Select a swimming pool dehumidification fan for 4–8 air changes per hour of the pool hall volume, with the supply air directed across the glazing, and with every wetted component built for chloramine attack — AISI 316L stainless steel or a two-pack epoxy coating rated to ISO 12944 class C4 or C5-M. The airflow is set by the moisture load rather than the sensible load: pool water at 27 °C in air at 29 °C evaporates roughly 0.10–0.25 kg/m²·h of water surface, and the fan must move enough air to carry that vapour to the dehumidifier coil or to outside.
Step 1: Estimate the evaporation load
Evaporation drives everything in a natatorium. Typical figures are 0.10–0.15 kg/m²·h for a still competition pool, 0.15–0.25 kg/m²·h for a public leisure pool with normal activity, and 0.30–0.60 kg/m²·h for a pool with wave machines, slides or spa jets. A 25 m × 12.5 m pool has 312 m² of surface, so at 0.18 kg/m²·h the moisture load is about 56 kg/h — roughly 38 kW of latent load.
Two rules keep evaporation under control before any fan is selected. Hold the air temperature 1–2 K above the water temperature, typically water at 27–28 °C and air at 29–30 °C, and hold relative humidity at 50–60 %. Letting the air fall below the water temperature can double the evaporation rate, and letting humidity rise above 65 % risks condensation on cold surfaces and rapid deterioration of the building fabric.
Step 2: Set the airflow and the outdoor air fraction
Total supply airflow is normally 4–8 air changes per hour of the hall volume: 4–6 ACH for a school or hotel pool, 6–8 ACH for a busy leisure centre with high bather load. Outdoor air is sized separately, commonly at 8–15 L/s per m² of wet area (pool plus surrounding deck), which is what actually removes chloramines and disinfection by-products from the breathing zone.
| Pool type | Evaporation (kg/m²·h) | Air changes per hour | Outdoor air (L/s per m² wet area) | Design RH |
|---|---|---|---|---|
| Hotel or school pool, low activity | 0.10–0.15 | 4–6 | 8–10 | 50–55 % |
| Public leisure pool | 0.15–0.25 | 6–8 | 10–12 | 55–60 % |
| Wave pool, slides, spa features | 0.30–0.60 | 8–10 | 12–15 | 55–60 % |
| Therapy pool, water at 32–35 °C | 0.25–0.45 | 6–8 | 10–14 | 55–60 % |
| Covered pool at night, cover fitted | 0.02–0.05 | 2–3 (setback) | 4–6 | 60 % max |
Step 3: Fan static pressure and air distribution
Pool air handling units are medium-pressure machines. A packaged dehumidification unit with a filter, evaporator, condenser and reheat coil typically needs 500–900 Pa external static once the supply and return ductwork is included, and 350–600 Pa for simpler ventilation-only units with a heat recovery core. Backward-curved centrifugal or plenum fans are the standard choice; forward-curved wheels are common in small packaged units but their non-overloading behaviour is poorer and their efficiency in the 700 Pa band is typically 8–15 points lower.
- Direct 50–70 % of the supply air upward across the glazing and roof lights, at a face velocity of 2–4 m/s from linear slot diffusers, to prevent condensation on the coldest surfaces.
- Keep the deck-level air velocity below 0.15 m/s so bathers do not feel a draught; above 0.25 m/s complaints are almost certain.
- Locate returns at high level on the opposite side, because chloramine-laden air is warm and rises; low-level returns near the water are used only for dedicated breathing-zone extract.
- Maintain the hall at 10–20 Pa negative to the surrounding building so damp, chlorinated air cannot migrate into changing rooms and corridors.
Corrosion: the real cause of pool fan failure
Chloramines and free chlorine, concentrated by condensation, attack steel, zinc coatings and even 304 stainless steel. Fans in this service should use AISI 316L for the impeller and shaft where budget allows, or a two-pack epoxy or polyester powder coating of 80–120 µm dry film thickness over hot-dip galvanised steel. Aluminium impellers are acceptable in the supply stream but corrode quickly in the humid return stream. Fasteners must be A4/316 grade throughout — a single carbon-steel bolt will bleed rust across a coated casing within a season.
- Specify ISO 12944 class C4 for indirectly exposed components and C5-M for anything in the return or exhaust airstream.
- Use sealed, greased-for-life bearings with an L10 life of at least 40,000 hours and shaft seals rated for continuous 90 % relative humidity.
- Choose motors of at least IP55, class F insulation, with condensation drain plugs and, on units that cycle, anti-condensation heaters.
- Slope every drain pan at 1–2 % and fit a properly trapped drain; standing water in a pool AHU becomes an aggressive chloride cell.
Keruite builds natatorium air handling units and matched fans at its 80,000 m² Dezhou plant with an in-house AMCA-210-style test laboratory, and every rotor is balanced to G2.5 and performance-tested before dispatch.
When to choose which system type
- Choose a packaged mechanical dehumidifier with heat recovery to the pool water when the pool runs year-round in a cold or temperate climate, because reclaiming 60–80 % of the latent heat into the water loop is where the operating cost is won.
- Choose a ventilation-only unit with a plate or wheel heat exchanger when the climate is mild, occupancy is limited, and capital budget is tight; recovery efficiency is typically 50–75 %.
- Choose an outdoor-air-only economiser mode when outdoor dew point is well below indoor dew point — often several thousand hours per year in continental climates — and let the dehumidifier idle.
- Choose variable-speed fans with humidity-based control whenever the pool has a cover or wide swings in bather load, since setback to 2–3 ACH overnight cuts fan energy by 70–90 % under the cube law.
- Choose a dedicated 316L exhaust fan for the breathing-zone extract when competitive swimmers train in the hall, because that duty sees the highest chloramine concentration of any airstream in the building.
Commissioning figures to verify
Check supply airflow within ±10 % of design, hall pressure at 10–20 Pa negative, glazing surface temperature at least 2 K above the room dew point, and deck air velocity below 0.15 m/s. Measure relative humidity at three points across the hall — a 5 % spread usually means the supply pattern is short-circuiting to the returns rather than sweeping the water surface.
FAQ
How much air does a swimming pool hall need?
Total supply is normally four to eight air changes per hour of hall volume, with four to six for hotel and school pools and six to eight for busy leisure centres. Outdoor air is sized separately at roughly 8 to 15 litres per second per square metre of wet area.
What temperature and humidity should a pool hall be kept at?
Hold the air one to two kelvin above the water temperature, typically water at 27 to 28 degrees Celsius and air at 29 to 30 degrees, with relative humidity between 50 and 60 percent. Air below water temperature can roughly double the evaporation rate.
What material should a swimming pool fan be made from?
Use AISI 316L stainless steel for impeller, shaft and fasteners where budget allows, or a two-pack epoxy coating of 80 to 120 micrometres over hot-dip galvanised steel. Specify ISO 12944 class C4 generally and class C5-M for return and exhaust airstreams.
Why must a pool hall be kept at negative pressure?
Negative pressure of about 10 to 20 pascals relative to adjoining spaces stops humid, chloramine-laden air migrating into changing rooms, corridors and structural cavities, where it causes condensation, mould and corrosion of hidden steelwork. It is achieved by exhausting slightly more air than is supplied.
How much heat can a pool dehumidification unit recover?
A packaged mechanical dehumidifier can typically return 60 to 80 percent of the recovered latent heat into the pool water loop, while a plate or wheel heat exchanger on a ventilation-only unit recovers roughly 50 to 75 percent of the air-to-air sensible energy.