Why an indoor pool is a moisture engine

An indoor pool continuously evaporates water into the air. The rate depends on water temperature, air temperature, air movement and the surface area of the pool. A warm pool in a cool room evaporates quickly; a covered pool evaporates far less.

All that vapour has to go somewhere. Without control it migrates to the coldest surfaces in the building, which are usually single-glazed windows, metal window frames, external walls and the underside of the roof. That is where condensation, and later corrosion and mould, begin.

The classic symptoms of poor pool hall humidity control

• Streaming condensation on windows and frames, sometimes running down the wall.
• Chloramine odour, the sharp chlorine smell that builds up in the air rather than in the water.
• Rust stains on fixings, door handles, lighting trims and steel structure.
• Peeling paint, blistering plaster and mould in corners away from the pool.
• A clammy feel even when the water temperature is comfortable.

Chloramines form when chlorine reacts with contaminants such as sweat and urine. They are volatile, so they leave the water and accumulate in the air when the space is not adequately ventilated or dehumidified. Fresh air and moisture removal together keep both the smell and the humidity in check.

How a pool dehumidifier differs from a domestic unit

A pool hall dehumidifier handles an unusually high and continuous moisture load, often running for long periods at a time. That means a corrosion-resistant heat exchanger, a cabinet designed for a warm and chemically active environment, and often the ability to work alongside ventilation or ductwork.

Ductable models are common in pool halls because they can draw humid air from the perimeter, where condensation risk is highest, and return treated air where it is needed. Some designs also recover the heat released during dehumidification and use it to warm the pool water or the room, which reduces the energy penalty of running the equipment.

Sizing for a pool hall

Pool dehumidification is sized on evaporation rate rather than floor area alone. In practice this means accounting for water surface area, water and air temperatures, and whether the pool is covered when not in use. A cover is one of the most cost-effective humidity measures available, because it removes a large part of the load at source.

Because the calculation is sensitive to several variables, it is worth working from the pool dimensions and the intended air temperature rather than from a simple room-size rule. Undersized equipment runs continuously and still fails to hold the setpoint on humid days.

Day-to-day measures that reduce the load

Equipment works best in a hall that is also managed well:

• Use a pool cover whenever the pool is not in use.
• Keep the air temperature slightly above the water temperature, which reduces evaporation and condensation.
• Maintain correct water chemistry so chloramine formation is limited.
• Keep doors and windows to the rest of the building closed during humid weather.
• Ensure air movement reaches the glazing and cold wall surfaces rather than short-circuiting back to the unit.

Frequently Asked Questions

What does a pool dehumidifier do?

It removes water vapour from the air in the pool hall. That keeps relative humidity in a range where condensation does not form on cold surfaces, which in turn limits corrosion, mould and the deterioration of the building fabric.

Why does my indoor pool smell so strongly of chlorine?

That sharp smell is usually chloramine, formed when chlorine reacts with contaminants in the water and then escapes into the air. Better moisture removal and adequate fresh air dilution both reduce it.

Is a normal dehumidifier suitable for an indoor pool?

A standard domestic unit is not built for the continuous load or the warm, chemically active air of a pool hall. Pool applications normally use corrosion-resistant, ductable equipment sized on evaporation rate.

Does a pool cover really help?

Yes. Covering the pool when it is not in use removes a large share of the evaporation that the dehumidifier would otherwise have to handle, which lowers running hours considerably.

What humidity should an indoor pool be kept at?

A common guideline is to hold the pool hall slightly below the level at which condensation forms on the coldest surfaces, with the air temperature kept a little above the water temperature. Exact values depend on the glazing and construction of the hall.

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