An indoor pool is a large, warm, open water surface, and all of it evaporates into the room. Without dehumidification the hall sweats: glazing streams, steel corrodes and gypsum fails. Sizing starts with the evaporation rate.
Where the moisture comes from
Evaporation from the pool surface, plus splash and wet decks, plus the moisture carried in by the fresh air the hall needs for occupants and chlorine control.
Why 50 to 60% RH
Pool halls are held at 50 to 60% relative humidity with the air a degree or two warmer than the water. Drier air pulls more water out of the pool and costs more to heat; wetter air condenses on every cold surface.
The evaporation formula
A worked example: 8 × 4 m at 28 °C
Surface 32 m². Water 28 °C, air 30 °C, target 55% RH, private use.
| Step | Value |
|---|---|
| Surface area | 32 m² |
| Vapour-pressure difference | — |
| Activity factor | — |
| Evaporation rate | — |
| Fresh-air moisture | — |
| Capacity needed | — |
Air changes and fresh air
Pool halls need fresh air for the people in them and to dilute chloramines; in a UAE summer that air arrives humid and must be dried. A ducted high-capacity unit handles both, see swimming pools and the high-capacity unit.
Key points
- Evaporation from the pool surface is the moisture load; it rises with water temperature, activity and dry air.
- Pool halls are held at 50 to 60% RH, with air slightly warmer than the water to limit evaporation.
Step by step
How to size a dehumidifier for an indoor pool
- 1
Measure the water surface
Length times width of the pool, plus any spa or wet deck that stays wet. An 8 × 4 m pool has 32 m² of surface.
- 2
Fix the water and air conditions
Water temperature, air temperature and the target RH (50 to 60%). Air kept a degree or two above the water reduces evaporation.
- 3
Choose the activity factor
A private pool used by a family evaporates less than a hotel pool with constant splashing; the factor scales the base rate.
- 4
Calculate the evaporation rate
Apply the ASHRAE evaporation formula: a coefficient times the water area times the difference between the vapour pressure at the water surface and in the room air, scaled by the activity factor, gives kg of water per hour.
- 5
Add fresh air and pick the unit
Add the moisture brought in by the required fresh air, convert to litres per day and select a high-capacity ducted unit with a margin.
Where this applies
