DOAS Archives | United CoolAir

February 27, 2019 by Matt Hanson

Choosing a Dedicated Outdoor Air System (DOAS)

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 methods used to help regulate temperature and humidity. Schools, hospitals, offices, and other commercial spaces can all benefit from a DOAS unit. 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.

Although DOAS units are not the primary source of cooling, they help the main HVAC unit work more efficiently. 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.

How to Get Started

Specifying a DOAS unit usually begins with selection software. This takes the basic parameters of the installation and focuses on the specific needs and requirements for fresh air. 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. What this does is reduce the load on the primary AC unit that will be used in cooling the space. Ultimately, this leads to greater efficiency and better indoor air quality with a lower overall cost.

Why Install Equipment Indoor?

Many HVAC manufacturers provide outdoor rooftop equipment as DOAS units, but occasionally a customer might require an all indoor packaged unit. This is the case when the building topography does not allow for outside mounting. Furthermore, codes that affect historic buildings often don’t allow for HVAC equipment to be installed outdoors. This is the case even if there were ample room on the roof or outside. The indoor packaged solution typically realizes a smaller footprint. This aides in new installations as well as for 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. Some examples would include 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.

By removing outdoor contaminants before they enter the building the indoor air quality will be greatly improved. Additionally, this will allow the main cooling unit to run more efficiently.
The dehumidification process removes moisture from the entering air which helps prevent the formation of mold. These elements can negatively affect the health of the building occupants.
Energy savings are also 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.

September 14, 2018 by Matt Hanson

Dedicated Outside Air Systems (DOAS) part 3

DOAS and VRF

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

Japan was the first to introduce the variable refrigerant flow technology in the 1980s. This was for a flexible way to condition spaces without the complexity of large HVAC systems. Furthermore, they did not 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. Other benefits are minimal maintenance, and the ability to run multiple evaporator units from a single condensing unit. With all its benefits, VRF systems have limited to no ability to satisfy ASHRAE ventilation requirements. Also, these systems cannot remove excess latent loads typical of certain geographical regions.

With rising energy costs and expanding energy consumption awareness, it’s no wonder VRF technology continues to popularize 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, estimates indicated the U.S. Market 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 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.

DOAS Are Not all Built Alike

Dedicated Outside Air Systems is 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. DOAS is ideal 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 installation of original HVAC systems in one of many configurations, require 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. Because they designed their units with special features to overcome even the most perplexing installation challenges.

United CoolAir’s DOAS design, like their entire line of air conditioning equipment, is 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. Unit sizing to accommodate floor-by-floor installations and multiple units are easily installed if more cooling capacity is needed. Easy mounting of air-cooled condensers indoors near an outside wall for waste heat removal. Additionally, a water-cooled condenser option is available if using a water tower.

One Size does not Fit All

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 break down into sections that fit through standard doorways, halls, and elevators. Lifting units by crane and modifying the building structure to accommodate equipment becomes a thing of the past. Additionally, this saves 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. The refrigerant couplings reattach during job site assembly and are ready for immediate operation. The elimination of Brazing, recharging, and testing saves additional time and money.

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

April 13, 2018 by Matt Hanson

Combining DOAS and VRF as an HVAC Solution

For decades, using Variable-air-volume (VAV) systems with air terminal units developed extensively in commercial/institutional buildings in the United States. Unfortunately, the optimized design of a VAV system with terminal heat is difficult at best. Mostly 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 that of maximizing thermal comfort, we avoid conflicting situations forcing 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.

In the simplest VAV system, mixing incoming outside air and return air in a central air-handling unit (AHU) and then pre-heated or pre-cooled occurs. For each zone, a terminal unit adjusts airflow based on cooling demand. When a zone requires heating, reducing supply-air flow to a minimum setting and heated, typically via a terminal-unit heating coil is the solution.

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, sizing does not 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 serve multiple zones. The arrangement of VRF systems provides energy recovery, moving heat from zones requiring constant cooling to zones that sometimes require heating.

Meeting Industry Standards

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, this requires some systems operations 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 maximization, the system-level intake/exhaust/relief-damper-reset sequence is set for space-temperature control. This aide in the coordination of building pressure with an air-economization-reset sequence. Such controls can are quite complex, particularly when considering pressurization between zones and interaction with exhaust systems requiring variable-volume control. 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. It can also include (“critical terminal unit” control), “ventilation optimization”1 (DCV), ventilation space-temperature setback, supply-air-temperature reset. Finally, included are 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.

Meeting Zone Requirements

Further complicating VAV-system design is the need to follow the ANSI/ASHRAE Standard 62.1 ventilation-rate procedure. This standard 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 flow changing follows 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, they often apply combining zone-level DCV with system-level ventilation reset. Ventilation reset is a control scheme by which the MSE is solved dynamically to change the system outside-air setpoint. This application with airflow-measuring stations, along with electronics and software to control dampers based on relative airflow, happens at the central system.

Maximizing Energy Efficiency

Central-system design usually is based on the static-condition (design) critical zone. While in the real world, the 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. This saves 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 as 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 in a single zone. This often leads to discomfort in certain rooms when other rooms in the same zone are not at the design load.

To Sum it Up DOAS and VRF

In summary, designing a VAV multi-zone HVAC system can be challenging to for DOAS and VRF, 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. However, technical solutions partly exist and are being developed. 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. The result is undersized or oversizing systems.

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