BACKGROUND

This paper is an updated version of a similar one published for Standard 62-2001.  Please study the complete documents discussed in this paper and review the bibliography.  This short paper should not be considered a substitute for the complete document, nor is it a shortcut to compliance with the standard.  There are many other issues to consider that are not discussed here.

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ANSI/ASHRAE Standard 62.1-2004 is an often misunderstood document outlining ventilation requirements intended to provide acceptable indoor air quality for new buildings or those with major renovations.  Because of the rate-based nature of both procedures allowed for compliance, this analysis focuses on the needs for reliable intake rate control and the risks of some popular indirect controls.  Design recommendations offered are intended to increase the potential for both predictable compliance and for the flexibility to accommodate future changes, while providing the greatest control reliability with the most energy efficient methods.

INTRODUCTION

This is a brief summary of ASHRAE Standard 62.1-2004, Ventilation for Acceptable Indoor Air Quality (IAQ) in Commercial, Institutional, Industrial and High Rise Residential Buildings [1], as it impacts and is influenced by ventilation control requirements, methods and equipment.  Operational implementation of these requirements can have a sizeable influence on energy usage, when applied improperly or incompletely.  Operational precision and design reliability are essential to minimize energy usage, when compliance with 62.1 and energy codes are simultaneous goals. 

This ANSI-approved standard has been developed by a Standing Standards Project Committee (SSPC) of the American Society of Heating, Refrigeration and Air-Conditioning Engineers, Inc. (ASHRAE), under a ‘continuous maintenance’ protocol.  At any point in time, the ’official’ Standard is comprised of both the most recently published parent document and all current addenda.  The latest parent document was republished earlier this year to combine 17 addenda that had been approved subsequent to the original release of the 62-2001 parent document in January 2002.  The result is a final version that is substantially different from the basic ventilation standard we have used since 1989.

In 2003, the scope of the Standard officially changed and a separate ASHRAE committee was formed to address the specific needs of Low-Rise Residential Buildings.  The existing Standard became known as 62.1 and the new residential standard became 62.2.

The long promised 62.1 User’s Manual was recently published and is now available at www.ASHRAE.org.  Work continues on Guideline 19P, which is intended to provide design guidance for methods of compliance that exceed the minimum requirements of the Standard.  Both of these supplemental documents should assist the designer and facility operator in the understanding of and compliance with the Standard.

ANALYSIS AND RECOMMENDATIONS

Our discussion of Standard 62.1 will mimic most of its structure, provide recommendations for compliance and highlight methods and assumptions to avoid.  Our objectives have determined the content. 

The Standard’s “Purpose” and “Scope” are covered in Sections 1 and 2.  To comply with the Standard, designers of mechanical ventilation systems are tasked to provide specific minimum rates of acceptable outdoor air to the breathing level of the occupied structures.  In doing so, an acceptable indoor environment may be achieved providing improved occupant comfort, productivity and health.  The procedures allowed for compliance with our national standard on ventilation are prescriptive or performance-based.  Your selection should be evaluated for IAQ risk by the design practitioner. 

DEFINITIONS

Section 3 addresses the definition of terms used within the Standard.  Noteworthy is the Standard’s definition of “acceptable indoor air quality” which is provided as

“air in which there are no known contaminants at harmful concentrations as determined by cognizant authorities and with which a substantial majority (80% or more) of the people exposed do not express dissatisfaction [1].”

This means that 62.1, like all ASHRAE Standards, assumes one out of five occupants (20%) may not be satisfied with the results of compliance and may express dissatisfaction with the indoor air quality, even if the Standard is followed perfectly.  Many sources have concluded that the majority of HVAC systems designed in the U.S. do not meet the minimum ventilation rates prescribed during operation.  In which case, the actual occupant dissatisfaction level is exponentially greater in practice [2].  It is not uncommon for rates to fall below levels that result in occupant dissatisfaction significantly greater than 50%.  Many systems cannot meet the minimum airflow requirements at the occupied space during operation because of design choices and equipment limitations, or due to the dynamic nature of mechanical ventilation systems and the constant external forces acting on the building envelope. 

The impacts from these continuously changing external conditions are not limited to Variable Air Volume (VAV) systems [2].  Outdoor airflow rates will also vary for systems that provide a Constant Volume of supply air (CAV) to the conditioned space, as a result of:

a) changes in wind and/or stack conditions on the intake system [3],
b) changes in filter loading,
c) changes in airflow requirements during an economizer cycle.

The lack of specific guidelines to overcome the effect of changing system dynamics on ventilation rates and air distribution for today’s HVAC systems is partially to blame for many design deficiencies observed.

Unlike thermal comfort, the effect of indoor air quality is difficult to measure.¹  Many believe that the outdoor air levels specified by ASHRAE are too low and should actually be increased, as indicated by published research and reflected in European standards from CEN Technical Committee 156 and their publication CR1752 [4].

[an error occurred while processing this directive] OUTDOOR AIR QUALITY

Section 4 of the Standard describes a three-step process to evaluate outdoor air for acceptability.  One of those steps requires examination of both the regional and local air quality by the building owner.  The section also specifies the documentation required to support the conclusions of this preliminary review.

If the outdoor air quality is found to be unsuitable per Section 4, then treatment may be required as indicated in §6.2.1.  Outdoor air treatment involves removal of the particulates and/or gases encountered that are in excess of the minimum standards cited by cognizant authorities in §4.1.  SYSTEMS AND EQUIPMENT

¹ “The work item that caused most controversy was an attempt to standardise [sic] design criteria for the indoor environment. The criteria developed in the process have been published as a CEN Technical Report CR 1752.  It specifies the levels of temperature, air velocity, noise, and ventilation for occupied spaces. Values are given for three categories of environmental quality: A - a high level of expectation, B - a medium level and C - a moderate level. Supporting information is given on the derivations of the specified values of the parameters as well as to enable alternatives, such as different clothing levels, to be accommodated in the design assumptions.  The most debatable section is on indoor air quality. Here, prominence is given to the evaluation of the required ventilation rate for comfort based on perceived air quality, the method developed by Professor Fanger and his colleagues in Denmark. While some data is presented, it is acknowledged that more research is needed to provide reliable information on pollution loads from materials and on the additive effects of emissions from multiple sources."  Source: http://www.aivc.org/frameset/frameset.html?../Air/20_3/jackman.html~mainFrame accessed June 2005.

Section 5 specifies the minimum systems and equipment required under Standard 62.1.  §5.4 states,

“Mechanical ventilation systems shall include controls, manual or automatic, that enable the fan system to operate whenever the spaces served are occupied.  The system shall be designed to maintain the minimum outdoor airflow as required by Section 6 under any load condition. Note: VAV systems with fixed outdoor air damper positions must comply with this requirement at minimum supply airflow [1].”  

Because the requirements in the Standard for compliance are set forth “under any load condition,” we are being asked to maintain a constant rate of outdoor airflow in dynamic systems.  Logically, there should also be a requirement for continuous airflow measurement at the intake of all air-handling units with automatic controls providing a space with a constant rate of outdoor air, regardless of the system size, type or point of operation.  Doing so would alleviate several issues, clarifying application and compliance questions in §6.2.7 Dynamic Reset; §7.2.2 Air Balancing; and §8.4.1.8 Outdoor Airflow Verification.  A continuous outdoor air measurement requirement was included in the draft Standard 62-89R before becoming politicized, much more complicated and vague to the point of confusion.  

The Standard encourages us to use direct measurement feedback for continuous control on all VAV designs, even those using a powered outdoor air system (i.e. injection fan, HRV/ERV, smaller DOAS, etc.).  Although not contained in the Society’s ‘minimum’ standard, ASHRAE is highlighting the potential source of problems and a few of the more obvious means to avoid them.

We believe and recommend that Section 5 of the Standard encourages the use of airflow measuring devices at the intake of all systems and in the supply air to critical zones of VAV systems.  This allows for not only improved operating savings, continuous verification of compliance and use as a diagnostic tool, but also may be used to reset intake rates based on changes in occupancy.  Polularly known as “Demand Controlled Ventilation,” the technology is available to count or estimate zone populations in real time and are surprisingly cost effective.  When compared to the potential energy savings without negatively impacting occupant productivity and health, there is no reason to avoid it.

PRESSURIZATION & MOLD

Problems identified immediately after publication of Addendum 62x (62-2001) generated Addendum 62ai (renamed 62.1a for the 2004 version and quoted below).  

The addendum only addresses positive pressure during periods of dehumidification.  However, it clarifies several issues, including exceptions to the 65% RH requirement and for labs and industrial spaces. 

“5.10 Dehumidification Systems.  Mechanical air-conditioning systems with dehumidification capability shall be designed to comply with the following:

5.10.1 Relative Humidity. Occupied zone relative humidity shall be designed to be limited to 65% or less at either of the two following design conditions: 1) at the peak outdoor dew point design conditions and at the peak indoor design latent load, or 2) at the lowest space sensible heat ratio expected to occur and the concurrent (simultaneous) outdoor condition when system performance is analyzed with outdoor air at the dehumidification design condition (that is, design dew point and mean coincident dry bulb temperature), and with the space interior loads (both sensible and latent) at cooling design values and space solar loads at zero.

Note:  System configuration and/or climatic conditions may adequately limit space relative humidity at these conditions without additional humidity-control devices. The specified conditions challenge the system dehumidification performance with high outdoor latent-load and low space sensible heat ratio. (cont’d)

Exception: Spaces where process or occupancy requirements dictate higher humidity conditions, such as kitchens, hot tub rooms that contain heated standing water, refrigerated or frozen storage rooms and ice rinks, and/or spaces designed and constructed to manage moisture, such as shower rooms, pools and spas.

5.10.2 Exfiltration. For a building, the design minimum outdoor air intake shall be greater than the design maximum exhaust airflow when the mechanical air conditioning systems are dehumidifying.

Exception: Where excess exhaust is required by process considerations and approved by the authority having jurisdiction, such as in certain industrial facilities.  Note: Although individual zones within a building may be neutral or negative with respect to outdoors or to other zones, net positive mechanical intake airflow for the building as a whole reduces infiltration of untreated outdoor air.”

Noting the proliferation of mold in buildings, the ASHRAE Board stated that sound moisture management should take precedence over energy cost savings when it issued Minimizing Indoor Mold Through Management of Moisture in Building Systems in June 2005 [5].  This Position Paper outlines recommendations by describing issues related to the topic and highlighting resources available through the Society.  This policy will eventually trickle-down through the society’s organization and eventually impact technical programs, research and standards.

Included in their recommendations for proper moisture management are:

• Building and system design, operation and maintenance provide for drying of surfaces and materials prone to moisture accumulation under normal operating conditions.

• Mechanical system design should properly address ventilation air.

• The sequence of operation for the HVAC system should contain appropriate provisions to manage humidity, control pressurization and monitor critical conditions [5].

[an error occurred while processing this directive] The flaw of ASHRAE’s position is in not recognizing the potential for high humidity alone to provide sufficient moisture content for mold growth.  Studies have shown that a temperature between 30^oF – 86^oF [-17^oC - 30^oC] and humidity of only 70% RH (non-condensing infiltration), mold growth has appeared on plasterboard, brick and concrete within 3 days.  At 65.3^oF [18.5^oC] (with adequate RH and “inadequate” substrate) mold grows on building materials after 6 hours.  It was shown to take only 1 hour with “adequate” substrate [2, 6].

Part of the solution to preventing infiltration of unfiltered and unconditioned humid air appears to be simple.  In 1996 the Florida Solar Energy Center first published a case study which identified that an extremely small negative pressure differential created conditions that lead to mold problems in a small commercial building.  This was later supported by a 2002 Journal article whose recommendations indicated that differential pressures as low as +0.004 to +0.008 in. WG [1 to 2 Pa] will prevent moisture infiltration problems [7].  This counter-flow overcomes most of the natural pressures that power moisture migration, namely: vapor, temperature (stack), and wind pressures.  Those periods when pressurization flow is insufficient to counter infiltration are generally limited in duration.  Thereafter, the flow of air to the direction of higher dew point temperature can remove any residual moisture in the wall cavity. 

Control precision and stability must be key objectives when energy usage is to be minimized and dynamic control of space pressurization is used.

The Standard should address wind and stack effect, and provide guidelines that reflect conditions that influence buildings in their normal environment.  In addition, increased humidity combined with wind and stack driven infiltration during periods when the ventilation system is not operating, may be a significant factor influencing mold and fungal growth, e.g. offices and schools during closures.  Designers and building operators should consider a limited night setback mode with provision for humidity and pressurization flow controls.  Such a provision would also tend to compensate for the building-generated contaminants by supplying a base ventilation rate, sufficient for minimal pressurization flow.

PROCEDURES

Section 6, Procedures, is the heart of the Standard.  For compliance, designers must claim using one or the other, not both.  You may choose between the Ventilation Rate Procedure (VRP) and the Indoor Air Quality Procedure (IAQP) to determine the minimum dilution ventilation rate required for design.  Designers and operators cannot selectively ignore the parts they do not like.  One entire procedure must apply.  Parts from each procedure cannot be combined to achieve ventilation rates lower than those determined by the VRP alone.  Great care should be given to the selection between these procedures.

The VRP as defined in 6.1.1: “…is a prescriptive procedure in which outdoor air intake rates are determined based on space type/application, occupancy level and floor area [1].”  The key phrase is “prescriptive procedure in which outdoor air intake rates are determined.”  Very simply, this implies the need for some form of airflow measurement. 

The alternative IAQP in 6.1.2: “…is a design procedure in which outdoor air intake rates and other system design parameters are based on an analysis of contaminant sources, contaminant concentration targets and perceived acceptability targets [1].”  Any analysis of this procedure quickly reveals the clear discussion of airflow rate requirements based on varying contaminant levels.

VENTILATION RATE PROCEDURE

The VRP detailed in §6.2 is rate based.  No question.  In fact, the entire Standard is rate-based, including the IAQP, which only provides the means to calculate allowable reductions in the design ventilation rate from those in Table 6-1 (see also Appendix D for alternate methods of calculation).  The explicit statement to this effect was removed last year in an effort to render the language in the Standard more ‘code enforceable.’ 

Designers claiming compliance with the VRP must be able to document and substantiate that minimum intake rates can be maintained during operation “under all load conditions,” at not less than the higher of either code-required levels or those indicated by table 6-1 and the calculations in §6.2.

Once the outdoor air is determined to be acceptable or has been treated for use indoors, we can begin to determine how much is needed under our specific design situation. 

First, we must calculate outdoor airflow requirements for the zone (V_oz), as detailed in §6.2.2.1 through §6.2.2.3 which can be summarized below with their corresponding equations and reference numbers:

• Calculate breathing-zone outdoor airflow -                         V_bz = R_pP_z + R_aA_z           (6-1)

• Determine zone air distribution effectiveness -                   E_z   = Table 6-2

• Calculate zone outdoor airflow at diffusers -                      V_oz = V_bz/E_z                   (6-2)

Then, determine the outdoor airflow requirements for the system (V_ot) and calculate minimum outdoor air intake flow.  We are given 3 general system types to choose from:

• Single-zone systems:                                                     V_ot = V_oz                                                (6-3)

• 100% OA systems:                                                        V_ot = å_{all zones} V_oz   (6-4)
  and

• Multiple-zone recirculating systems
  Outside Air Intake:                                                    V_ot = V_ou/E_v                                        (6-8)

In Multi-zone recirculating systems, the variables needed to solve for V_ot (V_ou and E_v), are determined in §6.2.5.1 – §6.2.5.4 summarized below, and will be examined in more detail later.

• Calculate the Zone primary outdoor air fraction:                 Z_p = V_oz/V_pz                    (6-5)

• Determine the Uncorrected outdoor air intake: V_ou = D å_{all zones} R_pP_z + å_{all zones} R_aA_z (6-6)
  and
  Accounting for Occupant Diversity:                            D = P_s/å_{all zones} P_z            (6-7)

One subtle change in the updated Standard includes the usage and definition of “breathing zone.”  In addition, rates will no be longer determined solely on occupancy “per person.”  Separate component rate requirements are included to address both occupant and building-generated contaminants.  

 “6.2.2.1 Breathing Zone Outdoor Airflow. The design outdoor airflow required in the breathing zone of the occupiable space or spaces in a zone, i.e., the breathing zone outdoor airflow (V_bz), shall be determined in accordance with Equation 6-1.

V_bz = R_pP_z + R_aA_z                                         (6-1)

where:

A_z = zone floor area: the net occupiable floor area of the zone m², (ft²).

P_z = zone population: the largest number of people expected to occupy the zone during typical usage. If the number of people expected to occupy the zone fluctuates,

P_z may be estimated based on averaging approaches described in Section 6.2.6.2.

Note: If P_z cannot be accurately predicted during design, it shall be an estimated value based on the zone floor area and the default occupant density listed in Table 6-1.

R_p = outdoor airflow rate required per person as determined from Table 6-1.

Note: These values are based on adapted occupants.

R_a = outdoor airflow rate required per unit area as determined from Table 6-1.

Note: Equation 6-1 is the means of accounting for people-related sources and area-related sources for determining the outdoor air required at the breathing zone.  The use of Equation 6-1 in the context of this standard does not necessarily imply that simple addition of sources can be applied to any other aspect of indoor air quality [1].”

Total intake rates at the air handler can be directly determined with hand-held instruments used in accordance with prescribed standards, or by using an appropriate and permanently installed airflow measuring device.  Total intake rates may be indirectly estimated by several other means (i.e. Supply/Return differential calculation, temperature balance, mass balance, steady-state CO₂ concentration, etc.).  However, the uncertainty of indirect techniques introduces a significant level of risk [2,7].  The designer, facility owner and occupants should carefully consider the method employed prior to implementing any CO₂-based Demand Controlled Ventilation scheme as the sole method of intake rate control.

The new VRP in §6.2 recognizes the magnitude of building generated pollutants, therefore added the “building component” in the zone ventilation equation.  Table 6-1 and the accompanying notes specify outdoor air requirements for specific applications.  Equation 6-1 (§6.2.2.1 above) is now based on the combination of ventilation rates per person (as CFM/p) PLUS ventilation rates per floor area (as CFM/ft²).  Therefore, systems that meet these requirements: