Update on the May 2003 article:
Ventilation Codes  ICC's International Mechanical Code vs. NFPA 5000, Analysis and Recommendations

Response to Letter to the Editor

INTRODUCTION

The International Code Council (ICC) was established in 1994 as a nonprofit organization dedicated to developing a single set of comprehensive and coordinated national model construction codes. The founders of the ICC are three previously independent model code bodies, who have just recently merged their efforts and organizations.  They include: Building Officials and Code Administrators International, Inc. (BOCA), International Conference of Building Officials (ICBO), and Southern Building Code Congress International, Inc. (SBCCI).

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The most noticeable absence from this group is IAPMO, the International Association of Plumbing and Mechanical Officials, which historically published the Uniform Building Codes (UBC).  The UBC and their Mechanical Code are mostly used in the western part of the U.S.  The jurisdictions still using the last update to the UBC will need to find another model code reference as the UBC will no longer be published.  In the alternative, the National Fire Protection Association (NFPA) has joined with IAPMO, ASHRAE and the Western Fire Chiefs Association and established an ANSI-accredited comprehensive “consensus”-based set of model codes (C3). Their new building code is called NFPA 5000 and combines with the National Electric Code, NFPA101 Life Safety Code, UPC, UMC, and NFPA 1. 

Because the ventilation requirements of NFPA 5000 depend largely on ASHRAE Standard 62 – Ventilation for Acceptable Indoor Air Quality, we will restrict the bulk of our discussion to the language in the current IMC2003. 

It is also significant to recognize the magnitude of influence that the IMC carries.  At last count, the IMC2000 or 2003 is effective statewide in 30 states.  Local governments in another 14 states have adopted it.  Lastly, it has been adopted but may not yet be effective in another 3 states.  A total of 47 states and territories have or will require compliance to the IMC somewhere within their jurisdictions.  Noticeably absent from the rolls are California, Hawaii, Minnesota, Montana and Puerto Rico.  Incoming Governor Schwarzenegger suspended California’s adoption of NFPA 5000 last year.  If this adoption is reversed, we estimate that 97.5% of the U.S. population (excluding PR) will be subject to the ICC’s Mechanical Code; otherwise only 85.3% of the country will utilize the protections included in the IMC model.

The International Code is on a 3-year development / review cycle, with the changes for the 2004 Agenda being approved at the ICC Final Action Hearings on May 17-20, 2004 in Overland Park, KS.  As I was present at those hearings and spoke against changes proposed to the ventilation section, our discussion will include only Chapter 4 of the IMC2003, which is the most current for building ventilation design and operational requirements.

IMC2003 ANALYSIS

SECTION 401 - GENERAL

The purpose of this section is defined in paragraph 401.3 below.

“401.3  When required.  Ventilation shall he provided during the periods that the room or space is occupied.”

Although the code references ASHRAE 62 and draws much of its content from the Ventilation Rate Procedure in ASHRAE Standard 62-1999/2001, there is no direct reference to a building pressurization requirement, either under occupied or unoccupied conditions, except when adjacent to Enclosed Parking Garages (Section 404.1 – 404.3).  This is a deficiency that has been initially addressed in addendum “x” to the current ASHRAE Standard 62 and a deficiency in the code that we hope to deal with in the near future.

SECTION 403 - MECHANICAL VENTILATION

[an error occurred while processing this directive] When mechanical ventilation is used, specific minimum rates of ventilation must be supplied, based on occupancy and structure type as defined in Table 403.3.  The section continues into 403.3.1 to specify that the ventilation requirements are allowed to be based on rates “per person” and restricted to the actual number of occupants present.  The original language inherently allowed ventilation reset for variable occupancy, or what is otherwise known as “Demand Controlled Ventilation” (DCV).  This was deemed insufficient, so a proposal was submitted to change the IMC last year and was approved “as Modified” by the ICC Mechanical Committee.  The changes/additions are underlined.

 Agenda Item M31-03/04

“403.3.1  System operation.  The minimum flow rate of outdoor air that the ventilation system must be capable of supplying during its operation shall be permitted to be based on the rate per person indicated in Table 403.3 and the design maximum occupancy or actual number of occupants present.

Exception: Where occupancy is variable or intermittent and the occupants are considered to be the primary source of pollutants in a space, the outdoor air provided by the ventilation system shall be permitted to be modulated such that it maintains the concentration of CO2 in the occupied space at a level that is not greater than 700 parts per million (ppm) higher than the CO2 concentration in the outdoor air.”

 The IMC modifications proposed above conflicted with requirements found elsewhere in the existing code and was not resolved by the current proposal.  A few of the reasons offered to disapprove the change are listed below, for example:

• 403.3 - CO₂ control has the capability of completely closing the intakes and may not “supply the required rate of ventilation air continuously during the period the building is occupied…”  Damper position alone does not determine flow rates.  Also, effective changes in space dilution significantly lag the sensing of changes in occupancy and CO₂ levels (by hours).

• Existing 403.3.1 – The proposal attempts to indirectly modify the foundation of the basic ventilation rate requirements by allowing rates to be based on unreliable and otherwise poorly substantiated estimates of occupancy, rather than the code required “actual number” or the design maximum.  And, there is no provision to limit the range of deviation from the control set point, or to specify any limitations on the use of the technology, as has been identified in existing research (NIST).

• Proposed 403.3.1 – A restriction on DCV to “assembly occupancies” is necessary and appropriate, but application limits are not defined in the section.  High occupant density, unpredictable, variable or intermittent occupancy are critical variables in any successful application of CO₂ -based DCV.  These factors are not considered in the ICC occupancy definitions.

• 403.3.2 – There is no indication how CO₂ -based DCV will satisfy multiple zones when supplied with ventilation air from a common source.  There is no indication if it can be used for multiple spaces or guidance on how to apply the Multi-space formula when DCV is used.  Whatever variant of CO₂ -based DCV that is eventually approved should be limited to single zone applications and dedicated systems only, until sufficient independent research is available to indicate if multiple zones can be served adequately by the control strategy, or the most appropriate method that would.

• 403.3.3 – In VAV systems, CO₂ -based DCV simply cannot “maintain the flow of outdoor air at a rate not less than that required by Section 403 over the entire range of supply air operating rates”.  This strategy of control has no direct relationship to intake rates or space pressurization requirements.  CO2 cannot tell us when intake flow is reduced below the minimum required.

• 403.3.3 – CO₂ -based DCV cannot provide ventilation for non-occupant generated contaminants.   CO₂ “only provides an estimate” and not a count of the actual occupants.  CO₂ “is just one of multiple contaminants of concern” as indicated in the IMC 2000 Commentary, page 96.  ASHRAE Standard 62 – 2005 will incorporate addendum “n”, which has already been ANSI approved and which has recognized the importance of addressing non-occupant generated contaminants.  This portion of the total contaminants of concern, even in buildings with the lowest contaminant-generating potential, can be a very significant and total about 50%.

• 403.3.4 and 405.1– Even if a design is deemed “capable”, without direct and dynamic control over intake rates, CO₂ -based DCV cannot provide the assurance that the minimum rates are “maintained” at “not less than” that required in Table 403.3. 

The current IMC language was sustained in the face of challenges proposed during the last proposal cycle.  The code language retained (until 2006) is as follows:

“403.3.1  System operation.  The minimum flow rate of outdoor air that the ventilation system must be capable of supplying during its operation shall be permitted to be based on the rate per person indicated in Table 403.3 and the actual number of occupants present.”

[an error occurred while processing this directive] Regardless of the “interpretations” presented by proponents of CO₂–based demand controlled ventilation (DCV) with CO₂, using the very generalized allowances for acceptance of modifications for individual cases in Section 105, and for the qualified statements made in the 2000 IMC Commentary for Section 403.3.1; this determination currently disallows the traditional and unlimited use of CO₂–based DCV strategies that have been proven to be problematic and which only “approximate” occupancy.  The mass balance/steady-state method, by definition, can only indirectly calculate ventilation rates– not verify them.  After estimating the number of occupants in the space based on a series of prerequisite and fragile assumptions, the mathematical relationship defined by the formula may exist. 

However, interior CO₂ levels have no direct relationship to the minimum intake rates required in table 403.3 or the actual rates introduced at the air handler. 

When distilled, this chapter of the IMC requires that the minimum rates specified in the tables must be provided “continuously” during occupation and controlled “to automatically maintain the required outdoor air supply rate”, and in VAV systems, “over the entire range of supply air operating rates.”  Not to exclude CAV designs, these requirements can only be satisfied with a dynamic and reliable intake measurement system used to directly control the airflow rates as changes internal and external to the system occur.  These are abilities that CO₂ sensing when used for control, and the misuse of the “steady-state” mathematical analysis, cannot provide. 

For the reasons given above and numerous others presented to the membership of the council at the 2004 Final Action Hearings last month, the committee-approved changes to the language of section 403.3.1, proposed last year to include CO₂ -based DCV, was overwhelmingly defeated.

To overcome the problems associated with indirect ventilation rate control using CO₂ alone, and in recognition of the need to provide design professionals with ways to allow for the diminished requirements for variable or intermittently occupied spaces; we intend to propose our own solution to this question, as well as for space pressurization and dynamic intake control for VAV systems.

The following equation should be familiar to everyone with any exposure to ASHRAE Standard 62.  The next section provides for the use of ASHRAE’s “multi-space” formula when a single supply system serves more than one zone.

“403.3.2  Common ventilation system.  Where spaces having different ventilation rate requirements are served by a common ventilation system, the ratio of outdoor air to total supply air for the system shall he determined based on the space having the largest outdoor air requirement or shall be determined in accordance with the following formula:

(Equation 4-1)    Y = X / (1 + X - Z) where:

Y =  V_ot / V_st   =  Corrected fraction of outdoor air in system supply.

X =  V_on / V_st   = Uncorrected fraction of outdoor air in system supply.

Z = V_oc / V_sc    = Fraction of outdoor air in critical space.

The critical space is that space with the greatest required fraction of outdoor air in the supply to this space.

V_ot       =          Corrected total outdoor airflow rate.

V_st V_on =          Sum of outdoor airflow rates for all branches on system

V_oc       =          Outdoor airflow rate required in critical spaces.

V_sc       =          Supply flow rate in critical space.”

The Multi-space Equation is a simplistic way of providing consideration for the needs of the occupants in the “critical” zone, while giving some credit for those spaces that will be over-ventilated by keying the intake rate for the building to the needs of a previously identified “critical” zone.  The latest addendum to ASHRAE Standard 62 – 2001 includes a new method of determining the intake rates for air handlers when supplying multiple zones.  It is located in Section 6.1, the Ventilation Rate Procedure.  ANSI-approved Addendum “n” includes consideration of several new factors that help make the calculation more effective, even if more complicated for the practitioner. 

The ICC has not yet considered these changes, but should in the next review cycle now that the most significant change to the document is ANSI approved.  The entire currently approved Standard is scheduled to be reprinted with all applicable addenda in January 2005.  The IMC and the 2000 Supplement recognize the need for dilution ventilation for building-generated contaminants, and the validity of requiring a non-occupant based control determinant, as has ASHRAE.  CO₂ -based DCV cannot readily provide ventilation control for dilution of non-occupant generated contaminants, which as we have indicated, can amount to at least 50% of the contaminant load.

Other extremely significant sections of Chapter 4 in the IMC include the following paragraphs.

“403.3.3.1  Variable air volume system control.  Variable air volume [VAV] air distribution systems, ……. shall be provided with controls to regulate the flow of outdoor air. Such control system shall be designed to maintain the flow of outdoor air at a rate of not less than that required by Section 403 over the entire range of supply air operating rates.”

“403.3.4  Balancing. Ventilation systems shall be balanced by an approved method.  Such balancing shall verify that the ventilation system is capable of supplying the airflow rates required by Section 403.”

[an error occurred while processing this directive] We can easily demonstrate with published research, together with modeling and full-scale testing, that intake rates on VAV systems will vary dramatically as supply rates change, and as internal system or external environmental forces change over time.  Fixed intake dampers, one-time air balancing and all indirect methods for calculation of intake flow rates have been shown to be extremely deficient for control purposes.  These methods persist, as there is usually no direct measurement available for comparison and to confirm conditions in real-time that can only be “guessed” without permanently installed instrumentation.

To rely on a one-time manual system set-up and balance requires the contractor (or user) to select the worst-case condition for operation and thereby increase the outside air delivered to normally excessive and unnecessary rates.  When reading these IMC sections, particularly the one on VAV systems and performance verification, it is difficult to believe that any means other than direct measurement of outside air intakes will satisfy this requirement.  This is especially true if the objective includes optimizing the energy used and minimizing operating costs.

The use of CO₂ -based DCV with VAV air distribution designs can easily lead to pressurization problems and mold growth in exterior walls.  It can both under ventilate, actually closing intake dampers in some circumstances, and over ventilate in others.  To avoid these significant drawbacks, to allow for needed methods to address energy conservation in variable and intermittent occupancy situations and to provide the flexibility to adjust the system as necessary in the future during the evolution of this code and existing standards; we intend to propose that a modified DCV method be used.  One that limits the ability of the system to exceed the design maximum intake requirements and also to prevent the system from closing the intake or reducing rates below those required for pressurization and to dilute building-generated contaminants.

SECTION 405 - SYSTEMS CONTROL

“405.1  General.  Mechanical ventilation systems shall be provided with manual or automatic controls that will operate such systems whenever the spaces are occupied. 

Air-conditioning systems that supply required ventilation air shall be provided with controls designed to automatically maintain the required outdoor air supply rate during occupancy.”

To provide effective and efficient control of dynamically changing systems, it is not reasonable to expect static or indirect control methods to provide acceptable results.  When the requirements of this section mandate controls that will “automatically maintain the required…rate” of ventilation air, I can only interpret that to mean a dynamic control mechanism. 

IMC2003 CONCLUSIONS AND RECOMMENDATIONS

The word “maintain” is unambiguous.  It definitely cannot be interpreted to mean “get close to” or “average” to determine the required rate.  At the most, it could be interpreted to mean “not less than” that required.  In fact, VAV requirements (403.3.3) provide that the control systems will maintain the required ventilation at a rate “not less than” that in the tables.  Therefore, any indirect method of control (fixed intake damper, adiabatic mixing, supply-return calculation, return fan speed-slaving, CO₂, etc.) will not be capable of insuring that the rates never drop below the minimums specified in the table and cannot provide verification that any specific level of intake volume has been provided. 

[an error occurred while processing this directive] An easy but often neglected conclusion is that the best and most efficient way to insure specific rates of outdoor air are introduced to the air handling system is to simply measure them directly.  With the appropriate instrumentation and subject to the conditions of measurement, one can choose to control a combination of system variables to maintain a fixed intake set point, at least to within the limitations of the instruments proven accuracy.  With this ability you may also log the data to verify compliance and to refute any claims that the operation has failed to provide the legal requirements for ventilation.  It may also be useful to troubleshoot operational or system performance problems.

The costs to provide the needed instrumentation and control components is minimal, compared to the overall cost of the control system, the annual amount of energy wasted by their absence and the HVAC system, to the risk embraced by the owner and the energy penalty that may be absorbed as a result of inadequate control mechanisms.  Yet, how useful are expensive control systems and equipment without reliable and repeatable inputs?  How scientifically valid are indirect control strategies? 

Some measurement technologies might be more expensive to apply than others, requiring modifications to the system, to the conditions where the air is being measured or realized in higher operating costs.  Some may have fewer application limitations, but most airflow measurement devices are capable of supplying these results within the conditional limitations of the specific technology and the component quality selected.

The IMC 2003 prescribes ventilation rates for legal compliance to the minimum requirements by the “authority having jurisdiction”.  The simplest way to prove compliance and provide consistent operational results is to directly measure and control the intake rate variable.

It should be clear to the design professional that the dynamic nature of mechanical ventilation requires dynamic control – a dynamic response to continuous changes.  Because the code and ASHRAE 62 are both “rate based” documentary requirements, continuous airflow measurement should be a central component of any effective control strategy to assure compliance, while simultaneously optimizing the energy needed to do so and the flexibility to adapt to changes, and to any changes in those requirements in the future. 

Text enclosed in quotation marks or otherwise indicated as quotations has been reproduced from International Mechanical Code and is copyrighted by the International Code Council, 1999 - 2003.

Letter to the Editor regarding the above article followed by Len's response.

From: Gene DeJoannis
Sent: 11:30 AM July 8, 2004
Subject: Ventilation Needs and the Requirements

Dear Ken,

I found your July ventilation article to be a rather one-sided opinion piece, which is not too surprising since the author works for a maker of air flow measuring stations, which he insists are a necessary part of any ventilation system. You did not point our the author's connection to the manufacturer. Is this an info-mercial or an unbiased article? I imagine makers of CO2 sensors might have a different opinion. One factor the author does not mention is that even if you can measure the OA rate accurately, no one can tell the designer or the control tech what the setpoint should be (other than the maximum design space occupancy). But this leads to a school with 500 occupants being ventilated to serve 2000, even when the gym, auditorium and cafeteria are empty. I don't think we want to do that.

[an error occurred while processing this directive] Certainly when CO2 is used to control ventilation for people-generated contaminants, we should clearly specify the minimum ventilation rate to be always supplied when occupied (exhaust + pressurization). The CO2 control forces the economizer to open beyond that point. But I think it is possible with current actuators and digital controls to set those minimum rates during commissioning at several different supply fan speeds (such as 35%, 70% & 100% of full speed) and operate with open loop control of the minimum damper position between these points.

I agree this is not the best method, and that a flow station would be desirable, and if there are local user-controlled exhausts in the space it is indispensable. However, not every job can afford a flow station, and if the local exhausts do not vary under the control of users, a static minimum ventilation rate should be satisfactory. That is not to say that it should be left alone for 20 years without rechecking, but outdoor air flow stations may add a little too much complexity for many owners and they have the disadvantage of needing a flow setpoint from another source (such as a human being who estimates the population or a CO2 sensor that detects it).

This sort of article casts doubt on the credibility of your publication.

Gene DeJoannis
vanZelm Heywood & Shadford
Mechanical & Electrical Engineers
29 South Main Street
West Hartford, CT 06107

Response

From: Len Damiano
Sent: 12:13 PM July 9, 2004
Subject: Ventilation Needs And The Requirements

Dear Ken:

This is in response to the email comments you received from Mr. DeJoannis of West Hartford.  (see above)

I am very happy that someone is reading the articles and thinking about what they say. However, the purpose of the “Ventilation Needs And The Requirements of the IMC…….” article was to inform and educate.  To sell a concept – not to sell product.  It was unequivocally non-commercial.  His final “shot” attempted to align your publications’ credibility with defects he perceived in the article.  He needs to be made aware that automatedbuildings.com has always tried to give equal time to opposing views - the hallmark of really good journalists, better publications and one of the best ways for us to learn from one another.

CO₂ sensor supporters and packaged equipment manufacturers have more than sufficiently promoted CO₂–based DCV over the past 15 years, creating much confusion and misunderstanding in the process from their unsubstantiated methodologies and loose interpretations of standards.  They have been so successful, that few understand how or when to use it.  They also thereby relieve me of any responsibility to give their claims “equal time”.  But none of this addresses the main point of the article – the IMC 2000 & 2003 versions have no provision to allow CO₂ monitoring for compliance with the intake rate requirements in ventilation section 403.  The only tenuous justifications presented to me are the very general allowances for code exception requirements and very qualified references to the control method in the IMC 2000 Commentary, which is not part of the IMC development process and should be read very carefully.

If bias was seen, it could only have been in regard to technology and method.  If bias is needed to communicate an appropriate opposing perspective – so be it.  Providing logical arguments to support a particular position are normally more successful when biased to that position.  I appreciate the writer’s comments, but I think Mr. DeJoannis of West Hartford also needs to keep an open mind and not believe everything that equipment vendors say when promoting CO₂ control options.  They may have their own self-serving interests involved.  It would not be the first time that control strategies or sequences supplied by equipment makers have been found to be less than that advertised – at least some of the time or for certain applications.  I believe that to be the case now.

I wonder if Mr. DeJoannis has considered any of the possible reasons that ALL major packaged equipment makers are promoting CO₂ control?  One reason might be that the use of CO₂ side steps all issues of equipment design and the capacity to handle the ASHRAE-required actual rate of outside air “under all operating conditions” (ASHRAE 62-2001, Section 5.3, Systems and Equipment).  Allowing direct measurement could magnify any capacity limitations or equipment deficiency.  CO₂ levels have no direct relationship with intake rates and its usage could provide such large intake set point errors that any claimed discrepancy can be easily disputed, or blame passed to deficient sensor quality.   All the while without having a reliable or continuous field measurement reference against which dynamic airflow control results could be compared.

My position and association with EBTRON, Inc. is clearly indicated near the title of the article and should not be a mystery to any reader.  My associations with ASHRAE, the USGBC, AMCA, and the ICC should also be clear from the details included in my Contributing Editor’s page. 

Links to my company’s web site under “White Papers” will give anyone with a few seconds more information than they bargained for on air measurement technologies and airflow control strategies that are known to work. 

The biggest objection Mr. DeJoannis appears to have with the underlying control position of the article is in the assumed “cost” of direct measurement, concluding that

  ….. a flow station would be desirable, and if there are local user-controlled exhausts in the space it is indispensable.  However, not every job can afford a flow station, and if the local exhausts do not vary under the control of users, a static minimum ventilation rate should be satisfactory”. 

In case he has not had the opportunity to check (and I cannot speak for all AFMS manufacturers), but EBTRON airflow meters for outside air control in smaller packaged equipment can be much more cost effective than using CO₂ sensors in the AHU or the zone, not to mention light-years ahead in reliability and providing the simplest means of air system performance verification and code compliance.  [Now that can be taken to be a promotional comment, but essential to correct an erroneous and unqualified statement by Mr. DeJoannis.]

[an error occurred while processing this directive] There are a number of control alternatives and opinions expressed in his letter that should be clarified or challenged, but not here.  We can save those for another discussion.  For some semblance of brevity, I have only addressed clarifications on the limitations and alternatives to CO₂-based DCV.

The most important reason to re-examine the way we use DCV is to remember that the real definition of DCV does not include the mention of CO₂ measurement.  It essentially describes optimizing energy usage by “monitoring and resetting the intake rate set point when conditions vary”.  Nor should we attempt to apply CO₂ in situations where it has been demonstrated to be difficult (if not impossible) to use.  CO₂ has yet to be convincingly demonstrated to be an accurate “people-counter”, nor even a rough equivalent for intake rate control in most situations, i.e. offices or schools.  CO₂ measurement has been shown to be “potentially” effective in demand controlled ventilation strategies ONLY in high-density spaces with unpredictably variable and intermittent occupancy.  Namely: churches, theaters, auditoriums, gymnasiums, lecture halls, retail stores and malls, etc.

This definition specifically excludes:  office spaces, most commercial and institutional buildings, labs, hospitals, nursing homes any healthcare facility or school classrooms.  This exclusion embraces any spaces that are occupied continuously during the majority of a day or in accordance with a predetermined occupancy schedule with predictable usage.  Simpler, less costly and more effective means for intake control are available in these situations.

 I do not advocate the traditional methods being promoted for CO₂-based Demand Controlled Ventilation proclaiming many of the same conclusions expressed by the California EPA-ARB, the DOE’s Lawrence Berkeley National Labs and NIST Indoor Environment Divisions.  Combined with my own conclusions, many are summarized below.  (references available on request)

• “Steady-state” conditions needed for ASHRAE 62 model validity rarely occur in variable occupancy facilities.

• CO₂ sensor error/drift result in significant intake errors, even under steady-state conditions.

• Outside CO₂ levels vary widely and result in significant error in OA intake flow rates.

• The CO₂ generation rate can over-ventilate during periods of high occupancy due to higher than assumed CO₂ production levels of occupants.

• Occupants can be overexposed to contaminants because CO₂ DCV can significantly under-ventilate during periods of low occupancy and the building generated contaminants represent a majority of the dilution air requirement.

• Occupants can be overexposed to contaminants because outside airflow rate changes can significantly lag occupancy and under-ventilate during startup hours of system operation.  This trait will also impact the methods energy-saving potential.

• Traditional CO₂ DCV methods cannot document compliance with ASHRAE Standard 62 ventilation rate requirements (only the interior CO₂ concentrations).

• DCV control involves significant reliability issues, compounding existing ventilation reliability problems already identified and studied (Calif. EPA-ARB, CEC, LBNL, LA Unified School Dist., NIST, etc).

• CO₂ concentrations and DCV control ignore building pressurization requirements, which are essential for energy conservation, comfort control, IEQ and the prevention of mold growth inside exterior walls.

• Temperature effect, air speed and temperature limitations (outdoor measurement) make using the “steady state” formula a problem in most climates that can reach 32 deg F.

• "Using multiple CO₂ sensors to determine the outside airflow rate is not possible due to the relatively large error associated with the absolute accuracy of commonly available sensors" (ASHRAE RP-980, 1999)

Many of these deficiencies can be countered by changing operational methods, by increasing sensor quantities, changing placements or increasing the frequency of calibration, etc.  But don’t these alterations contradict the simplicity and cost-saving advantages that have been promoted for CO₂ DCV – absent the greater security available with more precise, direct intake control? 

I will have another article on CO₂-based Demand Controlled Ventilation, which should appear in the ASHRAE Journal or ASHRAE Transactions this fall.  It has survived peer-review and is pending a publication decision.

So, what should we do for high-density spaces with unpredictably variable and intermittent occupancy?

1. Use Demand Controlled Ventilation in appropriate applications – but possibly with occupancy and airflow rate determinants other than CO₂ (e.g. direct velocity measurement, time scheduled occupancy, direct count by turnstiles or other passive methods, IR, monitored equipment usage, etc.) and reset intake rates dynamically according to changes in zone occupancy.

2. Design capacity should consider maximum occupancy at “worst-case” conditions, providing your maximum ventilation rate for control, never to be exceeded, as is not possible with CO₂ input alone.

3. Establish and insure a base (minimum) ventilation rate with direct intake measurement, regardless of occupancy level, in compliance with ASHRAE 62 addendum “n”.  No other means can provide the level of control feedback needed to “maintain” these lower rates “under all operating conditions”. This means dynamic control in opposition to internal and external conditions that could easily overcome mechanical systems and prevent this objective from being met.

4. Monitor occupancy for changes by reliable means and reset operating set points as needed to maintain per person rates.

What are the benefits of Direct Intake Control with Dynamic Reset?

• It is demonstrably the most accurate, reliable and most efficient OA control method to minimize energy while simultaneously complying with IMC (code) mandates, ASHRAE Standard 62 – or ANY rate-based operational intake control requirement.

• It provides the flexibility to adapt to operational conditions, occupancy changes, code requirements and changes in building envelope leakage over time.

• It can limit or eliminate the risk associated with IAQ deficiency claims by occupants – an uninsured risk.  Provides a readily defensible position against these claims for the owner or the engineer.

• Provides an integral part of the total solution to mold growth by proactively helping prevent damage in exterior building walls, by allowing use of air barriers to moisture infiltration.

• Adaptable and just as useful with ERV’s and DOAS, as with traditional VAV and CAV systems, when used to compensate for filter loading, wind pressure, stack effect, system effects or other dynamic changes that are not considered without a direct control input.  (NONE of which can be overcome with a fixed intake damper strategy).

• Provides the ability to reliably control and verify the mode and function of ventilation and/or pressurization systems during fire emergencies or CBR building security situations.

Designers should use any brand of airflow measurement they can afford and are comfortable with, but they are being professionally negligent if they do not seriously evaluate and consider direct measurement for intake rate control (especially in VAV systems), and avoid CO₂ -based DCV, except when appropriate and where the limitations of the sensor technology and control method will not risk damage to you, the building, the occupants or the owner.

 EBTRON, Inc.

Len Damiano
Vice President – Sales & Marketing

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