VAV Systems and Green Design - Part 2
Classical VAV systems have a reputation for compromising IAQ. Because they actively reduce ventilation to save energy. The degree to which they compromise ventilation often exceeds that which is permissible under the building codes. However, they are widely perceived to be efficient and cost-effective, to provide superior comfort control, and as a result, remain popular and in wide use.
Most VAV installations designed and constructed in jurisdictions subject to the International Mechanical Code (IMC) since 1998 fail to comply with code. This is especially true in ventilation-intensive occupancies like schools, where the required rates of ventilation are high. The mechanism for this problem was explained in Part I (January 2008, Engineered Systems) of this series. These system designs are still accepted by many code officials for two reasons: they seldom have the expertise to recognize and understand the problem, and they seldom know how to address the situation.
It is now widely recognized that shutoff VAV systems cannot provide adequate ventilation when used by themselves. The use and modification of this strategy will be discussed in a future article. The purpose of this article is to discuss the challenges of bringing classical VAV reheat systems into compliance with the requirements of ANSI/ASHRAE Standard 62.1-2004 “Ventilation for Acceptable Indoor Air Quality,”1 ANSI/ASHRAE/IESNA Standard 90.1 “Energy Standard for Buildings Except Low-Rise Residential Buildings,”2 and meeting the requirements of Section 4 of the IMC3. All of these requirements must be addressed and met to legitimately qualify for LEED® certification.
Challenges To Be AddressedAs the ratio of design space sensible load to occupancy sensible load decreases the need for VAV system sensitivity to the ventilation needs of the occupants increases Occupancies suitable for use with classical VAV reheat are characterized by high sensible heat ratios and often have high internal equipment gains.
However, increasing attention to wasteful equipment energy use by manufacturers, reduced lighting system Watt densities, greater building insulation levels, and more attention to glazing systems are effectively reducing sensible heat ratios. This is reducing the number of occupancies where VAV reheat systems are even viable. The actual needs of many spaces, like educational facilities, are now actually dominated by the need for ventilation. These applications play to the weakness of VAV reheat systems - their inability to provide adequate ventilation - making these systems increasingly inappropriate for use in those occupancies. This will be discussed in more detail in future articles.
The weakness of the classical VAV reheat system stems from the fact that these systems not only vary the airflow to individual spaces, but also due to the fact that in most areas of the country, the requirements of ANSI/ASHRAE/IESNA Standard 90.1 also require the use of an airside or waterside economizer.
The airside economizer is most commonly employed and varies the amount of outdoor air introduced for energy conservation purposes. When an airside economizer process is employed in conjunction with volumetric flow reduction in the space, to maintain a given rate of outdoor air introduction the primary outdoor air fraction of the air to the space (Zp) must increase as primary airflow to the space decreases. This means that as ambient conditions get colder, and the airside economizer reduces the primary outdoor air fraction at the system, more air must be processed, conditioned, and delivered to the occupied space to provide a given level of ventilation. As a result, using the mixed air path to introduce outdoor air is a very poor design choice.
Another factor that makes this challenge especially difficult for VAV reheat systems is that they provide primary supply air to the space based on the need for sensible cooling. In addition to people, this includes heat transmission through the exterior envelope and interior partitions, solar gains, and heat from lights and any kind of power consuming equipment. External thermal load characteristics vary with time of year as well as time of day. Designed equipment loads may be off, on standby, may have never even been installed, or are different than planned. Many HVAC engineers overestimate these loads in their computations “just to be safe.” Because “design” cooling loads rarely occur, the actual amount of primary air delivered to a space is usually less, and frequently much less, than design flows. As a matter of practice, overdesign overdrives minimum airflows and artificially increases the need for terminal reheat.
ASHRAE Standards 62.1-2004 and 90.1-2004 provide clear direction in this situation. Section 220.127.116.11 of Standard 90.1 prohibits the use of all simultaneous heating and cooling, even when the cooling is accomplished as part of an airside economizer. Reheat is permitted only under limited exception clauses. As a general rule, it can be easily demonstrated that to deliver a given amount of ventilation air, more energy is consumed in the form of fan and reheat energy by reducing the outdoor air fraction and delivering more air than by increasing the outdoor air fraction and reducing the total amount of air delivered. The exception to this is when a system’s outdoor air fraction is driven by an anomalous space, such as a conference room. There is also a point where the use of recirculation in a classical VAV reheat design becomes cost-ineffective and energy-inefficient.
Section 5.4 of Standard 62.1-2004 tells us the following:
“Ventilation System Controls. 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.”
It also tells us:
“VAV systems with fixed outdoor air damper positions must comply with this requirement at minimum supply airflow.”
This is why since the 1998 edition, the IMC has required “special controls” for VAV systems other than those delivering 100% outdoor air.
Special ControlsThe need for “special controls” has been touched on superficially in the ASHRAE Handbook – Systems and Equipment4 since 1992, but fails to tell designers how to actually accomplish this. This can, however, be derived by understanding the requirements of Standards 62.1 and 90.1 and the subject of air-handling system control.
Most HVAC system designers erroneously believe that minimizing the outdoor air fraction on a VAV system saves energy. However, when introducing outdoor air for ventilation through the mixed air path, precisely the opposite is true. Reducing total system mass flow and increasing the outdoor air fraction on the system reduces the amount of outdoor air which must be introduced, preheat energy, reheat energy, and fan energy requirements. It also reduces indoor air contaminant levels by breaking the cycle of concentration recirculation causes. The best economies actually occur when minimum airflows are delivered, and flow potential is most minimized when 100% outdoor air is used. The energy implications of this can be easily verified with a few relatively simple computations.
To effectively deliver adequate ventilation with a VAV reheat system, the system controls need to accomplish the following:
- The system must be able to measure and monitor, in real time, the delivered outdoor air fraction at the system. Theoretically, this could be accomplished several ways:
- Monitoring outdoor air and supply air totals.
- Monitoring outdoor air and return air totals.
- Monitoring outdoor air, return air, and mixed air temperatures.
- In all cases, flow measurement is required at the VAV box.
- Designers should also pay particular attention to the capabilities and limitations of the flow measurement technologies employed, and entrance conditions can make, or break, a design.
- The control system must know how much outdoor air is required at each space, and how much air is actually being delivered. When the amount of outdoor air required (Voz) at each space equals the computed ventilation requirements from Table 6-1 (Vbz) divided by the zone air distribution effectiveness (Ez) (Voz = Vbz / Ez). Voz may be a fixed quantity, or dynamically reset based on scheduled occupancy levels, direct sensing of occupancy such as card readers or infra-red and/or ultrasonic sensing technologies, or through indirect measurement of occupancy indicators such as CO2 sensing5.
- The zone primary outdoor air fraction (Zp) is determined by dividing the amount of outdoor air actually required (Voz) by the amount of total air delivered at the air terminal unit (Vpz) (Zp = Voz /Vpz). As airflow rates to the space decline, the value of Pz will increase. When the value of Pz exceeds unity (1.00), it means that less total air volume is being delivered to the space than required.
- The system air distribution effectiveness (Ev) is the lowest value of Evz computed for all zones served by the system (Ev = min Evz). Evz should be computed for each zone where Evz = 1+Xs – Zd where Xs is the primary outdoor air fraction at the air handling unit. Those familiar with Standard 62-1989 should recognize the above equation as the denominator from the multiple spaces equation. What this essentially means is that instead of making this computation once at the system level, Standard 62.1-2004 now requires it to be computed for each zone. This essentially gets us to the same end-point, although via a more roundabout and torturous route.
- For design purposes with VAV systems, this value must be computed at the minimum flow settings at the VAV box. Section 18.104.22.168 of Standard 90.1 limits the amount of air that can be reheated to no more than the greater of:
- The volume of outdoor air required to meet the requirements of Standard 62.1 0.4 cfm/sq ft
- 30% of the design cooling load of the system
- Since the 30% parameter usually varies, the value of Zd is going to typically be at least 3.33 times higher than for a constant volume system at design. For example, for a Standard 90.1 compliant VAV reheat system a critical zone with a Zd value of 0.25 at full flow, Zd would actually equal (= 0.25 / 0.30) 0.833. This value is off the scale for using Table 6-3, and the designer must then determine the amount of outdoor air on the system (Vot) by dividing the diversified amount of outdoor air for the system (Vou) by system air distribution effectiveness (Ev). So, the design outdoor air fraction on a system with a diversified outdoor air fraction of 20% would actually have to process (= 0.2 / (1 + 0.2 -0.833) = 55% outdoor air, almost three times the outdoor air actually required to meet the sum of the ventilation requirements of the space.
- The zone air distribution effectiveness (Evz) for each zone on the system must be continuously monitored to determine which space is critical and adjust the outdoor fraction at the AHU accordingly.
ConclusionsThe key to making VAV work is recognizing the fact that there are two variables which degrade the IAQ performance of these systems: Varying the amount of outdoor air introduced into the system, and varying the airflow to the space. To avoid control nightmares, one of those variables must be reduced to a constant.
One general approach to accomplishing this objective is the use of a technique known as “dual path” ventilation, which independently supplies outdoor air to each space. This permits the use of shutoff VAV for cooling. A second, 100% DOAS is used to meet minimum ventilation requirements. This technique substantially reduces heating and cooling capacity requirements, can functionally eliminate the need for reheat, saves significant energy, and reduces the delivery of outdoor air to each occupied space to a constant. On the other hand, it increases the total amount of air that must be delivered at peak flows and requires a minimum of two duct systems. It carries a small cost premium, which is quickly returned with improved life cycle cost characteristics.
The other approach is to make the VAV system 100% outdoor air and build it around full-range energy recovery. There are multiple options, and while some are vastly better than others, no single solution is correct in all applications. This approach requires designers to think outside the box.
If the reader is inclined to pursue this approach, there is a whole new world of performance potential waiting to be discovered, and that performance can be simply dazzling. On the other hand, while energy recovery is most effective and economical when used aggressively, aggressive use of energy recovery can become a technological minefield for the careless and unwary. If any readers of this article were able to follow the torturous logic of Standard 62.1-2004’s Ventilation Rate Procedure, which SSPC 62.1 has euphemistically described as a “simplified” multiple spaces equation, they would probably qualify for sainthood. And, for those inclined to think that this is some form of vast conspiracy to take away your favorite HVAC system, you may be onto something.
It is the author’s position that VAV reheat is an obsolete strategy with grievously serious problems achieving compliance with both Standard 62.1 and 90.1. The IAQ problems with these systems have been known for almost two decades, and there are legitimate reasons for them. Vastly superior strategies which have been tested and proven over the past 20 years are available. As individual designers, we can delude ourselves by believing that we have never had a problem in the past, but whether we want to acknowledge it or not, the truth is that we probably have had problems. VAV systems introducing outdoor air through the mixed air path have always achieved energy use reduction at the direct expense of ventilation and by virtue of physical configuration and underlying concept, are incapable of either efficiently processing or effectively managing the introduction of outdoor air through the mixed air path. You can provide adequate ventilation, but only at great energy expense.
ASHRAE Standards 62.1 and 90.1 are inexorably pushing designers in the correct direction toward more efficient and effective solutions. Those who have dedicated themselves to preserving the “tried and truly awful” strategies of the past are going to find it increasingly difficult and engineering intensive to justify continuing to use obsolete strategies. In doing so, they will handicap their clients with costly and inefficient designs, and watch their professional liability exposure go through the roof. Those who are smart enough to religiously go through the above effort will inevitably reach the conclusion, “There has to be a better way!” When they start looking for a better way, they will find to their delight that there are, in fact, not one, but many better ways.
Energy recovery and avoidance is green. The enemy is ignorance and the thermodynamic triple point. He lays in wait for careless and unsuspecting engineers. His tactics are known, but poorly documented. He can be outsmarted, but he should be considered to be armed, dangerous, unpredictable and particularly unforgiving of both design and installation error. The principles are not hard, but the devil is in the details. There is a long learning curve ahead, and designers need to take them one step at a time. ES
CITED WORKS1. American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), “Ventilation for Acceptable Indoor Air Quality,” ANSI/ASHRAE Standard 62.1-2004, 2004.
2. ASHRAE, “Energy Standard for Buildings Except Low-Rise Residential Buildings,” ANSI/ASHRAE/IESNA Standard 90.1-2004, 2004.
3. International Code Council (ICC), International Mechanical Code, Section 4, 1998: 27-30.
4. ASHRAE, 1992 ASHRAE Handbook - Systems and Equipment, Chapter 2, “All-Air Systems, Outside Air Requirements,” 1992:2.5
5. Mumma, Dr. Stanley, “Transient Occupancy Ventilation by Monitoring CO2,” ASHRAE IAQ Applications, 2004: 21-23.