Article - November 2000
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Len Damiano
Contributing Editor

Building energy usage can be limited while still complying with the new ventilation codes and standards. Engineers have found many creative ways to satisfy these superficially conflicting objectives.

Several government and academic studies have highlighted the potential in alternative design and equipment solutions. Halverson at Pacific Northwest Labs (DOE), Mudarri at the Indoor Environments Division of the EPA, Reddy at the Energy Systems Lab of Texas A&M, Bolin and Mumma of Penn State University have all published research documenting methodologies that demonstrate energy savings are possible, while simultaneous providing sufficient ventilation for acceptable IAQ.

The need to provide a safe, healthful and productive environment for employees should be an overriding concern for all organizations. The payback can be rapid and in magnitudes that dwarf the additional incremental costs required by factors greater than many hundreds to one.

On the down side: noncompliance with local codes; and dismissing, ignoring or "interpreting for convenience" a well established "standard of care" in mechanical design, can be very expensive, on several levels.

Suffice it to say that there are many well-documented financial reasons for continuously meeting the dilution ventilation rates required for the occupants of a particular building or space. But, risk management is a topic for another paper. For our purposes, let us assume that we all agree that it is in everyone's best interest to meet the ventilation requirements of ASHRAE Standard 62 and those requirements of your local code authorities. We can also agree that building owners and operators have economic motivations to minimize energy usage and maximize the value of and return from their space assets.

Therefore, What is really being asked of us by these legal requirements?

It is most important for everyone to recognize is that providing continuous outside air at the air handler alone cannot satisfy the requirements of ASHRAE Standard 62 and most codes. Summing the individual zone requirements at full flow conditions does not provide you with the total outside air needed in VAV systems for all zones, nor as conditions change in the system over time.

First examine the specifics of our national standard on "Ventilation for Acceptable Indoor Air Quality".

ASHRAE Standard 62, Section 6.1.3 reads

Indoor air quality shall be considered acceptable if the required rates of acceptable outdoor air in Table 2 are provided for the occupied space.

Sufficient rates of dilution air must be delivered to the occupied zone. If there is only one zone per AHU…no problem. When a single air handler serves multiple zones, however, the ASHRAE Standard provides a formula that must be followed for compliance (Equation 6-1, par.

The "Multi-Zone Equation" quantifies the individual 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. The formula is expressed as: Y=X / [1 + X - Z].

As a VAV system operates, the determination of the "critical zone" by the equation could wander from one zone to another and the total outside air requirement can vary dramatically. A reasonable method to counter these dynamic changes is with dynamic inputs to continuously monitor the zone changes, then automatically recalculate the ventilation requirement and reset the intake control set-point.

In order to have any chance of optimizing control in VAV systems, these airflow rates should be continuously and accurately monitored. The quantification and calculations specified in the Standard demand the accurate measurement of the airflow rates at the outside air intake, supply air and the individual zone air volumes.

The performance of the VAV terminal box airflow sensor is a key factor in successful VAV system design, especially as we would hope to use it within these requirements. Unfortunately, the typical transducer built into the box controller is not very useful for measurement purposes, as we can see from the calculations in the table below. 

VAV Box Transducer Error (typical)
 (add additional 5% for flow ring sensor)

VAV Box Transducer Error Table

Modeling indicates that the error rate of the "typical" VAV box airflow sensor can force an error in the outside air set-point as much as 50% of reading. This range of inaccuracy is determined easily by a calculation of the published inaccuracies of the transducer, as typically supplied with the box controller, is used to recalculate the error induced at the intake. A simple multi-zone model was used, including the 6-1 Equation from the Standard.

The "total accuracy "of the measurement can only be calculated by adding the uncertainties of the primary Pitot device, plus any environmental or installation conditions that deviate from the laboratory conditions that the device was rated within. This total is generally going to be greater that the inaccuracy of the transducer alone. We can therefore be fairly confident that the indicated range of resultant errors at the outside air intake are determined conservatively.

The total box flow sensor accuracy that produces this type of OA set-point error range may be suitable for comfort control, but its performance variance is far too wide to be acceptable for the control of dilution air at the zone level.

Why should this be significant to us?

The quantification and calculations specified occur within a dynamic system demand the accurate measurement of the airflow rates at the outside air intake, supply air and the individual zone air volumes. In order to have any hope of optimizing ventilation control in VAV systems, these airflow rates should be continuously monitored. In order to optimize energy usage, outside air intake set-points are dynamic in VAV systems and therefore must be continuously reset as zone flow rates change.

Inherent inaccuracies in the velocity pressure measurement technology and the specific limitations of the measurement components employed, leave us with such high rates of uncertainty that we could do as well by not measuring at all.

It is imperative that the designers give this aspect of a project's design and specifications sufficient attention in order to meet both indoor air quality and operating cost objectives for new or existing buildings. Measurement accuracy, repeatability, no maintenance requirements, no field calibration requirements and ease of installation should all be high priorities for terminal box specifications.

So, how does the ASHRAE Standard impact code requirements for ventilation?

The International Mechanical Code (IMC) excerpts the 6-1 multi-zone requirement directly from ASHRAE Standard 62-1989/99 (IMC2000 Section 403.3.2 Common ventilation system). The Southern Mechanical Code (SBCCI), The Uniform Mechanical Code (ICBO), and the BOCA Code will soon be combined and replaced by the International Mechanical Code (ICC). If adopted by the same jurisdictions that currently use these "model" building codes, this requirement would affect most of the country.

If your jurisdiction is considering or has already adopted the latest IMC, here are some of the requirements you should factor into your HVAC design decisions.

The IMC Section 403.3 "Ventilation Rates" states:

"The ventilation system shall be designed to supply the required rate of ventilation continuously during the period the building is occupied, except as otherwise stated in other provisions of the code."

No big surprises there, but the IMC goes on to effectively mandate control of outdoor air under Section 403.3.3 "Variable air volume system control":

"Variable air volume 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."

Section 405 "Systems Control" states:

"…..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 bolster these requirements, the latest addition to Standard 62 is Addendum "U", released for public comment by ASHRAE on August 11, 2001. It states in the forward:

"In many cases, an active outdoor air control system must be provided to ensure minimum intake rates are maintained."

A new Section 5.3 is proposed as follows:

5.3 Ventilation System Controls. Mechanical ventilation systems shall include either manual or automatic controls that enable the fan system to operate whenever the spaces served are occupied. 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 this proposed Addendum, ASHRAE has acknowledged and finally caught up with published research, some of which is more than 10 years old. We have demonstrated in other papers that all indirect methods of outside air control are deficient and incapable of providing sufficient reliability to be used for code compliance.

In fact, indirect control of outside air will be deficient to such an extent that we can expect large periods of time in a building's operational life, that the intake air rates will be negative or well below the codified minimums. We can also be secure that at times the building will be conditioning significantly more outside air than is needed for code compliance, and probably well in excess of the level where temperature and humidity criteria can be maintained for comfort. And, NO….these periods will usually not be of short duration and they will most likely not offset each other.

When these requirements are viewed collectively, as they will be applied, we are left with few economical alternatives that do not include direct measurement to control outside air intake rates.

Historically, direct control was considered impractical, if not impossible with the technologies available. So by default, indirect methods were employed, with mixed results. It is primarily because of this and the difficulty of getting accurate hand-held field measurements that the indirect methods have been sustained. The truth be known, it also provided some engineers and contractors with "plausible deniability". No one could readily challenge the performance of the building components and design if there were no accurate direct measurements available.

We feel that most installations with indirect control methods provide very unreliable results and at best are periodically and unpredictably deficient. Without the means of direct measurement and without conditions suitable to use hand-held instruments, we are guessing what is happening at the system's intake at most points in time.

Direct control of outside air intake rates is practical and affordable. Direct control appears to be the most efficient and effective method of accomplishing all of these objectives, while simultaneously providing a level of control flexibility not previously available. Several firms now make products using technologies that are capable of providing inputs for direct control. Our firm has been manufacturing products designed specifically for this purpose for over 15 years.

There are sufficient motivations to provide ventilation systems capable of meeting both design objectives and legal requirements. There are also significant motivations for an employer and/or the building operator to ensure that the interior environment provided is healthful and conducive to productive activity. There are competitive means available to provide more reliable control of air intake rates. Yet, many engineers are reluctant to use designs, methods or equipment specifications other than those that they have used for upwards of 30 years.

It seems that recently new products and methods of direct control of dilution ventilation are being introduced every few months. Many are less expensive to procure, install and/or operate than traditional ones. It is up to the design professionals to take up the challenge, become more innovative and provide solutions for IAQ and ventilation control that serve the interests of both the building's owner and its occupants.

Direct control of intake rates satisfies all those objectives, provides a more secure method of meeting ventilation codes without compromising construction budgets, and provides a much more flexible and reliable control solution.

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