United CoolAir

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Choosing a Dedicated Outdoor Air System to fill fresh air needs

In order to comply with current strict ventilation standards in the HVAC industry, manufacturers’ are introducing innovative new equipment to improve indoor air quality. Dedicated outdoor air systems (DOAS) are one such method used to regulate temperature and humidity in schools, hospitals, offices, and other commercial spaces. As these systems become more popular throughout the USA and Canada, technicians and end users need to understand the operation and importance of these units.

DOAS units are not the primary source of cooling but are often used in conjunction with other HVAC equipment. These units function by bringing in fresh air independently of other cooling and heating equipment. This pretreated air will ultimately save fan energy on the main HVAC unit and improve the overall indoor air quality. These units can be air-cooled, water-cooled and chilled water and may include variable capacity compressors.

Specifying a DOAS unit usually begins with selection software, which takes the basic parameters of the installation and then focuses on the specific needs and requirements for fresh air to be introduced. The software will calculate the performance expectations and recommend additional modifications to the basic package. A variety of fresh air units handle a range of CFM and tonnage and include options such as water-source heat pumps and modulating hot gas reheat. Other fresh air units could include direct expansion with an energy recovery enthalpy wheel. The unit should be able to handle the large latent ventilation load and deliver “neutral” air of 72°F to 75°F @ 50% RH. By separating the sensible load from the latent load, the unit will remove moisture from the primary source of fresh outside air. This, in turn, reduces the load on the primary AC unit that will be used in cooling the space and leads to greater efficiency and better indoor air quality with a lower overall cost.

Many HVAC manufacturers provide outdoor rooftop equipment as DOAS units, but occasionally a customer might require an all indoor packaged unit when the building topography does not allow for outside mounting. Additionally, codes affecting historic buildings may not allow for unappealing HVAC equipment to be installed even if there were ample room on the roof or outside. This indoor packaged solution typically realizes a smaller footprint and can be used for new installations as well as retrofitting as part of a VRF or conventional DX air conditioning project.

Ideally, the performance of your DOAS unit should meet or surpass the ASHRAE 90.1 standards of ISMRE 11.1 and ISCOP 8.0 and adhere to a sound rating of less than NC-40. Dedicated Outdoor Units also include options such as the installation of MERV 8 or 13 filters. Units can also include special coil coatings to prevent rust and corrosion when installed in harsh conditions such as marine or paper mill applications. Lastly, units may also include a manual on/off override switch and meet the power requirements of 208/230 single or three phase or 460/3 phase power.

When calculating the need for a DOAS unit it is important to understand all the benefits that can be realized with this type of installation.

  • By removing outdoor contaminants before they enter the building the indoor air quality will be greatly improved and will allow the main cooling unit to run more efficiently.
  • Much like the operation of a dehumidifier, moisture is removed from the entering air, which can prevent the formation of mold growth that affects the health of the building occupants.
  • Energy savings can be achieved by separating the ventilation requirement from the main cooling or heating unit.

Dedicated Outdoor Air Systems have become an integral component to an overall approach to HVAC installation. As concerns about climate change drive governments to enact more stringent energy requirements in the manufacturing process, DOAS units will continue to grow in importance. Ongoing developments will ensure that these types of units will become even more efficient and used well into the future.

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Dedicated Outside Air Systems (DOAS) part 3

DOAS and VRF

The choice of installing a DOAS, especially if the existing HVAC system already includes provisions for OA, is for operating efficiencies and the need to meet ventilation code requirements or occupant comfort. But we are now seeing new cooling technology being specified that has no provisions for OA and requires a dedicated outside air system for this purpose. One of these systems is the variable refrigerant flow (VRF) system.

Japan introduced the variable refrigerant flow technology in the 1980s as a flexible way to condition spaces without the complexity of large HVAC systems that require bulky mechanical rooms and expensive ducting. As technology improved, the American market embraced VFR systems but to a lesser degree.

Today’s VRF units offer the building owner/manager many benefits including low first cost, simple installation, minimal maintenance and the ability to run multiple evaporator units from a single condensing unit. But with all its benefits, VRF systems have limited to no ability to satisfy ASHRAE ventilation requirements or remove excess latent loads typical of certain geographical regions.

With rising energy costs and expanding energy consumption awareness, its no wonder variable refrigerant flow technology continues to gain ground in the U.S. marketplace. VRF has the potential to achieve significant energy savings compared to older HVAC systems, according to a study by the U.S. General Services Administration.

Reports going as far back as 2012 indicated the U.S. market was estimated to be worth over $7 billion for VRF systems. More recent estimates of the U.S market indicate a compound annual growth rate of 5.2 percent is expected through 2019. You can expect the sales of DOAS systems will grow as much if not better to match the upcoming surge in OA units.

Not All DOAS Systems Are Built Alike

DOAS systems are an ideal choice for new construction

Retrofit installations add to your existing HVAC system to improve performance. Traditional HVAC roof-top systems require adequate space for the air handler location and ducting large enough to handle both the ventilation/supply and return air. A DOAS is an ideal unit to handle the latent load requirements for OA requirements and dehumidification while the existing HVAC system manages the sensible load.

A DOAS system is ideally suited for retrofit applications where the new HVAC system (air handler and DOAS) must work within the confines of the existing space. The original HVAC system could have been installed in one of many configurations, requiring the design engineer and contractor to manage challenges as needed. Some manufacturers of DOAS units like United CoolAir Corporation have taken this serious problem into account and designed their units with special features to overcome even the most perplexing installation challenges.

United CoolAir’s DOAS, like their entire line of air conditioning equipment, is designed specifically for indoor installation. This feature is extremely important for multi-story buildings that have limited or no access to the roof or ground pad. Units are sized to accommodate floor-by-floor installations, and multiple units are easily installed if more cooling capacity is needed. Air-cooled condensers are easily mounted indoors near an outside wall for waste heat removal, or a water-cooled condenser option is offered if tower water is available.

Unlike the “one size fits all” approach by some manufacturers, the United CoolAir outside air system is made specifically for each project. Vertical configurations are ideal for small mechanical rooms, including closets while horizontal styles offer in-ceiling mounting, saving valuable floor space.

Having the ability to customize your DOAS unit to fit your installation requirements can save thousands of dollars in contractor charges and shorten installation time. United CoolAir’s OmegaAir II allows modification in air paths, component configuration, and utility placement before leaving the factory. They also offer a comprehensive list of factory-installed options to meet even the most complex cooling requirements.

Retrofit applications generally have limited access to the job site. United CoolAir’s indoor, customizable DOAS units can be broken down into sections that fit through standard doorways, halls and into elevators. Lifting units by crane and modifying the building structure to accommodate equipment becomes a thing of the past, saving time and thousands of dollars in building modifications.

United CoolAir charges and tests each of their units before leaving the factory. DOAS units include resealable refrigerant couplings between separable sections to preserve the factory refrigerant charge. Refrigerant couplings are reattached during job site assembly, and ready for immediate operation. Brazing, recharging, and testing is eliminated by saving additional time and money.

Dedicated outside air systems are as necessary to the safe and efficient cooling and dehumidification operation in commercial buildings as their HVAC cooling counterparts. It is important to investigate the many systems available to find a DOAS that best fits your unique retrofit installation requirements.

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Combining DOAS and VRF

Variable-air-volume (VAV) systems with air terminal units have been used extensively in commercial and institutional buildings in the United States for decades. Unfortunately, the optimized design of a VAV system with terminal heat is difficult at best because of limitations inherent in VAV and complications posed by design standards and regulations. One new approach involves the pairing of a dedicated outdoor-air system (DOAS) with a variable-refrigerant-flow (VRF) system. By separating the goal of achieving ventilation rates from the goal of maximizing thermal comfort, we can avoid situations in which the two goals are in conflict and efforts suffer from the resulting compromises. What’s more, we can simplify the design process and find system efficiencies that go far beyond those commonly achieved with VAV systems with 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 a supply-air fan to various occupiable zones at a temperature generally suitable for cooling. In each zone, a terminal unit adjusts airflow based on cooling demand. When a zone requires heating, supply-air flow usually is reduced to a minimum setting and heated, typically via a terminal-unit heating coil.

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

VRF systems use individual high-efficiency fan coils in interior spaces in combination with high-efficiency condensing units that can serve multiple zones. VRF systems can be arranged to provide energy recovery, moving heat from zones requiring constant cooling to zones that sometimes require heating.

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

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

Other common control sequences for large VAV systems include occupied-hours control, optimal start/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, smoke detection, fire alarms, and even security systems. The interplay between these control schemes vastly complicates efforts to optimize building energy use.

Further complicating VAV-system design is the need to follow the ANSI/ASHRAE Standard 62.1 ventilation-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 are changed to meet zone-level requirements. Once the critical zone has been chosen, heating-turndown requirements push designers to over-ventilate some zones by increasing heating minimum settings. This typically 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 with system-level ventilation reset, often is applied. Ventilation reset is a control scheme by which the MSE is solved dynamically to change system outside-air setpoint. This often is applied with airflow-measuring stations, along with electronics and software to control dampers based on relative airflow, at the central system.

Central-system design usually is based on the static-condition (design) critical zone, while in the real world, 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 in terms of building system design and building-operation hardware and software. Some commercial software packages can aid ventilation reset, calculating critical ventilation zone per ANSI/ASHRAE Standard 62.1. Calculating the minimum system outside air, however, usually requires the overriding of some terminal-unit turndowns, which changes the critical zone. This is allowed by ANSI/ASHRAE/IES Standard 90.1 because only a few zone overrides can change a system outside-air setting by many percentage 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. At the same time, other overrides, such as of supply airflow for makeup to areas with exhaust flows exceeding ASHRAE minimum rates, are necessary. These overrides and simultaneous variations result in the need for iterative calculations involving all environmental variables; such calculations currently are beyond the scope of commercially available software.

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

In summary, designing a VAV multi-zone HVAC system can be challenging to say the least. Overlaying the various requirements, exceptions, and system functions results in iterative design simulations that must be re-run whenever a room’s size changes. Complying with all codes while addressing the competing interests of ventilation and energy optimization is theoretically possible,2 and technical solutions partly exist and are being developed. But in practice, designing and redesigning systems with complex, iterative calculations 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 for ventilation and space pressure control, significantly undersizing or oversizing systems as a result.

Fortunately, a design paradigm is emerging to compete with the old VAV model. This new model can reward us with simplified system design while letting us achieve the increased system efficiencies that energy costs are demanding.

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