Commercial Energy Systems

Indoor Pools

Indoor Pools are enclosed swimming centers of which there are basically three pool types -- those with and without spectator facilities, and therapeutic pools. Chlorine is commonly used to disinfect the pool water, and the heated pool evaporates water releasing corrosive chlorine vapors into the pool area.

The pool enclosure and the HVAC system must be constructed of materials that can withstand the humid and corrosive environment and condensation. Even so, building maintenance costs tend to be high. Locating pools in the interior of the building reduces the design problems, compared with those with roof and outside wall exposures. Glass walls seriously complicate the design and operation, and add cost (particularly in cold climates) due to cold drafts and the need for condensation control.

Indoor air temperature in the pool areas is typically kept about 2°F above the pool water temperature and at 50 to 60% relative humidity. Recommended pool water design temperatures are:

- Competitive swimming 72 to 75°F
- Pleasure swimming 75 to 85°F
- Therapeutic 85 to 95°F
- Whirlpool / Spa 97 to 102°F

Air distribution should include return and exhaust grilles at ceiling level since warm moisture-laden air rises to these higher points in the pool enclosure. Supply air should be introduced at a low level to "wash" the exterior wall and window surfaces. Filters of 45 to 60% efficiency should be used to minimize wall and floor surface streaking from dirt coming in contact with moist surfaces.

Pools should be isolated from adjacent buildings, if possible by a negative pressure in the pool area. Air movement over the pool water itself should be kept to a minimum to reduce pool water evaporation. Evaporation rates vary with the indoor humidity, air and pool temperatures. For example, the evaporation rate for a 25-yard by 8-lane pool is:

There are several unique energy problems caused by the evaporation of the pool water. The pool water is cooled by the evaporation creating a large heating load to keep the water at the desired temperature for the swimmers. There are significant shower and laundry water heating loads that add to the load on, and fuel cost for, the hot water boilers. The high humidity loads in the pool area cause corrosive condensation and the large ventilation air intake requires additional heating in winter, adding more load and fuel cost. A reasonable base electric load and the sizeable heating load makes these facilities attracted to considering cogeneration, which may or may not be their best alternative. A heat pump pool dehumidifier system that dehumidifies the air in the pool area and also contributes to the pool water heating can reduce both maintenance and energy costs, while improving occupant comfort.


Ventilation is a primary method of pool air treatment with 4 to 6 air changes per hour recommended except 6 to 8 changes for pools with spectator facilities. Indoor air design for pleasure swimming is 75 to 85°F dry bulb and 50 to 60% relative humidity, for therapeutic swimming it is 80 to 85°F dry bulb and 50 to 60% relative humidity.

Relative humidities should not be kept lower than the recommended levels due to the evaporative cooling effect on the person coming out of the pool, and the increased pool evaporation and consequent higher pool water heating requirements. Higher humidities also increase corrosion and condensation problems as well as occupant discomfort.

Most Indoor Pools now use a pool dehumidifier system with the equipment specially designed to operate in the corrosive chlorine laden humid air and pool water.

Typical System

Conventional pool ventilation systems attempt to control the room humidity by removing the high humidity air and bringing in less humid outdoor air. The outdoor air brought in must be heated in winter or cooled in summer with the attendant operating cost and the energy used to heat the pool water is wasted.

When a pool dehumidifier system is used, a humidistat automatically operates the dehumidifier, which cools the air to remove the moisture and then reheats it before distributing to the pool area. In addition to the reheat coil, a water condensing coil uses some of the heat to maintain the pool water temperature. A separate water temperature thermostat actuates the pool water flow through this coil. When excess heat is available over and above that needed for reheat and to heat the pool water, a remote air-cooled condenser may be required to provide full capacity dehumidification regardless of the weather. Auxiliary heating is provided by an electric resistance coil, or by a hot water or steam coil served by a remote boiler. Ventilation air can be included in the dehumidifier cycle by duct, filter, and damper arrangements, controlled to minimize energy use when outdoor air conditions permit.

Locker rooms and offices should have separate air supplies and a positive pressure with respect to the pool area. Openings to the pool area should be minimized and an air lock arrangement is recommended.

Recommendations/Energy Services Opportunities

Existing ventilation only or HVAC facilities can be retrofitted with a pool dehumidifier system. Ozone treatment of swimming pool water can minimize the chlorine usage, therefore, reducing the HVAC loading.

Water Heating

Service hot water is used for cleanup, locker room and pool showers, and rest rooms. Hot water consumption varies significantly among individual facilities.

A separate and specially-designed pool water heating system is also required; this may be included with the pool dehumidifier system, which may also preheat the service water. The water heating system is installed prior to the return of treated water to the pool. A circulation rate of a 6 hours water change is usually acceptable. The system should include a pool water temperature control and a water flow safety switch to afford protection against inadequate water flow. Evaporation losses account for the greatest heat loss from the pool; remaining losses are conduction through the pool walls, plus radiation and convection from the water surface.

Typical System

Most service water heating is done separately from the main building heating system using heat pump, direct resistance or gas heaters, and in some cases, point-of-use heaters. Pool water may be heated by fuel-fired water heaters or boilers, electric boilers, tankless electric circulation water heaters, heat pumps and solar heaters.

Recommendations/Energy Services Opportunities

If existing water heating systems are inefficient or inadequate, replace with modern efficient equipment. Also add better insulation on storage tanks, or timer controls. Consider a heat pump water heater, also solar heaters can be added to reduce cost of pool water heating in sunny climates.


Pool lighting fixtures should be designed for installation under roof insulation. This will shield them from the cool winter attic air that might chill them below the dewpoint, resulting in condensation and dripping rusty water.

Recommendations/Energy Services Opportunities

Many older indoor pools still have incandescent and/or mercury vapor lighting systems. These should both be replaced with either metal halide or fluorescent systems. T-8 fluorescent lamps with electronic ballast should be used if fluorescent fixtures are selected. Shatter proof shields or covers will be required by most codes.

Photocell controls should be installed where the indoor pool has substantial "sun lights". The photocell should be used to control the lighting system to energize only those lights needed to reach the desired lighting levels.

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