Combining DOAS and VRF, Part 1 of 2

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HPAC Engineering

By GRANT BOWERS, PE, LEED AP; SSOE Group; Hillsboro, Ore.Mon, 2014-03-03 11:19

Separating cooling/heating and ventilation airflows simplifies system design and control

Variable-air-volume (VAV) systems with air terminal units have been used extensively in commercial andinstitutional buildings in the United States for decades. Unfortunately, optimized design of a VAV systemwith terminal heat is difficult at best because of limitations inherent in VAV and complications posed bydesign standards and regulations. One new approach involves the pairing of a dedicated outdoor-airsystem (DOAS) with a variable-refrigerant-flow (VRF) system. By separating the goal of achievingventilation rates from the goal of maximizing thermal comfort, we can avoid situations in which the twogoals are in conflict and efforts suffer from the resulting compromises. What’s more, we can simplify thedesign process and find system efficiencies that go far beyond those commonly achieved with VAV systemswith terminal heating.

Background

In the simplest VAV system, incoming outside air and return air are mixed in a central air-handling unit(AHU) and then pre-heated or pre-cooled. The tempered air is sent by supply-air fan to variousoccupiable zones at a temperature generally suitable for cooling. In each zone, a terminal unit adjustsairflow based on cooling demand. When a zone requires heating, supply-air flow usually is reduced to aminimum setting and heated, typically via terminal-unit heating coil.

In its simplest form, a DOAS is an AHU dedicated to ventilation, not sized to provide cooling air. DOASoften are supply-only systems with relief to outdoors; however, they also can include exhaust heatrecovery. Generally, they are not sized to provide 100 percent air economization (cooling using outside airin lieu of mechanical cooling).

More on DOAS: “Low-Cost Dedicated Outdoor-Air Systems”

More on DOAS: “Optimizing Dedicated Outdoor-Air Systems”

More on DOAS: “Dedicated Outdoor Air, Energy Recovery Rise to School’s VentilationChallenge”

VRF systems use individual high-efficiency fan coils in interior spaces in combination with high-efficiencycondensing units that can serve multiple zones. VRF systems can be arranged to provide energy recovery,

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moving heat from zones requiring constant cooling to zones that sometimes require heating.

More on VRF: “VRF Zoning Helps School District Achieve Energy- and Cost-Saving Goals"

Limitations of VAV Design

ANSI/ASHRAE Standard 62.1, Ventilation for Acceptable Indoor Air Quality, provides minimumoutdoor-air-flow requirements for design conditions. ANSI/ASHRAE/IES Standard 90.1, EnergyStandard for Buildings Except Low-Rise Residential Buildings, meanwhile, requires some systems to beoperated so ventilation capacity modulates to match ventilation load (i.e., demand). Designing systems tomeet both of these requirements is complex.

To help match capacity to load, ANSI/ASHRAE Standard 62.1 allows “dynamic reset,” but leaves thedetails to the designer. A common approach combines system-level outside-air-damper reset withzone-level demand-control-ventilation (DCV) strategies. For VAV-system energy efficiency to bemaximized, the system-level intake/exhaust/relief-damper-reset sequence for space-temperature controland building pressure must be coordinated with an air-economization-reset sequence. Such controls canbe quite complex, particularly when pressurization between zones and interaction with exhaust systemsalso requiring variable-volume control are considered. Weather conditions also can have a major impacton building pressurization and space ventilation.

More on DCV: “Single-Zone, CO2-Based Demand-Controlled Ventilation”

More on DCV: “Multiparameter Demand-Controlled Ventilation"

Other common control sequences for large VAV systems include occupied-hours control, optimalstart/stop, fan-pressure reset (“critical terminal unit” control), “ventilation optimization”1 (DCV),ventilation space-temperature setback, supply-air-temperature reset, dynamic space-pressure control,economizer, energy recovery, natural ventilation, and interfaces with building lighting controls, smokedetection, fire alarms, and even security systems. Interplay between these control schemes vastlycomplicates efforts to optimize building energy use.

Further complicating VAV-system design is the need to follow the ANSI/ASHRAE Standard 62.1ventilation-rate procedure, which asks designers to use the multiple-space equation (MSE) to calculate the“critical” zone, the space driving the overall (system-level) outside-air fraction. Zone-level flows then arechanged to meet zone-level requirements. Once the critical zone has been chosen, heating-turndownrequirements push designers to overventilate some zones by increasing heating minimum settings. Thistypically changes the critical zone and reduces the overall outside-air fraction at the central-system level,substantially affecting AHU components.

To respond to changes in zone population, dynamic reset of VAV systems, combining zone-level DCV withsystem-level ventilation reset, often is applied. Ventilation reset is a control scheme by which the MSE issolved dynamically to change system outside-air setpoint. This often is applied with airflow-measuringstations, along with electronics and software to control dampers based on relative airflow, at the centralsystem.


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Central-system design usually is based on the static-condition (design) critical zone, while in the realworld, zone population varies dynamically along with building HVAC load (internal and weather-related)over the course of a day. Maximizing energy efficiency under these conditions pushes the envelope interms of building-system design and building-operation hardware and software. Some commercialsoftware packages can aid ventilation reset, calculating critical ventilation zone per ANSI/ASHRAEStandard 62.1. Calculating minimum system outside air, however, usually requires the overriding of someterminal-unit turndowns, which changes the critical zone. This is allowed by ANSI/ASHRAE/IESStandard 90.1 because only a few zone overrides can change a system outside-air setting by manypercentage points, saving considerable pre-treatment energy and reducing central-system size.Meanwhile, inputs to these calculations vary with system load, which also can change the critical zone. Atthe same time, other overrides, such as of supply airflow for makeup to areas with exhaust flowsexceeding ASHRAE minimum rates, are necessary. These overrides and simultaneous variations result inthe need for iterative calculations involving all environmental variables; such calculations currently arebeyond the scope of commercially available software.

Zoning is another limitation. History shows building-construction economics can drive VAV-systemdesigners to combine up to several rooms on a single ventilation and temperature-control zone. Becauseheating and cooling loads can vary widely between rooms on a single zone, this often leads to discomfort incertain rooms when other rooms on the same zone are not at the design load.

In summary, designing a VAV multizone HVAC system can be challenging to say the least. Overlaying thevarious requirements, exceptions, and system functions results in iterative design simulations that mustbe re-run whenever a room’s size changes. Complying with all codes while addressing the competinginterests of ventilation and energy optimization is theoretically possible,2 and technical solutions partlyexist and are being developed. But in practice, designing and redesigning systems with complex, iterativecalculations is not very practical, and manual overrides generally do not fully optimize system designs.Meanwhile, designers often estimate “block” (net) load based on building-envelope heating and cooling,considering neither variations in internal and ventilation heating/cooling loads nor airflows needed forventilation and space pressure control, significantly undersizing or oversizing systems as a result.

In decades past, the basis of widespread use of VAV systems was their inherent high air-change rate,maintained during reheat. Now, standards push the minimization of reheat and air-change rates for thesake of energy conservation.

VAV systems require extensive space for ducts for distribution of low-pressure air downstream of terminalunits. This is particularly troublesome when interleaving return-air ducts, exhaust-air ducts, supply-airducts, terminal units, light fixtures, conduits, cable trays, and structure in ceiling space while still allowingroom for maintenance. Meanwhile, terminal units themselves can require considerable physical space.With codes prescribing use of bigger ducts and coils to reduce air-pressure losses, available space isshrinking.

Ceiling-space concerns are exacerbated by air-economizer requirements. Central VAV systems usuallyhave air-economizer dampers as needed to meet ASHRAE standards. With an air economizer, supply and


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return ducts must be sized to accommodate use of outside air for cooling when conditions are right.

VAV central fans are selected based on their flow characteristics. Fan curves generally demonstrate staticefficiency has diminishing returns at some fraction of design fan speed. This “maximum turndown”concept typically is used for selecting fans for best static efficiency at design conditions, but also helps toillustrate the problematic nature of maximizing efficiency in VAV systems under dynamic conditions. Iffan efficiency is not considered at maximum turndown (minimum speed), it is not being balanced againstall other system efficiency effects.

Centralized supply fans can be quite large and, thus, are likely to generate noise and vibration. Systems forabating this noise and vibration can be costly. Also, large fans require large seismic restraints and arelatively beefy structure for attaching those restraints. Repair, replacement, and modification of largecentral fans in VAV systems can require long lead times for custom equipment and to accommodateconstruction impacts, such as the removal of structure for access to large fan components.

Economizer dampers in central systems are affected by pressure losses. At the same time, maximizing theenergy effectiveness of a VAV system requires consideration of economizer cooling in real time, aformidable challenge when laying economizer control over other pressure controls. In short, thebest-designed economizer-damper controls can be quite complex, and these dampers and controls oftenare not well-maintained and, thus, can have a high rate of failure.3

VAV systems have other inherent pressure losses that constantly consume fan horsepower. These includeterminal-unit dampers, reheat coils, sound traps, and restrictive fittings in medium-pressure ducts.

VAV systems require intensive iterative technical balancing of heating-water, cooling-water, and air flows.This balancing can be costly and time-consuming, involving multiple design disciplines and buildingtrades.

Automated control of complex VAV systems is intricate, involving indoor/outdoor space pressurization,individual space pressures, relative pressure losses through ducts/dampers/diffusers/louvers, multipleoverlapping dynamic resets (airside and waterside), signal-select routines for combined zones, complexterminal-unit cooling/turndown/reheat schemes, heat-recovery control, defrosts, equipment start/stop,DCV, fire- and life-safety routines, lighting-control interfaces, and time-clock functions. Built-up VAVsystems tend to demand combination and optimization of these functions, while DOAS and VRF systemstend to deal with them separately, which can simplify a building-automation system by an order ofmagnitude.

Energy-recovery components in built-up VAV systems tend to have relatively high airflow losses becauseof the economic pressure to contain the size of VAV air handlers. Also, these energy-recovery systems canbe complex, involving bypass dampers, sometimes controls and additional maintenance for heat wheels orother energy-recovery devices, and pressure losses and controllability issues related to added dampers,coils, pumps, and piping.

Supply-air diffusers typically found in VAV systems are static, meaning their function changes whenterminal units reduce airflow. This is especially problematic with VAV heating turndown because air is notthrown as far into the room (Figure 1), sometimes leading to space-temperature stratification. Also, VAV


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room air throttling creates noise that can be particularly troublesome in office and educationalinstitutions. Larger and more expensive equipment, as well as components such as duct liner andsilencers, can be required, adding to capital and operating costs.

When mixed air is being cooled at an AHU and terminal supply air is being heated, some degree of“reheat” can occur, resulting in less-than-optimal energy efficiency.

Particularly high energy penalties can be paid where reheat occurs, which often is zones with highventilation requirements and low or intermittent cooling loads. For example, a typical conference roomhas variable occupancy, a high maximum occupancy density, and low unoccupied cooling requirements.In such a zone, delivery of minimum ventilation air that has been mechanically cooled at an AHU willcause subcooling, necessitating reheat. If the reheat is limited, then when additional occupants enter theroom, there may be a thermal lag, a delay in the cooled space-temperature sensor detecting the increasein occupancy. This will delay the signaling of the terminal unit to change from its minimum setting andcause ventilation deficiencies.

There are many buildings in which initial operating conditions have been overridden for any number ofreasons and not yet restored. Some consulting firms and organizations emphasize the need forretrocommissioning in these cases. Another option is to increase how “self aware” buildings are in termsof not only energy consumption, but ventilation performance, moisture-management performance, andeven pressure-relationship management.


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In the area of ventilation performance, accuracy is critical. This applies not only to control routines, butthe data on which they rely. If monitoring data for, say, carbon dioxide (CO2) is sloppy and inaccurate,how can control-procedure modifications be any different?

Historically, the primary driver of the use of VAV systems has been energy savings. In theory, fan energy issaved by slowing a system fan; in practice, at low speeds, reduced system-level airflow can make achievingcode-required ventilation at all zones difficult, despite reductions in the amount of supply air provided. Atthe same time, large fans and associated variable-speed drives have limited turndown, so commanding afan to slow further may not produce expected energy savings.

A lot of buildings have and will continue to have VAV systems. Thus, there is a continuing need to improveVAV-system operation and reduce risk and uncertainty in the delivery of ventilation. Several approachesare available.

Techniques for increasing mechanical advantage include selecting more mechanically efficient fans andusing premium-efficiency motors with improved variable-frequency drives. Another approach is to use adirect-drive fan array for improved airflow turndown and accuracy.

Additionally, we can provide control routines that coordinate VAV terminal-unit airflows with occupancyschedules and occupancy sensors to ensure optimal start for room flush, similar to “morning warmup”routines.

Another approach is to heighten building self-awareness with respect to ventilation performance.Fortunately, intelligent monitoring systems designed to help make buildings aware not only of ventilation,but temperature, moisture management, and pressure relationships across building zones and even filtersin AHU are in development.

Smart building controls require additional sensing of temperature, pressure, and occupancy. Forexample, airflow sensors with control routines that coordinate outside-air flow with other requirements,such as cooling, space pressure, and dynamic changes in economizer operation, can be provided inoutside-air intakes to maintain pressure relationships and meet airflow requirements. Of course, theamount of outside air entering a HVAC system can be useful to know, but it indicates nothing about howthe air is distributed and how that relates to the distribution of people. A far smarter approach, fosteredby the U.S. Green Building Council (USGBC) in its LEED (Leadership in Energy & Environmental Design)green-building rating systems and ASHRAE, is to implement accurate CO2 monitoring in spaces with thehighest occupancy to assess the amount of ventilation being provided. A goal of smart building controlsshould be to monitor ventilation performance in enough spaces to determine how well outside air isreaching the breathing zone of all spaces (as opposed to knowing merely how much outside air is enteringthe HVAC system and how much is provided in conference rooms).

VAV terminal units often are linked to occupancy sensors through control routines. What this approachlacks is a feedback loop indicating whether additional ventilation is being provided upon occupancy. Justgenerating a signal that occupancy has occurred is not a fail-safe solution, as there still needs to be


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programming to interpret the signal and instruct the HVAC system to modify its performance. What’smore, the HVAC system needs to be optimized to be able to modify ventilation performance.

Smart building controls and building intelligence in general should go beyond the mere sending of signalsto assessing whether desired operational details are being implemented. A truly smart building needs toinclude diagnostic feedback on “end point” performance as part of its ongoing commissioning capabilities.In other words, it needs to go beyond merely asking whether a signal was generated to determiningwhether an appropriate amount of ventilation is being provided as occupancy varies. ASHRAE’s and theUSGBC’s impetus to provide DCV (often measured with a few local CO2 sensors) is a beginning, but moreneeds to be done.

An aspect of controls frequently overlooked is the quality of sensing data. If buildings are to achieveautonomics, by which controls are aware of key performance parameters, either sensors need to becomesignificantly more accurate or we need to develop ways to collect more high-quality data.

In general, the industry needs to expend the extra effort to develop VAV-system controls that not onlyprovide sufficient accurate data, but optimize tradeoffs between all HVAC operating functions. Suchcontrols are being considered and partly developed.

Fortunately, a design paradigm is emerging to compete with the old VAV model. This new model canreward us with simplified system design, while letting us achieve the increased system efficiencies thatenergy costs are demanding. Next month’s article will review the DOAS/VRF design paradigm in depth.

References

1) Stanke, D., & Bradley, B. (1998, January). The threefold challenge of …ventilating single-duct vavsystems. Trane Engineers Newsletter. Available at https://www.trane.com/commercial/uploads/pdf/673/en27_01a.pdf

2) Murphy, J. (2011, October). High-performance vav systems. ASHRAE Journal, pp. 18-28.

3) WHPA. (2013, March). Economizer experts gather to solve the problem. Retrieved fromhttp://performancealliance.org/Newsletters/March2013/tabid/348/Default.aspx

Grant Bowers, PE, LEED AP, is a mechanical engineer with SSOE Group, provider of architecture,engineering, construction-management, and specialized services worldwide. Over the last 20-plusyears, he has developed expertise in the design of mechanical systems and management of mechanicaldesign and construction projects for manufacturing, cleanroom, laboratory, office, government,medical, and education facilities. He can be contacted at 503-439-8777 or [email protected].

Did you find this article useful? Send comments and suggestions to Executive Editor Scott Arnold [email protected].


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Combining DOAS and VRF, Part 1 of 2

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