In a two-part series, RTM Principal Doug Brewer shares his expertise on natatoriums, commonly referred to as indoor swimming pools. In this post, Brewer shares the complexities of natatoriums and outlines the importance of preventing condensation and achieving desirable indoor air quality.
Natatoriums are complex buildings that involve several unique design considerations. Providing a safe, healthy and comfortable pool environment while minimizing operating costs and delivering the best possible air quality is no simple task. The system design and equipment must perform reliably, day in and day out, preferably with minimum energy consumption and maximum energy recovery, to provide precise control of the environment regardless of the ambient temperatures or pool activity.
Condensation is a major concern for all types of building construction as it permits mold and mildew to grow. If allowed to occur inside the building walls or roof, condensation also can cause deterioration and can devastate the structure if it freezes during colder months.
When it comes to natatoriums, the heat, humidity and chemicals in pools combine to create harsh environments where condensation can occur and produce complications for buildings and equipment. To avoid major issues caused by condensation, the natatorium’s envelope design and construction must be suitable for 50% – 60% relative humidity year round. This includes vapor barriers throughout and thermally broken window/door frames to avoid transfer of moisture and condensation.
Additionally, off-gassed chloramines have a strong attraction to airborne humidity and will combine with moisture in the air. Consequently, any condensation of the space humidity will become corrosive, making it critical that the space humidity levels be controlled such that condensation is prevented. A successful design will blanket the building elements with warm supply air to prevent condensation.
Achieving desirable indoor air quality
The factors that impact the indoor air quality (IAQ) in a natatorium are pool water chemistry, outdoor air, air distribution/stagnation and humidity.
The chemical off-gassing from the pool water is the pollutant that causes all the IAQ issues in a natatorium. A successful HVAC design will adequately remove these chemicals and provide good IAQ by properly addressing the airside design requirements.
The treated and conditioned air must be delivered down into the breathing zone where the patrons are if there is to be any reasonable expectation of providing good air quality and a comfortable experience. It also is recommended to provide some air movement at the water surface to dilute the higher concentration of chemicals where swimmers breathe. However, too much air movement at the water surface (above 30 fpm) will increase the evaporation rate and negatively affect swimmer comfort.
The introduction of fresh outdoor air is an essential part of maintaining good IAQ in the pool area and is a code requirement. The purpose of outdoor air introduction is to dilute chemicals off-gassed from the pool water. The natatorium needs to be maintained at a slight negative air pressure with regard to the surrounding spaces. Warm “energy rich” air needs to be exhausted.
We typically consider two approaches to control a natatorium environment. Energy consumption, geographic location, budget and desired control of space conditions are among some of the key decision making criteria.
- Refrigeration-based dehumidifiers use both sides of the refrigeration cycle (evaporator and compressor waste heat) simultaneously. The evaporator is used to dehumidify (and cool when needed) and the waste heat is used to reheat the process air and/or heat the pool water.
- 100% outdoor air (single pass) ventilation systems can be set-up as heating only (for comfort) or as cooling (for comfort and dehumidification) and heating (for comfort). It is important to note that heating only systems may not be able to maintain the desired indoor design conditions in the summer months as the outdoor conditions may not be cool or dry enough.
ASHRAE recommends 4-6 volumetric air changes per hour in a regular natatorium and 6-8 volumetric air changes in facilities with spectators. Properly designed refrigeration-based systems circulate more air throughout the space than single pass ventilation systems, thus providing better distribution to all areas and added prevention of condensation on the building surfaces. Refrigeration-based systems have a higher first cost, but are considerably more sophisticated in terms of controls and offer the best year-round control.
Regardless of the system type, they are equipped with energy/heat recovery between the exhaust and outdoor airstreams to reduce the cost of utilities. This option typically realizes a payback of less than one year.
To read more from Doug Brewer, check out another recent blog post: Q&A on Geothermal Systems.