Dehumidifying and cooling large indoor pools, or natatoriums, can be quite a substantial challenge because of the heavy sensible/latent heat load placed on refrigeration equipment.

Most dehumidification equipment does a reasonably good job of recycling waste heat to the pool water and space to supplement heating costs, rejecting the balance to an external condenser when operating conditions have been satisfied and space cooling is required. One dehumidification equipment manufacturer found a way to accomplish both tasks while actually lowering equipment and operating costs.

PoolPak International (York, Pa.) has dubbed this environmental control process "flywheel air conditioning." The process is not a mechanical feature added to an existing refrigeration system, but rather a control strategy designed to utilize existing components to remove heat without the need of an external condenser. Simply explained, flywheel air conditioning uses the thermal storage capacity of the swimming pool to hold excess heat created during air conditioning of an indoor pool facility.

During occupied times, the PoolPak unit is designed to cool natatorium air by removing the sensible and latent heat from the air, placing it into the pool water. The pool water's temperature is allowed to raise a maximum of 2 degrees F above its normal set point to avoid bather discomfort.

A proprietary microprocessor control software is designed to maximize environmental conditions while keeping operational cost to a minimum. According to PoolPak, the cost advantages of utilizing flywheel air conditioning have proven to be quite substantial, translating into as much as 10 percent on a new installation.

This, the company said, is accomplished by eliminating the need for an outdoor condenser because heat removal is achieved through the pool water condenser. Additional first-cost savings are realized by eliminating contractor time, copper piping, wiring, and refrigerant associated with the condenser installation.

Daily operational savings are also significant, said the manufacturer. A controller is designed to sense when outdoor conditions are cooler than the inside, especially during summer months.

The PoolPak dehumidification system switches modes from mechanical cooling to economizer cooling, eliminating the need for the refrigeration system. The economizer not only cools the space, said the company, it also removes heat stored in the pool water through flywheel air conditioning.

How It All Began

The origin of flywheel air conditioning was the brainchild of Ed Hill, a representative for PoolPak in its Atlanta region. In the early 1980s when air conditioning was not yet a feature on PoolPak dehumidification systems, Hill experimented with an existing PoolPak installation, where he manually set the operating conditions to achieve space cooling. He allowed the dehumidification unit to mechanically cool the space during the day, recycling waste heat to the pool water.

In the evening, Hill reset the system in the 100 percent ventilation mode, designed to allow excess pool water heat to dissipate through evaporation. The pool water temperature was measured both morning and evening over a period of weeks and his results were surprising.

Hill's experiment showed that on moderately hot days with cooler evenings, his strategy could cool the space sufficiently. But during hot, muggy days followed by hot nights, the strategy didn't work as well. His cooling strategy warranted additional investigation, especially for cooler climates and it was adapted into control software, which is the key to its successful operation.

Flywheel air conditioning is most effective in geographic locations where evenings are generally cooler than daytime temperatures. This is more prevalent in northern U.S. climates, such as Michigan, Oregon, and Pennsylvania. Southern cities like Atlanta may not have a high enough percentage of cooler evenings to justify its operation.

When outside air is cooler than the air in a pool that is enclosed, and the space requires cooling, PoolPak units will not use mechanical cooling but will utilize the economizer mode for cooling. During these times, the space is being cooled without heat being put into the pool water.

Many Factors Involved

There are a number of other factors that influence the efficient use of flywheel air conditioning.

Pool size is among the most important issues. The greater the pool's water mass, the more heat it can hold without significantly increasing pool water temperature, said the manufacturer. The average pool tank depth is about four feet, which is sufficient for normal flywheel operation. Deeper pools will hold more heat without a significant temperature change while shallow pools have less capacity for heat storage.

Another factor to consider for the successful operation of flywheel air conditioning is surface area. The greater a pool's surface area, the higher its evaporation rate. A large surface area allows the pool to get rid of its heat faster than a small pool surface area, thereby cooling the water back to its set point after a day of mechanical cooling, the company said.

External sources of heat must be taken into account. Natatoriums with high internal or solar sensible heat gain hinder flywheel air conditioning from working at its highest capacity. This is very common with a facility that has many skylights and windows on the south, west, or east walls. The additional solar heat gain can significantly increase the sensible heat load, making it difficult to cool using flywheel air conditioning, said the company.

Consideration must also be given to the color of the pool bottom. A pool tank with a dark-colored bottom will tend to trap solar heat more than a white bottom and thereby increase the pool water's thermal load, said the manufacturer.

Heat removal from pool water can also be accelerated under certain conditions. As an example, if the pool is heavily used by swimmers while flywheel air conditioning is cooling the air and placing heat into the pool water, surface water turbulence caused by swimmers will increase the evaporation rate and accelerate heat removal from the pool water.

Conversely, heat dissipation will be slowed if the temperature differential is not great enough. Pool water set point temperatures typically fall between 80 degrees to 84 degrees, and a pool maintained at a temperature of less than 80 degrees is considered cool. Since cool water does not evaporate as rapidly as in a warm pool, it will take longer for heat removal when the system is operating in the economizer mode, said PoolPak.

Flywheel air conditioning has proven successful at the natatorium in West Ottawa High School (Holland, Mich.) due to proper insulation levels and architecture that makes use of very little glass.

Success Stories

Flywheel air conditioning has been successfully adapted to many installations across the country. Jim Wedeven, P.E., of Design Plus - a full-service architectural engineering firm based in Grand Rapids, Mich. - has used this technology in a number of school pool installations.

Two of these installations are at the natatorium at Howell High School in Howell, Mich., and the natatorium at West Ottawa High School located in Holland, Mich.

"Flywheel air conditioning works well with our regional climate because even on heavy pool usage days, the natatorium's temperature doesn't start to climb until late in the afternoon or early evening," said Wedeven.

"Typically, most Michigan bathers prefer their water temperature around 84 degrees to 85 degrees, so only minimal adjustments are necessary for comfortable space heating and cooling."

Wedeven said flywheel air conditioning works well at the West Ottawa and Howell schools because of Michigan's summer climate and because each natatorium is constructed with very little glass.

He enjoys the fact that these units feature heat pumps to help maintain an even temperature and recover/recycle heat to keep bathers comfortable in and out of the water.

Wedeven also likes the thermal flywheel effect during evening hours, which utilizes the economizer to help cool the pool area by one or two degrees for proper temperature balance. The economizer function flushes the pool enclosure with fresh air to provide cooler air and also prevents the build up of chloramines (chlorine gases), said Wedeven.

For more information, visit

Publication date: 08/09/2004