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Requirements for compliance with ANSI/ASHRAE Standard 62-2001.
This article replaces "Providing 'Ventilation for Acceptable Indoor Air Quality'"
What are some of the practical implications of compliance with ASHRAE Standard 62-1999?" published in September 2000.
After reviewing the Position Paper on IAQ, issued by ASHRAE June 28, 2001, no one should be surprised by the direction that Standard 62 is taking. ASHRAE's "position" reads in part:
2.2 HVAC Systems While ventilation is not the only determinant of IAQ, perceived air quality and health outcomes generally improve as ventilation rates increase (Seppanen et al. 1999). In current practice, minimum ventilation rates are recommended by ASHRAE Standard 62. Interpreting more recent research, there are health, productivity and perception benefits from increasing ventilation rates above the current ASHRAE values (Apte et al. 2000; Milton et al. 2000; Sundell et al. 1994; Wargocki et al. 2000) .
HVAC systems also affect space pressure differences that control the inter-zonal transport of pollutants, and this may prevent or promote water vapor condensation in walls .
Even if you have never read the Standard before, one can conclude from the Position Paper that any ventilation standard developed by ASHRAE would primarily:
be a rate-based standard,
be very dependent upon the consistent maintenance of intake rates,
be a true "minimum" standard (considered insufficient in some conditions),
use building pressurization as a key component in the prevention of mold growth; and
use space pressurization to isolate contaminants from areas of occupancy.
Surprise! That is exactly what the new standard provides.
The "new" Standard 62
The updated Standard 62 - 2001 was published as an ASHRAE-approved document at the Winter Meeting. This version became effective January 24, 2002. It takes all of the 1999 Standard and has incorporated the following addenda into the "parent" document:
j - replaces the
performance requirement for natural ventilation systems with a prescriptive
l - adds a new section to the standard on construction and ventilation system startup
m - creates a new section on operation and maintenance procedures
p - clarifies previous requirements for providing combustion air
q - modifies several definitions for clarity and deletes others that are not used
s - clarifies and updates requirements for equipment related particle filtration
w - defines performance criteria for air stream surface materials in ventilation system equipment and ducts
It is significant to note in this version that ASHRAE has formally confirmed the importance of system operation to the sustainability of acceptable levels of IAQ. Start up and preventative maintenance requirements are now specified and included in the Standard (Addenda "l" and "m" created new Sections 7 and 8).
Several additional addenda were approved by ASHRAE for publication at that meeting, thereby automatically become a part of the current ASHRAE standard, and have just been announced (6/12/02) as part of the ANSI national standard. They are:
i - describes
situations for using IAQ procedure
t - condensate management
u - control of ventilation systems
v - balancing, air distribution capabilities and efficiency requirements
ab - addresses equipment that generates contaminants
ASHRAE News Release 1/24/02 and Announcement of 6/12/02
These addenda are now officially part of the ANSI/ASHRAE Standard but will most likely not be incorporated into the consolidated "parent" publication until the winter of 2003/04. This does not mean that we should ignore it until then. Any "standard of care" test for design will refer to the official Standards that represent the state-of-the-art in HVAC design knowledge. That body of knowledge and any codes that refer directly to ASHRAE Standard 62, include these addenda.
Copies of finalized addenda are available to the public for downloading in PDF format from the Standards and Codes section of the ASHRAE web site. [http:/www.ashrae.org/STANDARDS/62-2001_add_menu.htm]
Ventilation System Design, Control, Operation and Performance Verification
The actual requirements of ANSI/ASHRAE Standard 62 can only be determined by reading the entire document, including the current addenda. To do otherwise, ignores the cumulative impact of all sections of the Standard that would apply equally to any design. Reading an individual section, paragraph or requirement might lead one to believe that there is a broad range of potential options available for compliance, but with an understanding of other sections one the range of alternative solutions usually narrows.
This paper will examine those parts of the standard that have the greatest influence on design issues, especially those that affect dilution ventilation control. Why? Because if Standard 62 is anything, it is an intake rate-based standard. Even when it allows reduction of intake rates through the treatment and mitigation of specific contaminants, the Standard still refers the reader to the tables in Section 6, or formulae whose solution is measured in terms of CFM of dilution air and supply air.
Of the addenda just approved through ANSI, the most significant one relative to the design and control of ventilating systems is Addendum "u". It replaces Section 5.3 on Ventilation System Control.
The Foreword to Addendum "u" states:
" ... It [this section] specifically addresses VAV system controls for outdoor air intake airflow. The intake control requirements recognize that at low supply volumes sufficient outdoor airflow may not be maintained if a fixed outdoor air intake damper position is used. In many cases, an active outdoor air control system must be provided to ensure minimum intake rates are maintained."
But wait. It then reveals the replacement for Section 5.3 to require that:
"The system shall be designed to maintain the minimum outdoor airflow as required by section 6 [procedures for compliance] under any load condition. Note: VAV systems with fixed outdoor air damper positions must comply with this requirement at minimum supply airflow."
Aside from providing for a very inefficient "worst case" solution, I am unable to suggest how a reasonable engineer can comply with the Standard in designing a new or renovated system without direct inputs for the variable supply and intake rates to be controlled.
Prior to the ANSI review process, the 1/24/02 ASHRAE approved addendum read: The system shall be designed to maintain the hourly-average supply airflow and the hourly-average minimum outdoor airflow as required by section 6 under any load condition. Note: VAV systems with fixed outdoor air damper positions may not meet this requirement.
In either version, the phrase that was retained, "Under any load condition", implies that building systems are subject to multi-state conditions and are dynamic. Pressure and flow conditions upstream and downstream of a VAV fan system change during normal operation. Mixed air plenum pressures are therefore variable and respond dynamically to these influences. It has been shown numerous times in published research that fixed damper methods of control will not satisfy the minimum intake rates of the Standard or the needs of the occupants. It has also been shown that fixed dampers, and many other traditional methods of intake control on VAV systems, are also not effective (ASHRAE RP-980, et. al.)
Both the "static" test results (above-left) and the predicted "theoretical" errors (above-right), show only the impact from supply fan turndown. Neither factors the effects from the system or from internal or external environment, which we have known for years can have dramatic impacts on the ability of an air system to intake air.
The graph below models an intake in a VAV system with a 400 FPM minimum intake design velocity at the damper, set up in the winter with a 15 MPH direct wind and operated in the summer with a 15 MPH cross wind. All reasonably mild conditions, but the effects on the intake are dramatic. If extended, the intake would go negative at just less than 50% of Supply design flow. How can the SSPC62.1 committee ignore these conclusions which have been echoed by other researches during the last 12 years?
A packaged system using fixed dampers as the method of intake control cannot reasonably be set-up to provide the minimum rates required, anticipating the worst combinations of environmental conditions and loads. Attempting to set up for the worst-condition [minimum supply air flow] would effectively negate much of the energy savings advantages that VAV is expected to provide building operators.
Maybe we can find a voice of reason in another recently approved addendum. One that may help to give us a more focused picture of the real requirements of this Standard is Addendum "v". The Foreword to Addendum "v" includes the following:
..note that balancing is not a one-time event but must be reevaluated over time as indicated in other sections of the standard.
.and, it then supplies a new section 5.2.1 Designing for Air Balancing.
"The ventilation air distribution system shall be provided with means to adjust the system to achieve at least the minimum ventilation airflow as required by Section 6 under any load condition."
"Air Balancing" is the only method of adjustment specifically endorsed by the Standard, but not the only acceptable one. Permanently mounted instrumentation appears to be a more subtle and infinitely more economical solution in any life cycle cost analysis. The mere fact that 5.2.1 appears to require "balancing" as load conditions change, seems to beg the question - How is that to be accomplished? The frequency needed for this type of activity does not correspond with the economic and practical limitations of system "balancing" under dynamic conditions. This is a continuous system control issue and not a periodic "set & forget" activity.
For the first time ASHRAE 62 addresses the humidity component to IAQ, as well as building pressurization. Addendum "x" provided us with new and more direct requirements in Section 5.10.
5.10 Dehumidification Systems. Mechanical systems with dehumidification capability shall comply with the following:
5.10.1 Relative Humidity. Such systems shall be designed to limit occupied space relative humidity to 65% or less at peak outdoor dew point design conditions.
5.10.2 Building Pressurization. Such systems shall be designed to maintain the building at net positive pressure with respect to outdoors, in the absence of wind and stack effect, during all hours of dehumidification.
A lack of building pressure control can foster infiltration and condensation within the building's exterior walls when the dew point of the outside air is greater than that of the building envelope. Although proper building pressurization may not overcome vapor pressure gradients, a negatively pressurized building will contribute to condensation and should be avoided, especially during periods when the dew point of the outside air is high. Moisture from condensation is the prerequisite for mold and fungal growth.
Unfortunately, wind and stack pressures can influence infiltration significantly and it is unclear why the Standard only requires net positive pressure "in the absence of wind and stack effect". Also, high dew points can be troublesome at night or in schools during the summer, when many systems are not in operation and hence no mechanical pressurization is available. Again, one must question the statement "during periods of dehumidification" which may not be active when the building in unoccupied. In many cases, mold and fungal growth from condensation is the direct result of shutting down of ventilating systems.
Improved sensor technology has made available products that allow control to very low differential pressures (< 0.001 in. WG) by measuring a bleed airflow between spaces or across a fixed orifice. They can be used to compensate for wind and stack pressure variations. Low-pressure measurement could also allow for limited night set-back operation with ultra-low outside air intake for pressurization (low set point pressure = low outside airflow for pressurization).
Section 22.214.171.124 Multiple Spaces
This section applies to all systems that serve more than one space. Simply summing the outside airflow requirements of each space does not result in Standard compliance. A VAV design is a multi-state system. As a VAV box changes in response to thermal load, the amount of air to the space varies. If the outside airflow rate is not varied at the AHU, the space may become either over or under ventilated with dilution air for IAQ.
The Standard provides a formula to determine the most "critical zone" of the total being served by a single air handler, which in turn governs the set point for minimum outside air required at the intake. Compliance with the Standard for these situations requires applying the formula in all multi-zone situations (Equation 6-1).
The "Multi-Space Equation" quantifies the zone dilution air requirement based on the outside air portion of all zones at a given airflow rate to each, and rewards the system for unused recirculated outside air from over-ventilated areas. In order to have any chance of optimizing control in VAV systems these airflow rates should at least be continuously monitored. To have any chance of optimizing equipment costs, we must consider outside air intake set points to be dynamic in VAV systems and therefore must be continuously reset, as zone flow rates change.
On VAV systems, the Multi-Space Equation should be used (a) for dynamic reset of outside airflow rates or (b) to determine the "worst-case" outside air set-point for the AHU. Since outside airflow calculations from 6-1 can be extremely variable, dynamic reset is the best way to comply with the Standard during operation.
Compliance with 126.96.36.199 can be accomplished by using high accuracy airflow measuring stations in critical zones, for total supply and at the outside air intake of the AHU. The airflow measurement accuracy provided with VAV boxes is inadequate, mostly due to limitations in the differential pressure sensor on the host controls, and should not be used for this application. Not all VAV box controllers can accept electronic airflow measurement signals. Check with your controls provider for compatibility or use the high accuracy airflow measuring station for equation 6-1 reset only.
According to 188.8.131.52 Recirculation Criteria, airflow rates may not be reduced below the requirements specified in Table 2. This section directs the reader to the [Indoor] Air Quality Procedure, specifically the Air Cleaning, Section 6.2.3. Unfortunately, this is a contradiction of the Section 6 summary, which states: "[The Indoor Air Quality Procedure] does not prescribe ventilation rates or air treatment methods". Designs that use CO2 measurement as a method of reduction of outside airflow rates are not valid if compliance with the Ventilation Rate Procedure is claimed [IC62-2001- ]. This is not a trivial point, since is significant risk when claiming compliance using the Indoor Air Quality Procedure 6.2.
Completely NEW Sections 7 & 8
New sections have been added with requirements for the operation, maintenance and verification of the intake system's actual performance. CONSTRUCTION AND SYSTEM START-UP in Section 7.2.2 on Air Balancing requires that systems be balanced "at least to the extent necessary to verify conformance with the total outdoor air flow and space supply air flow requirements of this standard." A field verification requirement must be implementable and we can refer back to 5.2.1 Designing for Air Balancing.
"The ventilation air distribution system shall be provided with means to adjust the system to achieve at least the minimum ventilation airflow as required by Section 6 under any load condition."
Under OPERATIONS AND MAINTENANCE Section 184.108.40.206 requires the recalibration of any sensor used to dynamically control outdoor air, on a 6-month cycle, or as indicated in the manufacturers' O&M Manual. Permanently calibrated sensors will have a decidedly huge economic advantage over any other type of sensor. The maintenance labor required to recalibrate even a small quantity of sensors every 6 or 12 months throughout the economic life of the mechanical system far exceeds any initial cost inducement.
Section 220.127.116.11 Outdoor Air Verification includes specific provisions for periodic field verification of compliance under operating conditions. This is another issue with potentially high, on going maintenance cost implications. Permanently mounted air velocity meters, particularly those without periodic maintenance requirements and calibrated to standards traceable to some national or international reference, have a significant advantage over hand balancing. This type of instrument exceeds the applied performance of any hand-held device currently used to balance HVAC systems.
Section 18.104.22.168 specifies a 6-month verification cycle for dynamic control instrumentation, but in Section 22.214.171.124 an identical unit without dynamic control, verification of intake rates is required only "every 5 years" by one-time air balancing. Does this seem backwards to anyone else? The more reliable and repeatable measurements are usually those done automatically without human error or intervention. Is there some kind of a disconnect in this Standard? Has the project committee anticipated some unusual differences in the performance between these measurement methods and forgotten to tell us in the published Standard?
The latest Standard adds "effectiveness" and will soon include "efficiency" in the list of variables that must be considered. Addendum "n" is in it's 2nd Public Review and we believe will be adopted in the very near future. It will replace Section 6.1 Ventilation Rate Procedure, including all of Table 2. Therein, "efficiency" is defined by its use in the formula that is provided in the addendum. It is detailed in the proposed new Appendix H.
System Ventilation Efficiency (E):
Based on the maximum value of z for the zones served by the system, determine the system ventilation efficiency E from Table 6.3 or Appendix H.
Table 2 is being revised to allow a base ventilation rate, per structure and occupancy types, based on the total floor area. This is additive to the occupancy requirements and provides for dilution requirements of building generated contaminants. It is required without consideration of occupancy. This one change goes a long way to improve the standard and make the application of dilution ventilation more realistic and effective. What it also does is insure that the ventilation rates never fall below an identified minimum base rate. In the strictest interpretation, it would require that the system never be completely shut down, as this would allow contaminants generated by the building to accumulate. (There is no indication if a purge cycle prior to occupancy would be allowed or how it would equate.)
The Standard's Table 2 rates assume 100% effectiveness in distribution and system efficiency. Understanding that it is not normally possible to expect every system to reach this level of performance, the standard identifies types of systems and their impact on effectiveness through the use of other formulae (ANSI/ASHRAE Standard 62, Appendix E, p. 27). A Stratification Model is used to identify aspects of ventilation design or operation that degrade the ability of the specific system to deliver the required intake rate to the occupied zones. Therefore, the amounts of outside air that maybe actually required are being increased due to system factors. There is now a greater incentive for building operator to become as efficient as possible in air handling operations and optimize all systems to minimize the amount of outside air that is introduced. To date, this is accomplished by shutting the intake dampers, to the detriment of the occupants.
Overall, these methods and requirements indicate that significantly larger volumes of intake air should be provided. The wise engineer will attempt to provide an HVAC design that allows more precise and effective control of the outside air intake, so that the system treats only the amount of intake air that is needed as conditions change.
Why should we be concerned with these requirements? After all, many codes do not refer directly or indirectly to Standard 62. In the past, engineers and owners had more latitude to "interpret" the standard in ways that might be more advantageous with a particular design. Beware, if you think that there is still some "wiggle room" to define and interpret the Standard to suit your situation or method.
Section 5.3 (Addendum "u") is unambiguous about what is required for compliance and it applies equally to both procedures in Standard 62: the Ventilation Rate Procedure (6.1) and the IAQ Procedure (6.2). Therefore, regardless of the method chosen, we are still asked to ensure that minimum rates are provided to the occupied space and breathing level. Designers, equipment suppliers, component vendors and owners must prepare to deal with it.
It is important that we recognize the obligations that are imposed on new construction and major renovations. If not as a codified, minimum standard of design, it is the design standard-of-care. It is the benchmark minimum against which all designs will be measured when any system does not perform as expected.
It is also important to understand that all Federal and most state construction requirements include compliance with the current version of ASHRAE Standard 62. The GSA specifically cites the ASHRAE Standard in their Facilities Standards for the Public Building Service (#PBS-P100, Nov 2000), which sets design minimum standards for engineering professionals to follow in all GSA owned or leased facilities.
In Section 5.4 HVAC General Requirements, we find the following:
"..Compliance with the latest versions of ASHRAE Standard 90.1 and ASHRAE Standard 62 is required for the elements of the project (architectural, mechanical and electrical).
Facilities Standards for the Public Building Service (#PBS-P100, Nov 2000), pg. 126
Also, in Section 5.9 Air Distribution Systems (VAV) there are the following references to intake rates:
.the minimum [intake] volume setting should equal the larger of the following:
(1) 30% of the peak supply volume;
(2) 0.002 m3/s per m2 (0.4 cfm/ft2) of conditioned area; or
(3) Minimum m3/s (cfm) to satisfy ASHRAE Standard 62 ventilation requirements. Outside air requirements shall be maintained in accordance with the Multiple Spaces Method, equation 6-1 of ASHRAE Standard 62 at all supply airflow conditions.
Facilities Standards for the Public Building Service (#PBS-P100, Nov 2000), pg. 145
Finally in Section 5.16 Meters, Gauges and Flow Measuring Devices, the guideline states:
"Airflow measuring grids are required for all central air-handling units. Measuring grids shall be provided at the supply air duct, return air duct, and the outside air duct. Airflow measuring grids must be sized to give accurate readings at minimum flow."
Facilities Standards for the Public Building Service (#PBS-P100, Nov 2000), pg. 165
Why should we comply with the requirements of ANSI/ASHRAE Standard 62?
It is referenced directly by many local building codes and some model codes.
Compliance will reduce your IAQ liability.
Compliance will minimize energy costs (by optimizing the system).
Compliance will generally: improve occupant productivity, reduce healthcare costs and reduce sick days lost.
Compliance is required in all Federal and many State construction specifications.
Potential Methods of Compliance
With the range of imperatives and the unambiguous nature of this evolving standard, it appears that the consensus of our peers really want to insure that a constant rate of acceptable ventilation air is supplied to the occupied zones, under all operating conditions.
Most of the geographic areas of the country can condition the required volumes of intake air and function acceptably well without dedicated intake conditioning equipment. It can be shown that dedicated intake equipment is neither the most economical, nor the most dependable method for providing ventilation control for all environmental conditions.
Another consideration is that dedicated intake air handling equipment does not eliminate the need for direct measurement of intake rates. Whether using ERV's, PTAC's, fan coil units, unit ventilators or zoned AHU's; the primary determinant of intake flow rates is the mixed air plenum pressure, not supply fan speed. Mixed air plenum pressure is directly affected by internal system changes (system resistance and stack effects), and external environmental conditions (wind and temperature). Therefore, all air distribution systems are affected by these dynamic changes. Intake rate measurement would provide a systemic solution for continuous dynamic control and Standard 62 compliance.
Dedicated intake systems may be somewhat simpler to comprehend, but are not the most economical in most climates. It can be argued that the addition of multiple fan systems unnecessarily increases: energy usage, potential down time, service interruptions and maintenance costs. It can also be argued that it is easier (and less expensive) to keep a single large system running and available for >99% of the time, than it is to keep 98% of a large number of small fan coil units operating.
It seems that the simplest and most direct method to insure that the prescribed intake rates are provided under all conditions, is to dynamically maintain the flow rate with closed loop, direct feed-back control. This assumes that the latent loads are economically manageable with the same equipment used for space sensible load conditioning. It also means that permanent instrumentation on dedicated intake equipment may be necessary, if not only prudent in most situations.
In this latest version of Standard 62, ASHRAE has acknowledged and finally caught up with published research, some of which is more than 10 years old. It has been demonstrated by numerous published research studies and other published data, that all indirect methods of outside air control are deficient and incapable of providing sufficient reliability to be used for code compliance.
A negative cannot be proven, therefore it is impossible to show that current system designs are inherently deficient (without direct intake control), if they are not now being directly monitored. Other means of determining flow rates are time-specific and not as reliable as permanently mounted instrumentation.
When viewed as a whole, we get a better idea of what is really required for compliance with the Standard. We are not allowed to selectively designate which sections we want to use for compliance or which sections we choose to apply to a specific situation.
We can summarize the newest and most significant ventilation provisions of ASHRAE 62 - 2001 as follows:
Maintain the minimum outdoor airflow as required by section 6 under any load condition
Duct designs must allow for air "adjustments" and to verify conformance with the intake rate table.
Intake rates are based on zone occupancy (and soon will include a base rate for zone floor area in pending Addendum "n")
Multi-space equation must be used to determine outside airflow requirements, when a supply system serves more than one space.
If cleaned, recirculated air is used to reduce the outdoor airflow rate below the values shown in Table 2, the Air Quality Procedure, 6.2, must be used (Appendix E provides a formula to determine the amount that the rates may be decreased in specific situations).
Measures shall be employed to reduce the migration of construction-generated contaminants to occupied areas with barriers and zone pressure control.
Verify the accuracy of sensors for dynamic minimum outdoor air control every 6 months or per O& M manual.
The verification requirements of the Standard include repeated demands for air balancing, sensor recalibration, and potentially unnecessary filtration/intake treatment, needing continuous maintenance. Even if most commercial buildings can be constructed for compliance with Standard 62 without a significant change in the initial cost, doing so could place a significant cost burden on operations throughout the life of a building.
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