Mon, Jun 17, 2019
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Article: Pool Room Dehumidification


What Method is best for me?

If asked about the method of dehumidification used in their indoor pool facility, the majority of respondents would probably have no idea, let alone how it works and what it does. The reality is if an indoor pool facility did not have some means of removing excess moisture, the space would be practically unbearable to use. 

The poolroom environment varies based changes in occupancy as well as external temperature and humidity conditions throughout the year. Outside temperatures affect the inside heat loss/gain but usually resulting in the need for more energy to maintain occupant comfort. The same applies for ventilation, where colder incoming air places an additional load on the heating system.

Most pool facilities need heat a majority of the time for both pool water and air to maintain a typical condition of 82°F water temperature, 84°F air temperature and 50% to 60% RH. Energy consumption is a function of the variables necessary to satisfy the occupant and protect the facility, and this can result in a fairly high cost of operation.These variables include space heating and cooling, water heating, humidity removal and ventilation.

Evaporation of the pool water is another issue that must be carefully managed. Evaporation takes place as the pool water is heated: the higher the temperature, the greater the evaporation rate. It is also influenced by the difference in air temperature and humidity as compared to the water, or vapor pressure. And if that wasn’t enough, the evaporation rate is also increased through agitation. As you increase the water surface area exposed to air, you increase evaporation.

Evaporation removes heat from the water, which causes the pool water to cool.  To maintain water comfort conditions, additional energy is needed to sustain the water’s original set points. Heat energy must be added back into the pool water or it will cool down below a comfortable temperature.

And of course, with evaporation there lies the possibility of condensation formation. Water vapor will return to a liquid as it comes in contact with cold surfaces. In many cases, the condensate includes chloramines, a corrosive byproduct of the chlorine pool chemistry. Chloramines are very destructive, and aggressively attack metallic surfaces including fixtures and structural components.

The big question comes down to ‘What dehumidification system will provide the best management of temperature and humidity at the lowest initial cost as well as lowest operational cost?’

What Types of Equipment are Available?

The most common technologies employed in pool room dehumidification equipment are ventilation-based and/or mechanical refrigeration-based systems. These packaged dehumidification units are built specifically for pools and incorporate more sophisticated mechanical design and software than conventional HVAC systems to minimize the cost of operation and increase reliability.

Ventilation-based dehumidifiers use outside air to remove moisture. The volume of air needed is a function of the amount of moisture to be removed and the moisture content of the incoming air. Mechanical dehumidification systems utilize a refrigeration system much like a standard air conditioner, but with one major exception! They typically incorporate a heat pump cycle capable of reclaiming the heat energy during dehumidification and use it to heat the pool water and/or space.

Ventilation-Based Systems

The most basic dehumidification option is a heating and ventilation system (H&V). Moisture removal is accomplished through the dilution with outside air, and operates under the premise that enough drier, outside air will offset the effects of pool evaporation. This system may or may not have a means of cooling during the hot summer season, especially when the temperature delta between outside and inside is minimal or the outside temperature is hotter.

During colder seasons, an H&V system must raise the inside diluted air temperature from the outside ambient to 84°F, the desired space temperature. Considering typical winter conditions require heating the outside air, the cost of operation could be quite high. Capital cost is the lowest for a packaged H&V system, but the temperature and humidity control during the summer season are not very good with a ±30% variation.

A more cost effective way to increase an H&V unit’s efficiency is the addition of a heat exchanger to recover heat from the exhausted air for tempering the incoming air. Air-to-air heat recovery systems are somewhat passive, and include heat pipe, plate, heat wheel and glycol run around loop designs. Although sensible energy recovery is high, heat is not completely removed from the water vapor, which comprises a large portion of the exhausted air. Space heating requirements can be lowered by 50% but the capital cost premium over standard H&V systems can increase by approximates 25% to 30%, depending on the unit type. In some extremely hot climates, the addition of plate-type or run-around heat recovery unit can also be beneficial during the cooling season.

As noted earlier, ventilation systems will maintain accurate heating setpoints with reasonably good moisture control, especially during colder seasons, but they have no means of cooling below the ambient temperature during the summer. Where chilled water is available, a cooling coil typically sized for the sensible load can be added for limited cooling as well as dehumidification. Temperature and humidity control is more accurate, with ±5% variation from the desired set points, making it ideal for dry, northern climates.

It would appear not cost effective to add a direct expansion (DX) refrigeration system to a ventilation dehumidifier because of the limited hours it operates compared to its first cost, and it will be very similar to the alternative type of system we will be discussing next.

Mechanical Refrigeration Systems

Packaged mechanical refrigeration or direct expansion systems dehumidify by passing the warm, wet air of the pool facility over an evaporation coil where the air is cooled, moisture condensates out and energy is captured. They also incorporate a ventilation system to supply the facility with outside air to meet code requirements.

Mechanical poolroom dehumidifiers offer the best all-season comfort. They will maintain humidity setpoints regardless of the moisture content outside, manage water and air heating, offer cooling during summer seasons and provide ventilation as needed.

Conventional air conditioners reject waste heat to an exterior condenser whereas a pool dehumidification system, using hot gas reheat, reintroduces it to the supply air for space heating. If equipped with a pool water condenser, the waste heat will also heat the pool water very efficiently. This type of system is similar to a conventional heat pump cycle, but it recovers heat from the excess moisture in the poolroom air rather than an outside air or geothermal source. 

The benefit of this approach is the high coefficient of performance (COP), while providing superior control of the poolroom. A heat pump pool dehumidification system heating COP of 5 is not uncommon, which simply means that the system heat output energy is 5 times the input energy. And this is very crucial when you consider a pool facility requires water and space heating most of the year.Capital cost of this system increases over ventilation units because of the addition of a refrigeration system.

Mechanical refrigeration with heat recovery is probably the most common dehumidifier system used for small to medium pool applications due to its control of the space temperature as well as overall good life cycle cost.  These systems are complex and more expensive to install and maintain than ventilation method design, but their ability to recycle energy assures lower operating cost over the life of the system.

Which System Is Right For My Facility?

Indoor pools are complex environments that need constant care and maintenance, and balancing comfort, cost and appearance all have trade-offs. Most, if not all, decisions for specifying dehumidification equipment is completed by engineering professionals, but it is helpful to know what types of dehumidification equipment is available along with the benefits for each.

For simplicity, North America can be divided into three distinct environmental zones when discussing pool dehumidification. There are regions dominated by long, hot seasons and high humidity, long, hot seasons and low humidity and regions where the temperatures tend to be colder with reasonably low humidity. But not only do these regions overlap, longer term weather trends can change entirely from the prevailing conditions, at least for a short period of time.

Regions with long, hot seasons and high humidity

High temperatures and high humidity, especially during the summer months, are quite a challenge for indoor pool facilities. The dewpoint for a typical poolroom is generally 65°F to 70°F.Ventilation systems have limited application in this zone because the dewpoint temperature during summer design conditions can reach 75°F, limiting its ability to dehumidify. Simply put, the outside air at a 75°F dewpoint contains more moisture than the inside air with its lower dewpoint. And since the outside temperature may be warmer than the poolroom, the indoor temperature will climb. Ventilation systems can work effectively during the cooler seasons because the dewpoint lowers considerably.

Dehumidification systems using mechanical refrigeration may be the only practical choice for this region. They will dehumidify during the cooler days, recycling the waste heat back to the pool water and/or air. During the hot, summer months, the waste heat can be rejected, thereby cooling the space.

Regions with long, hot seasons and low humidity

A ventilation-based system can work very well in this environment for dehumidification, but perhaps too well. During dry periods, dehumidification may go below 50% RH, the lower end of the desired humidity range. Evaporative cooling occurs causing a chill to occupants as they exit the pool. Conversely, the air temperature during the peak summer season can easily exceed 100°F, making it very uncomfortable for the pool users.

If chilled water is available from another source, a cooling coil can be installed in the ventilation system sized for the sensible load. It can be used for cooling as well as dehumidification without greatly increasing the cost of the unit.

During peak summer conditions, a mechanical poolroom dehumidification system can take advantage of cooled air, which is the byproduct of dehumidification, and use it for space cooling rather than returning it to the pool water/air. It does this by rejecting the waste heat to an external condenser, much the same as a traditional air conditioning system.

On days when the outside temperature is below the desired indoor space temperature, the compressors can be shut down and ventilation increased for free cooling and dehumidification.

Northern regions

The northern U.S. and southern Canadian region have generally cold winters and short, hot summers. The average annual dewpoint conditions are low, making this region a good candidate for dehumidification using the ventilation method.

When equipped with an air-to-air heat recovery system, space heating can be reduced by up to half as compared to one without during the colder months. Pool water heating must be done independently by a standalone unit. And during summer months, temperature and humidity control are limited, approximately a ±30% variation from setpoint conditions.

A ventilation system is also a good choice if lower first cost is important and variations from summer setpoint conditions are acceptable. Since the cooling season is short, a cooling coil can be installed if chilled water is easily available.

Mechanical dehumidification units, on the other hand, may have higher initial cost but have a lower operating cost during the life of the equipment. Since most pools require heat for a majority of the year, these units can meet the heating requirements without ever using an auxiliary heating system. This is especially significant where electricity costs are lower compared to other fuel sources. But mechanical dehumidification systems offer other methods to reduce the cost of operation in these conditions.

During summer operation, mechanical poolroom dehumidification systems generally require an outdoor condenser for the removal of excess heat once the pool water and air have been satisfied. However, in climates where the evening temperatures are cool, a method called “flywheel” air conditioning can be used to temporarily store the excess heat, eliminating the need for the outside condensers.

In the flywheel air conditioning process, excess heat generated from dehumidification is channeled to the pool water rather than rejecting it outside even thought the water temperature has been satisfied. During the evening hours when the outside temperature drops, the refrigeration systems is shut down and ventilation is used to remove the excess heat generated through evaporation. The daytime water temperature rise is minimal, generally one to 2 degrees, and is quickly removed through ventilation.

Another control strategy available on specific dehumidification equipment that works well in northern climates is “smart economizing”. Unlike regular economizing where outside air is used for cooling instead of the refrigeration system when conditions permit, smart economizing is used to capture the maximum amount of heat when available.

As warm, moist air from the poolroom passes over the evaporation coil, the air is cooled to 55°F and moisture is removed. In a typical heating mode, the waste heat generated from dehumidification would be added back to the cold air to heat the space.

In smart economizing, the controller monitors the outside temperature and humidity level, choosing the outside air when its temperature is higher than the air coming off the evaporator and the humidity is low enough. Rather than reheating the cold, dry air, it is exhausted and replaced with the warmer outside air, which is heated instead. The result is more heat at no additional cost.


It should be clear to you now that a pool dehumidification and environmental control system is much more complicated than commercial HVAC environmental systems. Selecting the right dehumidification method can be very complicated and is best accomplished by using a design professional familiar with pool room applications. The choice of systems usually comes down to the operating cost of the equipment over the long term.

In general, comparing the cost of fossil fuel to electricity has to be weighed against capital cost and long term operating cost just like any other mechanical system. Recognizing that the performance of ventilation equipment with energy recovery is less than mechanical-based systems, the cost of fossil fuel becomes important. Additionally, if high quality servicing capability is not available for the mechanical-based systems, the ventilation system with heat recovery may be your best choice.


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A Word About Chemistry

Pool water chemistry is critical to the proper operation of the pool, with chlorination as the most common pool treatment method. It controls pathogens and organic contaminants introduced into the water by the normal activity of bathers. Under-treated pool water can result in unsanitary conditions while over treated water can facilitate the off-gassing chlorine compounds, such as chloramines, into the air.

Dehumidification/ventilation equipment is not designed to remedy the effects of poor pool chemistry. They are designed to remove excess humidity and deliver prescribed ventilation to manage smaller amounts of pollutants generated from normal pool activity. If left uncontrolled, poorly maintained chemistry will have adverse effects on the structure, equipment, occupants and practically any metallic surface within the facility.

A strong chlorine odor is an indicator of improper pool water chemistry, and is generally offensive to the occupants. The Pool-Spa Operator’s Handbook recommends that the chloramine levels not exceed 0.2 ppm. If the level is above 0.2 ppm, the corrosion process caused by the chloramines in the air is accelerated.

Newer non-chemical sanitization systems such as UV and Ozone are coming on the market and are used in conjunction with chlorine-based systems. These systems can reduce the amount of chlorine needed making it easier to maintain proper chemistry.