects.”
Fortunately, having addressed both issues for some time for our clients, I would like to suggest a solution that builds upon information already presented in Standard 62.1. First, however, let’s address the issue of occupant dosage while at work and at home.
It is also possible under some Standard 62.1-2004 provisions that safe contaminant concentrations can, at times, be exceeded by other or assumed contaminants of concern. They can be introduced into circulating air (e.g., chemicals released in office areas via document duplicating machines), resulting over time in an unacceptable dosage to sensitive or otherwise healthy building occupants through undetected exposure.
Accordingly, it is important that a provision be added to Appendix A to provide a written record of the specified minimum ventilation rates for representative occupancies listed in Table 6-1, along with a list of contaminants assumed and whether or not they are the same as values listed in Table B-1 and B-2 (Hodgson, A.T., et al., 1989). This document should accompany the final as-built documents delivered to the client, particularly in dealing with speculative office buildings where tenant improvements are often made by others.
In any event, where planned tenant operations are found to exceed initial design assumptions in order to comply with the above-referenced Standard 62.1 purpose and or above-referenced Appendix B tables, the HVAC designer should be advised of the need to compute a contaminant time-weighted average concentration to determine if the initial design based on specified minimum ventilation rates is still applicable.
Unfortunately, the above-referenced Standard 62.1 may not adequately address “risk management” issues that in complying with either of the two procedures required in dealing with known or unknown contaminant(s) which over time could result in “adverse health effects” to building occupants. One way to better manage that risk is to directly compute a time-weighted concentration of the potentially harmful contaminant(s) employing applicable OSHA-Z, NIOSH, ACGIH (ACGIH, 1994), EPA, or World Health Organization (WHO) guidelines containing allowable exposure data developed by cognizant health professionals. There are both short and long time exposure issues to be concerned with, namely:
- Short-term duration: Estimate building occupant exposure time within but not exceeding a 8-hr working day, or
- Long-term duration: Estimate most probable building occupant exposure time if undetected initially and/or lasting several days, weeks, months, or longer.
D = Br x Ctw x Vbz (microgram-liters/minute) (Equation 1)
Next, refer to Appendix G, Figure C.2 of Standard 62.1-2004, to identify the appropriate building occupant activity level for a given occupancy and select the corresponding value of Br given in the abscissa. The value of Ctw can be determined after first identifying the desired HVAC system type(s) listed under Table D-1. And then taking the first derivative of the corresponding “space contaminant concentration” Cs(t) equation indicated in Table 1 yielding the first derivative of the corresponding equation currently shown in Table D-1.
Based on the maximum probable time duration M (in minutes), a building occupant(s) can be exposed to Cs(t) within a 8-hr working day, one can compute Ctw(t) as follows.
Plot Cs(t) on ordinate vs. M on abscissa in IP units (with values of M ranging from 0 to 480 minutes). Since the area under the above referenced curve represents the cumulative product of Ctw x M in units of microgram-liters by dividing the graphical area under the curve by M, we obtain Ctw directly. If the contaminant of concern likely to be present beyond a short duration of 1-2 days, it must be designated as a process issue which is clearly beyond the scope of the above Standard 62.1.
Accordingly the HVAC system practitioner is advised in his design basis document transmitted to client to either advise client to:
- Retain an industrial hygienist to apply applicable ACGIH Standards (ACGIH, 1994).
- Employ appropriate containment/direct exhaust methods to insure a safe environment for affected building occupants or address the problem directly if desired.
In achieving acceptable IAQ with any of the VAV air distribution systems, Figure D.1 is useful in characterizing the particulate and/or gaseous filtration utilized either in positions identified as “A” and “B” allowing one to index their effect on Table D-1 space contaminant concentration state points.
Example: Value Added IAQ Simulation MethodologyTo demonstrate use of Table 1, let us assume the following representative design problem.
Assume that a new (Grot, R.A. et al.1991) or recently remodeled office building (Meckler, M., 1993) will contain two adjacent 1,000-sq-ft conference rooms, located along a common exterior wall, and served from a constant outdoor air VAV system, equipped with a medium efficiency particulate and gaseous filtration system generally located as defined by “B” in Figure D.1.
Accordingly, the maximum anticipated space contaminant concentration relationship can be found under the “required recirculation rate” heading under the “B/VAV/Constant” heading in Table D-1.
Referring next to Table 1, notice that using the same B/VAV/Constant heading the same corresponding time-varying space contaminant concentration heading is also listed.
In view of the extensive amount of wood flooring, wall paneling, and furniture planned, the MEP consultant retained by client, then proceeds to Table B.2 containing the Standard 62.1 contaminant concentration concerns for off-gassing formaldehyde (HCH0).
After being unable to locate readily some of the information listed under the corresponding “Reference” column, our MEP consultant decides to contact some local vendors of wood, flooring, paneling, and furniture to try and get a better handle on an order of magnitude value of an overall HCHO emission rate, for a newly completed conference room with extensive use of wood products and finishes.
This is required since the health effects listed under the corresponding “Comments” column of Table B.2 suggested potential problems from acute and chronic inhalation exposure.
Based on the information obtained from above-referenced local vendors, the MEP consultant is now able to compile an maximum probable value of N, the contaminant generation rate estimated to be 4.44 micrograms HCHO per cubic meter–minute at initial occupancy exposure.
Next, assuming our MEP consultant has the benefit of the Table 1 time-vary space contaminant concentration relationships, he can readily solve the Class VI HVAC relationship utilizing one of the many commercially available spreadsheet programs, e.g., Excel for each hour of a typical working day, after first compiling part-load cooling information on a time-of-day basis for each of the representative seasonal conditions likely to occur in his microclimate employing the extensive ASHRAE Handbook database.
Next, employing Table 6-1 under listed default values for conference room peak combined outdoor air rate, our MEP consultant would be able to compile the necessary remaining corresponding hourly values of variables - Fr, R,Ev, Vo, etc. - to solve directly for Cs(t) also listed in Table 1.
Our MEP consultant has now determined that he needs to specify a more efficient HCHO air-cleaning system (ASHRAE, 2004), rather than the 15% gaseous removal efficiency type he originally budgeted, to comply with the current one-hour reference exposure level (REL) for HCHO. This follows since 94 micrograms/cubic meter (or 76 parts per billion (ppb)) and which is below a HCHO contaminant exposure level of 33 micrograms/cubic meter (or 27 ppb) derived for an 8-hr exposure period (WHO, 1987).
Further Reducing Occupant Exposure To IAQ ContaminantsTogether with incorporation of the above-recommended additional IAQ protocols, exposure to potentially harmful contaminants can be further reduced, provided the installed VAV systems are equipped to both sense and react promptly to unanticipated excursions in contaminant concentration due to spills, initial off-gassing of furnishings and finishes, solvents introduced by nocturnal cleaning crews, and potential acts of outdoor or indoor IAQ terrorism.
For example, through use of commercially available photo-ionization detectors (PIDs) one can measure volatile organic chemicals and other toxic gases at airborne concentrations ranging from ppb to 10,000 parts per million (ppm), and by simultaneously solving for human and PID meter sensitivity, a logical program of atmospheric risk reduction based on a given PID response in both known and unknown chemical environments can be developed.
That relationship is:
PID sensitivity + Human sensitivity = Decision to Alarm (Equation 2)
However a more direct approach since PID sensitivity can be expressed in terms of a correction factor (CF) is:
CF + Exposure limit = Decision to Alarm (Equation 3)
CF’s provide the key to employing PIDs for assessing varying chemical contaminant mixtures and unknown environments (Application Note AP-221, 2000) since they can be used as a measure of PID sensitivity to a particular contaminant gas, e.g., HCHO.
CFs also permit calibration of one gas while directly reading the concentration of another, thereby eliminating the need for multiple calibration gasses. However, CFs are instrument- or manufacturer-specific, therefore, it is important to employ CFs provided by the manufacturer of the PID.
To make an assessment of toxicity risk using a PID, both human and PID sensitivities must be understood and then related. Human sensitivity is usually expressed in exposure limits defined by organizations such as OSHA, NIOSH, and ACGIH.
HCHO exposure limits are listed under above-referenced Table B-2 as 0.081 ppm. Corresponding to a HCHO contaminant concentration of approximately 100 micrograms/cubic meter), based on “irritation of sensible people to a 30 minute exposure ” or to 0.05 ppm (corresponding to a somewhat lower HCHO contaminant concentration of approximately 62 micrograms/cubic meter). This is based on avoidance of “irritation in allergic and asthmatic individuals …” and “as a value that is reasonable to achieve in light of HCHOs potential carcinogenicity.”
Another benefit of the above PID approach is that it can be used to record an alarm condition in lieu of setting off an audible alarm (to relieve building occupants of any health concerns) should transient contaminant excursions exceed safe exposure limits. This is accomplished by employing the above-referenced PID’s decision ability to immediately respond by employing a low-cost bypass particulate and gas removal filtration system (VAV/BPFS) capable of operating:
- In a outdoor air demand control mode, thereby significantly improving the response time of conventional VAV systems.
- To reduce excessive contaminant concentrations to pre-determined safe concentration levels automatically without traumatizing building occupants with distracting alarms from time to time (Meckler, M., 1993).
EPACT Energy Saving Incentives For IAQOne potential candidate for an EPAct incentive is the VAV/BPFS, which has been thoroughly tested by researchers at the ITTRI and proven capable of operating in an outdoor air demand control mode to significantly improve the response time of conventional VAV systems.
The VAV/BPFS concept was studied and shown to reduce excessive contaminant concentrations to pre-determined safe concentration levels automatically and without traumatizing building occupants with occasional distracting alarms (Meckler, M., 1993).
For example, the above-referenced VAV/BPFS system test results published by IITRI researchers (Moschendreas, J.D., et al., 1996) confirmed short- and long-term chamber “statistically significant” test energy savings respectively of 35% to 40% and 20% to 32% for corresponding energy loads. IITRI scientists also reported VAV/BPFS system chamber statistically significant test results regarding improved particulate and gaseous space contaminant reductions as follows:
- Average particulate matter (PM) removal rate was 50% higher.
- Average total volatile organic compound (TVOC) removal rate was 20% higher than for the corresponding conventional VAV system also assembled for chamber testing with identical TVOC and PM loading.
- IITRI researchers concluded “that the VAV/BPFS system is a promising alternative to the conventional VAV system because, under the conditions tested, it is capable of reducing and maintaining good indoor air quality and decreasing outdoor supply rate” to help a building owner qualify for a EPAct tax deduction.
The cost/benefit ratio results from only having to provide high-efficiency filtration/gaseous cleaning media to treat approximately 30% of total supply airflow only when necessary. This avoids the need to design the same capability to treat 100% of the supply air in the event such emergency, which obviously would require substantially higher initial first cost, more frequent filtration/gaseous media replacement costs, and higher fan-related operating costs over time.
Summary And DiscussionIn summary, HVAC system type conforming IAQ simulation models can be employed in combination with Standard 62 when dealing with unanticipated high concentrations of contaminants, (e.g., HCHO, CO) in new and remodeled buildings (Meckler, M., 1993). The above-referenced IAQ simulation models and protocols allow the HVAC consultant to independently verify whether the worst-case scenario VAV system part-load condition will meet satisfactory compliance of maximum space contaminant concentrations indicated by application only of the Table D-1 “state point relationships.” They can also be used to estimate the time of day at which maximum contaminant concentrations are most likely to occur so that the correct selection of particulate filtration and gaseous air cleaning systems can be made (ASHRAE, 2004).
Equations currently listed under heading Space Contaminant Concentration Table D-1 on page 17 of ANSI/ASHRAE Standard 62.1-2004 for various listed constant flow and VAV systems are incapable of providing an estimate of real-time or time varying airborne concentration levels. For certain type HVAC systems, e.g., variable flow supply VAV systems, this can expose building occupants to unsafe time-related exposure or dosage of unhealthful airborne contaminants which would otherwise remain undetected until symptoms could subsequently be traced to work-related indoor environmental conditions, at potential non-reversible health and treatment cost to occupants and consequential financial exposure to their employers and HVAC system designers, as well (Diamond, M. 2001).
The proposed time-varying IAQ simulation modeling can ensure compliance with allowable contaminant concentrations at all times, while emphasizing the important role that air cleaning and filtration play in attaining allowable contaminant concentrations and acceptable and cost-effective IAQ. This approach can also provide a useful means to evaluate HVAC system operations, especially for VAV systems at part-load conditions prior to commencing construction.
The ability to synchronize time-varying contaminant concentration levels in occupied building spaces with available time-varying space thermal heating and cooling loads in those same spaces, along with affected VAV and air circulation quantities, is not presently taught under the latest edition of Standard 62.1 available to our members and users. Hopefully that can be remedied soon.
For example, the above recommended methods can be readily incorporated through issuing new Addenda for both Standard 62.1 and ASHRAE/ARI/NIST 62.1 Users Manual, along with an explanation/illustration of how they may be utilized employing some of the new IAQ protocols and VAV system cautions, without requiring another ANSI review.
In this way, the ASHRAE Standards Committee can readily achieve greater beneficial use by society members and other users of an otherwise excellent standard within the framework of ASHRAE’s Board of Directors-mandated continuous maintenance doctrine intended to allow Standard 62.1 to continue to improve its usefulness. The positive take on this is to provide additional insights and enhance guidance to our members and other users. After all, Standard 62.1 is intended to continue as a work in progress, particularly as new health and other important information, data, and needs are identified.
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