Indoor Air Quality

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Indoor Air Quality

  • Pool Water Chemistry
  • Chlorine Smell
  • Foul Odor in the Pool Area
  • Pool pH Levels
  • Total Dissolved Solids
  • Humidity and Corrosion
  • Outdoor Ventilation Air
  • Exhaust Air
  • Air Change Rate

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Good indoor air quality can be a challenge in an indoor pool. However, designers that follow ASHRAE guidelines as well as those addressed in this manual should have every expectation of a great space condition and pleasant overall experience for the patrons of the facility. There are steps a design engineer must take in their design to minimize the chances that a patron experiences a foul smelling pool odor and stinging eyes. There are many factors that impact the IAQ in a natatorium: pool water chemistry problems, inadequate outdoor air, air stagnation, poor air distribution, high humidity, mold, mildew, condensation and corrosion.

Four of the five key factors having the most direct impact on Indoor Air Quality (IAQ) are under the control of the design engineer.

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 four key airside design requirements.

Pool Water Chemistry

Pool Water Chemistry is the key variable that impacts IAQ and is not under the responsibility of the design engineer.

Good pool water chemistry by the building operator is critical in order to achieve levels of human comfort and health. Also by maintaining optimal pool water conditions you will have the best possible indoor air quality and ensure optimal performance from the mechanical system.

Poor water chemistry is the single biggest source of indoor air pollution and corrosion problems in a Natatorium.

Ultraviolet light (UV) treatment of pool water has shown to have a very positive impact on the water chemistry and can help reduce, if not totally eliminate chloramines. This approach to enhance water treatment is gaining popularity and as positive data continues to come should become more and more common.

Chlorine Smell

When you walk into an indoor pool and smell a strong chlorine odor, you naturally think it's caused by too much chlorine in the water. The odor is actually caused by chloramines (combined chlorines) off gassing from the pool water surface. Chloramines are formed in the pool water when there is insufficient free chlorine in the pool to address the nitrogen-containing compounds brought into the pool water by the swimmers. These nitrogen compounds are naturally-occurring and contained in sweat, urine, body oils and other proteins that get released into the pool water. If the introduction of these nitrogen compounds outpaces the introduction of free chlorine the chlorine becomes combined with the nitrogen compounds rather than fully oxidizing them. The chloramine (combined chlorine) levels increase in the water, resulting in an increase in chloramine off-gassing, which creates the odor of chlorine in the room. There are three different types of chloramines that can form: monochloramine, dichloramine and trichloramine. Trichloramine is the most volatile and will off-gas most quickly.

The powerful chlorine smell that is often associated with indoor pools is NOT the result of too much free chlorine in the water; it is TOO LITTLE free chlorine that is the culprit!

To avoid chloramines it is imperative to maintain proper free chlorine and pH levels as outlined in table 5. Off-gassed chloramines also have a strong attraction to the airborne humidity which will combine with moisture in the air. Consequently, any condensation of the space humidity will become corrosive.  An example of this can sometimes be seen at pools with chloramines problems where the ladder rails outside the water corrode while the portion under water does not.

Foul Odor in the Pool Area

When you walk into an indoor pool and smell a strong chlorine odor, you naturally think it's caused by add to much chlorine. The odor is actually caused by insufficient amounts of chlorine, better known as combined chlorines and can lead to high levels of algae and bacteria growth in the pool water. To avoid foul odor it is imperative to maintain proper levels of chlorine and pH as seen in table 5. Airborne chloramines also have a strong affinity to pure water such as condensate. Consequently any condensation will become corrosive and further damage the structure.

Pool Water Chemistry Parameters Recommended by NSPI
Pools Whirlpools
Desirable Range Desirable Range
pH 7.4 - 7.6 7.4 - 7.6
Alkalinity 80 - 100 PPM 80 - 100 PPM
Free Chlorine 2.0 - 3.0 PPM 3.0 - 4.0 PPM
Combined Chlorine 0 PPM 0 PPM
Dissolved Solids 100 - 300 PPM 100 - 300 PPM
Total Hardness 225 - 250 PPM 175 - 275 PPM
Table 5 - National Spa and Pool Institute Recommended Levels for Water Quality

The proper balance of outdoor air and room exhaust air along with air movement at the water surface is also crucial to ensuring chemical concentration levels are maintained within acceptable levels.

Pool pH Levels

High pH levels (alkaline range) encourage scale formation, which reduces pool water heater efficiency. With low pH levels the water is acidic and corrosive. This may damage the metal parts in pump, water heaters and piping. Maintaining pH levels between 7.2 and 7.6 will ensure the longest life for the pool equipment.

Total Dissolved Solids

It is imperative that pool water filters are routinely back washed and that total dissolved solids monitored. High quantities of total dissolved solids will overrun the free chlorine levels and form chloramines.

A proper pool water test kit should monitor the parameters seen in table 5.

Humidity and Corrosion

As previously mentioned, off-gassed chloramines have a strong attraction to the airborne humidity and will combine with moisture in the air. Consequently any condensation of the space humidity will become corrosive.  It is critical that the space humidity levels be controlled such that condensation is prevented as it will damage the building and mechanical system.

Seresco units are built for a pool environment. All electrical and refrigeration components are out of the pool air and in a protective service vestibule. The components in contact with the pool air stream are protected with the best possible corrosion resistant paints, coatings and materials. Seresco units are built to last!

By design, indoor pool environments are warmer and have higher moisture levels compared to traditional spaces.

Engineers and architects must understand the consequences of this and pay special attention to the special requirements on the entire HVAC system and building envelope.

Outdoor Ventilation Air

The amount of outdoor air to be introduced to the facility is determined by construction codes. Most codes adopt ASHRAE Standard 62. Outdoor air is critical towards diluting airborne chemicals and maintaining good indoor air quality.

Facilities that introduce outdoor air per ASHRAE Standard 62 and have proper air distribution have the best IAQ.

  • More outdoor air than required by ASHRAE Standard 62 is not recommended (except water parks)
  • Outdoor air requires a significant amount of heating energy in the winter and must be included in heat load calculations.
  • Heat recovery should be considered between the exhaust air and outdoor air streams.
  • Exceeding code amounts is not recommended, as it creates extremely high operating expenses. Engineers must ensure condensation is avoided. Wintertime outdoor air is cold and in summer it can be very humid. Both scenarios can lead to condensation.
  • Introduce the outdoor air at the factory provided intakes on the air handlers
  • Locate outdoor air intakes away from sources of airborne contamination such as exhaust fans or plumbing vents.
  • The outdoor air must be preheated to 65°F - If more than 20% of the total airflow is outdoor air or if the winter design temperature is below 10°F
  • A certified air-balancing contractor must balance the system airflow.

All Seresco units are equipped with an outdoor air connection, filter and balancing damper. Motorized dampers and time clocks are also available.

Exhaust Air

ASHRAE recommends the room be maintained at 0.05-0.15" WC negative pressure relative to surrounding spaces.

Ten percent more exhaust air than outdoor air is a good rule of thumb.

More exhaust air than recommended by ASHRAE will not reduce or stop moisture migration through the building envelope to outdoors in cold weather. Vapor migrates based on vapor pressure differential. There is effectively a 10” WC pressure differential between indoors and outdoors on cold winter days. There is no amount of negative airside pressure you can put into a space to stop vapor migration. The building envelope must be designed by the architect to adequately deal with this by placing vapor retarders in appropriate locations in the entire building envelope.

Figure 5 illustrates how the strategic location of the exhaust grille can also significantly improve the air quality in the space. A spa or whirlpool should have the exhaust air intake grille located directly above it. This source captures and extracts the highest concentration of pollutants before they can diffuse into the space and negatively impact the room air quality.

Figure 5 - Exhaust Air
Figure 5 - Exhaust Air

Air Change Rate

ASHRAE recommendations for proper volumetric air changes per hour are important, ensuring that the entire room will see air movement. Stagnant areas must be avoided, as they will be prone to condensation and air quality problems.

Short circuiting between supply and return air must also be avoided as it significantly reduces the actual air changes within the space.

ASHRAE recommends:

  • 4-6 volumetric air changes per hour in a regular natatorium
  • 6-8 volumetric air changes per hour in facilities with spectators

A quick calculation will determine the supply air requirement.

  • Supply air required (CFM) = room volume (ft3) x desired air changes / 60