Energy-saving technological advancements renew interest in outdoor air ventilation systems

March 1, 2014

Sept. 17, 2012-Setting Seresco NV-008 at Wulf Rec Ctr-Evergreen 010[1]
Instead of a mechanical dehumidifier, an outdoor air-based ventilation-only system (OAVOS) was installed at this aquatic facility because the area has a dry, cooler climate that can maintain comfortable indoor pool conditions nearly year-round.

By Ralph Kittler

Recent technological advancements in dehumidifier design are now being applied to 100 per cent outdoor air-based ventilation-only systems (OAVOS) for environmental control in natatoriums, and as a result, many facility operators are considering this equipment again for conditioning indoor swimming pool environments.

An indoor swimming pool environment can be precisely controlled for much of the year using an OAVOS approach. This method is not suitable for every facility, however, as there are certain times of the year (mild weather and summer) when the space conditions might get more hot and humid than could be desirable. For facilities where this period of time is short or where the patrons would not mind elevated conditions during the warmer weather, this is definitely an option again.

When the weather is drier and colder, a well-designed OAVOS can control the space conditions quite precisely. It is important to properly model a system’s year-round performance and establish what the local climate can deliver and whether or not it is acceptable for the operator and patrons.

If the indoor conditions cannot be maintained in an acceptable range during milder/warmer periods of the year, the facility operator would need to use a compressorized mechanical dehumidifier to control their space precisely year-round at the conditions desired. Until fairly recently an OAVOS had high operating costs and did not provide stable space conditions, even in cold weather.

An early conditioning method

Long before the development of modern compressorized dehumidifiers in the ’70s, indoor swimming pools were conditioned using simple, 100 per cent OAVOS. These systems generally moved a constant volume of outdoor ventilation air through the pool area year-round. Most did not modulate the amount of outdoor air, nor did they recovery any energy from the exhaust air. As a result, there were dramatic variances in space conditions in addition to seasonal and high energy costs.

For instance, during this time a typical indoor swimming pool would have had a space too dry in the middle of winter and too warm and humid than patrons would have liked in the summer. As energy prices increased, heating large amounts of outdoor air during winter operation started the move towards more energy-efficient alternatives to controlling indoor pool conditions. Thus, the compressorized dehumidifier was born.

A compressorized dehumidifier provides precise, year-round temperature and humidity control, while even contributing to pool water heating. While the initial cost for these systems was higher, their energy savings and precise condition control capabilities made them the product of choice for indoor swimming pools.

Standards change over time and now natatorium designers are seeing building codes calling for increased amounts of outside air as a minimum requirement to providing the best possible indoor air quality (IAQ). With this trend of increasing the volume of outside air in a facility, people have been looking at OAVOS again.

Indoor swimming pool environments are typically maintained at 27 to 29 C (82 to 85 F) and 50 to 60 per cent relative humidity (RH). There may not be too many days during the summer in Canada where the outside air is not able to control the space.

An important tool and critical first step to help establish whether or not an OAVOS is a consideration is the ability to model the year-round space conditions. Software has been developed by manufacturers that can model what the space conditions would be throughout the course of a year using local weather data. This information will help a facility operator decide if this approach to environmental control is something to consider.

Natatorium environmental control has seen many technological developments in recent years, including chloramines source capture removal, waterside ultraviolet germicidal irradiation (UVGI), exhaust air heat recovery, dedicated-duty, direct-drive fans, and microprocessor operational control and monitoring. Depending on the system, it may even have Internet connectivity. These new innovations have been game-changers for the outdoor air ventilation approach.

Perhaps the most notable air comfort and efficiency development for the OAVOS approach has been the introduction of modulating controls. These controls monitor indoor and outdoor air conditions precisely and introduce only the required amount of outdoor air to maintain the best possible IAQ. Before these controls were developed, natatoriums with an OAVOS provided more outdoor air than necessary during the winter, which resulted in indoor RH levels to drop too low. Low RH levels create an uncomfortable chilling effect on wet skin and also raise operational costs. Bringing in more outdoor air than needed means more outside air must be heated and the pool water heating requirements also increase dramatically.

New natatorium projects as well as heating, ventilation, and air conditioning (HVAC) retrofits must decide between OAVOS and mechanical dehumidifiers. Computer modelling can help determine the level of comfort an OAVOS will maintain throughout the year and in seasonal weather conditions common to the natatorium’s respective geographical location. Armed with the best information computer modelling can provide, a facility operator can then make the best business decision for occupants based on comfort and economy trade-offs.

Modulating outdoor air via OAVOS

An OAVOS generally ranges in size between 113 to 1982 cubic metres per minute (CMM) (4000 to 70,000 cubic feet per minute [CFM]) and consist of many important components, which help minimize operating costs, while still providing the best possible space control.

The biggest problem with the old OAVOS was the constant volume of outdoor air as it rendered them expensive to operate, while over drying spaces in colder weather. Newer systems, however, closely monitor the indoor and outdoor conditions and introduce only as much outdoor air as is needed to control the indoor conditions at the set point levels.

This control strategy, facilitated by advances in microprocessor capabilities and better sensing devices, has a dramatic impact on lowering the operating costs, while providing more comfortable conditions for patrons in cold weather. This type of control was not previously available.

An effective OAVOS will introduce only as much outside air as is needed to provide the best possible space conditions at the lowest possible operating conditions.

Heat recovery

All commercial buildings must bring in outdoor air as mandated by local building codes. These codes are generally based on the recommended values from American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE)[2], Standard-62, Ventilation for Acceptable Indoor Air Quality (IAQ). The Centers for Disease Control (CDC) has also recently published some suggested outdoor air requirements in its Model Aquatic Health Code (MAHC), which are substantially higher than ASHRAE values. Regardless of which is used, heating cold outdoor winter air to 27 to 29 C (82 to 85 F) is very expensive.

ASHRAE guidelines also recommend all natatoriums be maintained at a slight negative pressure. While moisture will always try to pass through the building envelope due to a pressure differential (it is critical an effective vapour barrier is in place as a preventative), maintaining negative space pressure assures potentially damaging pool air with inherent moisture and chemicals is not pushed through the natatorium’s walls or doors leading to the building’s non-pool areas.

Consequently, the amount of exhaust air must always exceed the HVAC system’s outdoor air. The exhaust air is humid and warm, making it extremely energy rich and ideal for energy recovery, which can be used to preheat the code-required outdoor air. This can reduce outdoor air heating costs by 60 to 75 per cent.

Most natatorium HVAC systems and equipment can easily use heat recovery because the outside air and exhaust air streams are in close proximity. This makes accomplishing heat recovery easy on just about any project.

A--NV Unit Indoor Pool Ventilation unit by Seresco Tecnologies[3]
Ventilation units using a glycol runaround loop (GRAL) have a considerable smaller footprint than plate heat exchanger style pool ventilation units. The former might make good retrofit systems since they are able to fit into smaller spaces.

Another recent technological advancement is the development of a glycol runaround loop (GRAL) for heat recovery. The GRAL uses corrosion protected fin/tube coils and circulates a glycol fluid for heat exchange between exhaust and outdoor air streams. It is a simple and highly efficient form of heat recovery (compared to other methods) and has several additional advantages. The coils, for instance, which manage the condensate quite effectively, are compact, resulting in equipment with an extremely small footprint. The smaller unit size makes it attractive for retrofits and a valuable space-saver in new construction projects.

Further, relative to other forms of heat recovery, the coils are lightweight, can be cleaned and serviced easily, and are fully corrosion protected. Finally, the low-energy requirement for the heat recovery loop’s fan makes the overall annualized system efficiency higher compared to other forms of heat recovery.

Chemical source capture devices and UV

HVAC systems using 100 per cent ventilated air are not immune to air quality issues. To ensure optimum IAQ, the following design criteria should be provided regardless of the humidity control system being considered:

Chloramines are byproducts of chlorine (Cl) molecules that attach to contaminants in the swimming pool water. Chloramines, especially trichloramines, are heavier than air and hover just above the water surface in the swimmers’ breathing zone. Chloramines are also suspected in respiratory ailments associated with swimming pools such as Lifeguard Lung (a respiratory ailment caused by prolonged exposure to chloramines in natatoriums).

To combat this, a source capture/exhaust plenum-type device was recently developed to draw and exhaust chloramines directly off of the swimming pool surface. The exhaust air from the source capture/exhaust system can be ducted directly outdoors or exhausted through a dedicated air path in the OAVOS.

Another great benefit of the glycol runaround loop heat recovery method mentioned earlier is the exhaust air heat recovery coil can be mounted on the unit or remotely, allowing energy to be recovered from the exhaust air regardless of where the airstream is located. There is no energy penalty for using this source capture/exhaust technology. In fact, its effectiveness is currently being considered by code officials so indoor swimming pool facilities employing a source capture/exhaust system can operate at lower minimum outdoor air requirements than other natatoriums.

The chloramines source capture/exhaust system performance can be optimized when located at the swimming pool’s edge, nearest the natatorium’s return air grille, which helps draw chloramines toward the evacuation system.

Another option to fight chloramines as well as other biological contaminants that foul swimming pool water is the use of a UVGI system. Ultraviolet-C (UV-C)[4] radiation can sanitize pool water, air and surfaces, and reduce chlorine use. It works by destroying the deoxyribonucleic acid (DNA) of algae, bacteria, cysts, and viruses as the pool water circulates past a UV lamp. The highly concentrated electromagnetic energy destroys organic matter and eliminates the formation of chloramines.

UVGI is a trusted form of sanitizing. Many large metropolitan areas use it to disinfect drinking water, while various waterparks also use it to sanitize water as per National Swimming Pool Foundation™ (NSPF™) standards. A registered sanitizer (e.g. chlorine or bromine [Br]) must be used in conjunction with the UV system (to sanitize water outside of the UV system chamber/piping) as recommended by health authorities in Canada and the United States.

The bottom line is, however, these methods help improve IAQ by reducing chloramines and lessening the need for 100 per cent outdoor air at all times, which saves energy. Instead, outdoor air can be reduced to levels recommended by ASHRAE.

Dedicated-duty, direct-drive fans

Aa-Direct Drive Plenum Fans v.1[5]
Direct-drive plenum fans with variable frequency drives (VFDs) are an example of an emerging technology being used for dehumidifiers that is now also making outdoor air-based ventilation-only systems (OAVOS) more attractive efficiency-wise.

The introduction of dedicated-duty, direct-drive exhaust fans and direct-drive plenum fans with variable frequency drives (VFDs) is another example of an emerging technology being used for dehumidifiers that is now also making an OAVOS more attractive efficiency-wise.

The dedicated-duty approach only operates as many fans as needed compared to systems with two large fans operating at full airflows. The supply fans deliver a constant volume of air 24-7; however, since the amount of outdoor air needed varies with weather conditions, this approach ramps up and down the amount of exhaust fans in use in concert with the amount of outdoor air needed to control the indoor space conditions. This also dramatically reduces energy use by the fans.

A-Direct Drive Plenum Fans[6]
Plenum fans are different in that they deliver air more efficiently than centrifugal-style fans, which are typically found in traditional dehumidifiers.

Plenum fans are different in that they deliver air more efficiently than centrifugal-style fans, which are typically found in traditional dehumidifiers. Compared to traditional belt-driven fans, the direct-driven approach provides greater efficiencies with significant energy reductions and reduced maintenance.

A direct-drive plenum fan with VFDs can amount to as much as 15 per cent in fan motor energy reduction. Considering OAVOS supply fans operate 24-7, the savings over the equipment’s lifecycle are significant. The payback is instantaneous since direct-drive plenum fans with VFDs are comparable in price to belt-driven systems.

For example, a 906-CMM (32,000-CFM) OAVOS operating 24-7 with an electricity rate at 0.095 cents per kWh, direct-drive plenum fans would save approximately $3,200 annually versus using a belt-driven, centrifugal blower. When adding the on-demand exhaust fan operation to the savings, a natatorium would be looking at approximately $6,000 to $8,000 in fan energy savings compared to using older technology. This is a substantial sum over a 20-year period.

While this technology has been available for years, it was only recently introduced into the indoor pool HVAC market and still not all manufacturers offer them. The age-old concept of fan belts connecting the fan motor to the blower requires more maintenance such as regular adjustments and fan belt replacements. Conversely, the direct-drive method connects the motor directly to the fan shaft, thus minimizing maintenance and noise, as well as friction and power transfer inefficiencies associated with belt drives.

When VFDs are added to direct-drive technology there is potential for even more efficiency with the ability of ramping up or down the fan speed. Adjusting the fan speed via a VFD also adds more flexibility for air balancing.

Another potential advantage VFDs offer is the ability to ramp down the plenum fan speed during off-peak hours when less overall supply airflow might be a consideration.

Remote monitoring for optimum dehumidifier performance

Technological improvements do not deliver their full savings unless they operate at optimal performance levels for which they were designed. Like all technology, controls, dampers, fans, and sensors on an OAVOS need oversight and fine-tuning for minimizing operating costs, while maximizing efficiency and air comfort.

A factory-trained technician should ideally be servicing and adjusting the system a few times a year to assure parameters such as RH, temperature, and other operating conditions are within an efficient range.

The aforementioned outdoor air modulation, for example, is critical to a natatorium’s air comfort and energy usage. A facility operator who is not aware that outdoor air and humidity were being inefficiently controlled could result in compromised air comfort. Further, less efficient use of fans, air modulation, or space heaters can cost a facility hundreds or even thousands of dollars in energy costs before it is discovered.

The expense of periodic maintenance checks by a factory-trained technician might seem high enough that facility operators may be inclined to let things slide, especially for facilities with limited budgets. This is not the best approach since the risk of higher operating costs and repairs makes a proper service and maintenance program a good investment. Thanks to recent technological advancements, support has never been better and service costs have been dramatically reduced. Manufacturers have developed an on-board monitor/control microprocessor that can send all of the OAVOS vital operating statistics to their factory via the Internet. These manufacturers offer a free daily monitoring service and even have smartphone applications where an authorized user can access a unit’s current and historical performance statistics from a remote location. The manufacturer can alert the facility operator of any issues and help the local service contractor set-up and adjust the unit to ensure optimum performance. In the event of an alarm, troubleshooting can be assisted by a factory engineer, which ensures a quick resolution to any problem.

An OAVOS can be a viable option if the geographical climate delivers acceptable conditions. Only proper computer modelling will provide the information upfront whether or not this is an approach an operator can consider. Advancements in technology have dramatically reduced the operating costs while providing excellent cold weather space condition control. However, knowing how the facility will perform in the warmer weather is a vital part of making an informed decision as to what approach a facility operator would like to consider for conditioning a natatorium.

The modern dehumidifier’s advantage is precise space temperature and humidity control. These systems mix outdoor air—as well as recirculated indoor air—that is mechanically cooled and dehumidified to provide optimal indoor air comfort year-round.
Using refrigerants and compressors, much like modern air-conditioning, but on a larger scale, indoor pool dehumidifiers maintain a cool coil that wrings the moisture out of the air as it passes through to maintain an optimal 50 per cent relative humidity (RH). If the space needs cooling, the refrigeration cycle also provides air conditioning.
These systems also use the compressor waste heat to contribute to space heating to provide considerable energy savings.



Kittler_HeadshotRalph Kittler, P.Eng., is co-founder and vice-president of sales and marketing of Seresco USA in Decatur, Ga., a subsidiary of Seresco Technologies Inc., an Ottawa-based manufacturer of natatorium dehumidifiers and outdoor air ventilation only systems (OAVOS). He has 25 years of experience in the heating, ventilation, and air conditioning (HVAC) industry and a degree in mechanical engineering from Lakehead University in Thunder Bay, Ont. Kittler recently produced a free ‘Professional Development Hour’ (PDH) video available at, which targets the continuing education requirements for engineers, but also serves as an invaluable primer of indoor pool design and operation basics for facility managers. He can be reached via e-mail at[7].

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  2. American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE):
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