Knowledge · swimming pools

Indoor pool humidity, condensation and energy

An indoor swimming pool evaporates water all day long. That vapour goes into the air. Without proper dehumidification you get misted-up windows, a stuffy hall and, in time, damage to the building. This page explains why, and what to look for when you choose an installation.

Have your pool calculatedSee the pool in the 3D world (in Dutch)

Why does so much water evaporate?

Water evaporates at the surface of the pool. How much depends mainly on:

• the size of the water surface;
• the water temperature, and the temperature and humidity in the hall;
• how busy it is: playing swimmers, waves and water features enlarge the water surface;
• wet floors around the pool;
• whether the pool is covered outside opening hours.

Designers calculate the evaporation with the German guideline VDI 2089 or with the method in the ASHRAE Handbook. We use those methods too: based on the water surface, the temperatures and the use of your pool.

Dew point and condensation: where things go wrong

Air can only hold a certain amount of water vapour. When humid air cools down, it reaches its dew point. There the vapour turns back into water.

An example: hall air at 30 °C with a relative humidity of 55% has a dew point of about 20 °C. Any surface colder than 20 °C gets wet. Think of windows, frames, thermal bridges and the roof structure.

Calculated with the Magnus formula and rounded to whole degrees. The more humid the air, the higher the dew point and the sooner the glass mists up.

Wet glass is a nuisance. Condensation inside the structure is worse: moisture in the insulation, rust on steel and mould. Pool air also contains chlorine compounds, which makes condensation in a pool hall extra harmful to metal.

Comfort for swimmers and staff

Air that is too humid feels stuffy. Air that is too dry is not good either: more water then evaporates from the pool, and wet swimmers feel cold. That is why the designer chooses the air temperature and the humidity in the hall together with the water temperature. The VDI 2089 guideline provides the starting points.

Air quality matters too. Trichloramine causes the typical smell of chlorine. Fresh air and a good airflow over the water help. The X-Cross Blue Line measures trichloramine continuously and controls on it.

The energy story

Evaporation takes heat. Every litre of water that evaporates from the pool takes about 0.68 kWh. That heat comes from the pool water and the hall air. It is then held in the water vapour as latent heat.

If you blow the humid air outside, that heat goes with it. According to the Dutch knowledge centre Stimular, the air in a swimming pool is typically refreshed 5 to 7 times an hour. A heat exchanger keeps part of the heat inside. You only recover the latent heat when the vapour condenses, for example on the cold evaporator of a heat pump.

That is exactly what the X-Cross Blue Line does. It recovers heat from air that has to be dehumidified anyway and returns it to the pool water. The figures per unit are on our page about dehumidification and heat recovery for swimming pools.

What to look for when you choose

• A calculation of the evaporation from your own pool, not an estimate per m².
• The latent and the sensible load shown separately, so you can see where the energy goes.
• Heat recovery from the exhaust air, preferably including the latent heat.
• Where the recovered heat goes: to the air, to the pool water or to both.
• Materials that can withstand pool air.
• Controls that connect to your building management system.
• Room for maintenance: filters and refrigeration equipment must be easy to reach.
• For businesses taxed in the Netherlands: an air handling unit for swimming pools is on the RVO Energy List. Read more about Dutch tax incentives.