Indoor swimming pools are among the most demanding environments for moisture management in industrial facilities. A heated body of water with a large surface area enclosed in a confined volume creates a continuous and intense evaporation process. This evaporation cannot be prevented; it can only be controlled. Uncontrolled humidity erodes the building structure, threatens user health, shortens equipment life, and increases energy costs. On the other hand, excessive dehumidification disrupts the temperature balance and user comfort. For this reason, pool humidity control is not just a technical problem but an integrated facility management issue.
Evaporation Calculation
The amount of evaporation in an indoor pool environment depends on various parameters, including the difference between water temperature and air temperature, air flow velocity, pool surface area, number of users, and activity type. To calculate the evaporation rate, a simplified formula based on the German Association of Engineers' VDI 2089 standard is used:
W = evaporation (g/h) · β = usage-dependent evaporation coefficient [g/(m²·h·hPa)] · A = pool surface area (m²) · ps = saturation vapor pressure at water temperature (hPa) · pL = partial vapor pressure of the hall air (hPa)
Evaporation Coefficient by Usage (β)
Swimmers, water agitation and wet body surfaces increase evaporation. VDI 2089 accounts for this effect with a β coefficient selected according to the type of use:
| Usage | β [g/(m²·h·hPa)] | Description |
|---|---|---|
| Calm surface (pool not in use) | 5 | Surface evaporation only |
| Private pool (villa, residential) | 15 | Low-intensity use |
| Public indoor pool | 20 | Normal use |
| Leisure pool | 28 | Waves, slides, heavy agitation |
Sample Calculation
An 8 × 4 m (32 m²) villa pool with water at 28 °C and hall air at 30 °C and 60% RH:
Pool not in use (β = 5): 5 × 32 × 12.3 ≈ 1,970 g/h ≈ 2.0 kg/h
In use (β = 15): 15 × 32 × 12.3 ≈ 5,900 g/h ≈ 5.9 kg/h
2 hours of use per day: 22 × 2.0 + 2 × 5.9 ≈ 55 L/day
With a 20% safety margin ≈ 66 L/day (e.g. Microwell DRY 500 SILVER: 66 L/24 h at 30 °C and 60% RH)
Under the same conditions, evaporation from a 500 m² public pool (β = 20) reaches about 123 kg/h during use; at this scale the load is covered by several units or an air handling unit. If the hall humidity is lowered to 50%, evaporation from the villa pool rises by about 35% (≈ 74 L/day); raising it to 65% reduces evaporation, but the dew point rises to 22.7 °C and the risk of condensation on cold surfaces increases.
Effects of Humidity Problems
Health effects: Chlorine in the pool water reacts with nitrogenous compounds carried by swimmers, such as sweat, urine and cosmetic residues, to form chloramines; the most volatile of these is trichloramine (NCl₃). Because trichloramine is heavier than air, it accumulates just above the water surface; insufficient air change and high humidity increase this accumulation. These compounds cause respiratory tract irritation, worsening of asthma symptoms, and eye and skin irritation. Swimmers and pool staff who spend long hours in the hall are the most exposed to these effects.
Structural damage: Concrete carbonation and rebar corrosion, drywall moisture absorption and swelling, surface rusting on metal profiles, condensation and moisture accumulation on window frames, and paint peeling occur.
Comfort and equipment: High humidity increases the perception of heat; users feel a stifling environment. Mold and fungal growth on walls and ceilings leads to respiratory problems. Corrosion in electrical panels, moisture penetration and early failure in lighting fixtures, shortened control panel and sensor life, and condensation-related early failure in HVAC equipment are observed.
Chlorine Damage Mechanism
The free chlorine (hypochlorite, ClO⁻) in pool water remains dissolved in the water and does not evaporate; the main compound that passes into the air is the volatile trichloramine (NCl₃). In a high-humidity environment, this compound and chloride ions accumulate on building surfaces together with the condensing moisture.
The moisture film in which chloramines and chloride ions accumulate creates a corrosive environment. On concrete surfaces, they cause a drop in pH, carbonation, and increased porosity. On metal surfaces, the chloride ion breaks down the passive oxide layer, initiating galvanic and pitting corrosion. For this reason, the dehumidification equipment used in a pool environment must be specially protected against corrosion.
Standard drywall (gypsum) should not be used in an indoor pool environment. Moisture absorption sets the stage for swelling, loss of structural integrity, and mold growth. Non-hydrophilic surface coatings or special cement-based panels should be preferred in pool areas.
VDI 2089 Standard
The German Association of Engineers' VDI 2089 standard defines comprehensive technical requirements for indoor swimming pools. This standard is a fundamental reference document in many countries and addresses humidity control, air change, and comfort parameters together.
In indoor swimming pools, relative humidity should be kept in the 50–64% range; the air temperature should be 2–3 °C higher than the water temperature and should not exceed 34 °C (ASHRAE recommends a 1–2 °C difference). These conditions provide comfort and balance evaporation and condensation.
| Parameter | Value / Range | Description |
|---|---|---|
| Air relative humidity | 50–64% | Swimming area target RH |
| Air-water temperature difference | +2 ~ +3 °C (max. 34 °C) | Air should be slightly above water temperature |
| Air change rate (min.) | 1–3 room volumes/hour | CO₂ and chloramine removal |
| Fresh air per swimmer | 30–40 m³/h | Minimum air per person |
| Air flow velocity (swimmer zone) | Max. 0.2 m/s | Preventing the sensation of cold |
| Window surface | Should be minimized | Condensation and heat loss risk |
Pool Types and Requirements
Each pool type requires different moisture loads and target conditions depending on water temperature and usage profile. The table below summarizes design values for common pool types:
| Pool Type | Water Temp. (°C) | Air Temp. (°C) | Target RH (%) | Moisture Load |
|---|---|---|---|---|
| Olympic pool | 26–28 | 28–30 | 50–60 | High |
| Therapy / Rehabilitation | 32–36 | 34 (max.) | 55–65 | Very High |
| Children's pool | 30–32 | 32–34 | 55–65 | High |
| Jacuzzi / Hot tub | 36–40 | 34 (max.) | 60–70 | Very High |
| Diving pool | 26–28 | 28–30 | 50–60 | Medium |
| Hotel / SPA pool | 28–32 | 30–34 | 55–65 | High |
Dehumidification System Design
Capacity Calculation
In pool dehumidifier design, the following components are summed to determine the total moisture load:
- Pool surface evaporation: The baseline load according to the formula.
- Usage effect: the VDI 2089 usage coefficient (β), reflecting swimmer numbers and activity.
- Wet surrounding surfaces: Deck, changing area, showers, and corridors.
- Infiltration load: Door openings and building leakage.
- Safety factor: 15–25% additional capacity.
Air Flow Design
Air flow design is critically important in the pool environment. Incorrect air distribution both reduces dehumidifier performance and negatively affects user comfort. Basic design principles:
- Air inlet and outlet points should be placed asymmetrically; short-circuit flow should be prevented.
- Air flow sliding along the pool surface should carry away moisture without accelerating evaporation (without supplying cold air).
- A warm air curtain in front of windows prevents condensation.
- The exhaust outlet should be placed at a low point near the water surface where chloramine accumulation is highest.
Special Coating Technologies
The indoor pool environment — high humidity + chlorine + acidic pH — typically exceeds the durability limit of standard aluminum or steel coatings within 1–2 years. Equipment with Blue Fin or cataphoretic coating can operate reliably for many years in this environment.
Blue Fin Coating
Blue Fin is a protective coating applied to aluminum coil surfaces. Under chloramine-laden, moisture-heavy, and acidic conditions, aluminum surfaces erode rapidly; this both degrades heat transfer and shortens equipment life. The Blue Fin coating is specifically designed against these conditions:
- Chemical resistance: High resistance to chloramine and acidic vapor environments.
- Heat transfer efficiency: The thin coating layer does not affect heat transfer.
- Application: Possible under factory conditions or as a field application.
Cataphoretic Coating
Cataphoresis (electrophoretic painting) is based on the principle of immersing metal parts in a paint bath and drawing paint particles to the surface with an electric current. The result is a homogeneous, pin-hole-free coating layer on all surfaces — including curved geometries:
- Homogeneity: Equal-thickness coating even on recessed and protruding geometries.
- Salt spray resistance: High salt fog resistance.
- Flexibility: The coating does not crack during thermal expansion/contraction.
- Pool-specific criticality: Standard paint coatings degrade rapidly in a chloramine environment; cataphoresis is a widely used solution for this environment.
Energy Recovery
Pool dehumidifiers can be used to heat the pool water or the facility's hot water circuit if the condenser temperature is high enough. This feature allows the system to operate not only as a dehumidifier but also like a heat pump. A significant part of the heat released during dehumidification can be recovered back into the facility.
Although systems with an energy recovery module require a higher initial investment, the operating cost advantage pays back the investment over time.
Maintenance and Service
A special maintenance protocol should be applied for dehumidifiers in the pool environment:
- Monthly: Surface cleaning of the condenser and evaporator coil (acid-containing solvents should be avoided; a pH-neutral cleaner should be used on Blue Fin coated coils).
- Quarterly: Filter replacement, drain cleaning, control panel and sensor calibration.
- Annually: Compressor pressure check, refrigerant charge check, coating integrity check.
Indoor pool humidity control is achieved by correctly performing the evaporation calculation, determining target humidity and temperature values according to standards such as VDI 2089, selecting the right corrosion-resistant equipment, and carefully designing the air flow. When all these elements are addressed together, the structure is protected, user comfort is ensured, and energy costs are minimized.
As NKT – Nem Kontrol Teknolojileri, we supply Microwell DRY WAVE series pool dehumidifiers for your indoor swimming pools, therapy pools, and SPA facilities and provide technical support in selecting the right model.
