Five important factors to determine
operating conditions in indoor
swimming pools

In order to calculate the humidity load on an indoor swimming pool project, you first need to determine the operation conditions.
In doing this, there are a number of factors to take into consideration. In this article we will look at the five most important ones.

Swimming pool with Dantherm dehumidification

For a detailed guide to determining conditions and humidity loads, sign up to download our comprehensive 44 page “Selection Guide – Dehumidification of Swimming Pools” here

1. Setting the room temperature

In consideration of both comfort and budget, the air temperature around a swimming pool should be higher than the water temperature. This minimises evaporation and the dehumidification requirement. However, an increased room temperature will of course impact the heating bill.

In general, it is recommended that the room temperature is set to 2°C higher than the water temperature.

Air temperature is recommended to be 2 °C hugher than the water temperature

2. Setting the water temperature

When setting the water temperature, the following benchmarks will often be a good starting point.

  • Private pools and hotels:       26-30 °C
  • Public swimming pools:         26-28 °C
  • Competition swimming:         24-27 °C
  • Therapy baths:                     30-36 °C
  • Spa:                                    36-40 °C

3. Getting the relative humidity right

The relative humidity is an expression of the air's actual water vapour pressure compared to the water vapour pressure when the air is saturated. It is stated as a percentage, and the set point when managing the operating conditions for a dehumidifier should be an RH value of 50–60%.

You can also choose to operate with a differentiated set point, so that it is low during winter (50 %RH) to avoid condensation problems on cold surfaces, and high during summer (60 %RH) to reduce the operating costs. When the temperature is high during the summer, condensation problems do not occur.

Going above 60 %RH will impact the comfort level negatively and increase the risk of condensation problems and fungal attacks. An RH value any lower than 50 %RH is not appropriate either, since this will increase evaporation leading to greater dehumidification requirements and increased operating costs. 

4. Water in the outdoor air

The water content in the outdoor air varies a lot over the year, more or less all over the world. From more than 12 g water/kg air during the summer down to 2 g water/kg air during the winter.

At Dantherm we use the guideline VDI 2089 by The Association of German Engineers (VDI) when setting operating conditions for internal condition, outside condition and amount of outdoor air.

VDI has chosen 9 g water/kg air as the value that characterises outdoor air in Northern Europe and we use this figure in our calculation program DanCalcTool.

However, other values can be used for outdoor air in DanCalcTool, if the values deviate significantly from the Northern European standard.

In addition, VDI 2089 recommends that the absolute water content of the indoor air does not exceed 14.3 g water/kg air (54 %RH with an air temperature of 30°C), if the water content of the outdoor air is less than 9 g water/kg air, which will be the situation during winter.

During the summer, a higher water content in the indoor air can be permitted, as the temperature in the air that enters from outside is high and thus does not contribute to condensation.

VDI 2089 states that the volume of outdoor air must be at least 15 % of the circulating air. This applies for public and commercial pools. There are no specific requirements in VDI 2089 for the addition of outdoor air in private pools.

Water content in the outdoor air is an important factor, that typically changes with the seasons

5. The significance of the outdoor air

The outdoor air is obviously not a factor you can influence or set, but it is still important to take into consideration since the outdoor air temperature and the water content in the outdoor air affect the choice of dehumidifier type. For example, a low outdoor air temperature will favour a ventilation solution with a heat pump.

The water content in the outdoor air (RH value) influences the overall humidity load and if the water content in the outdoor air is higher than the water content in the air in the pool room, the condense dehumidifier must have a larger capacity.

In general, a dehumidifier should be installed according to daytime operations since the evaporation is greatest during the day, when the pool is used the most.

When outdoor air is incorporated in the installation, you must use the summer load for determining the humidity load, as the water content in the outdoor air is highest during the summer. You must ensure that the dehumidifier is large enough to deal with the humidity load with 100 % outdoor air volume during the summer.

During winter the outdoor air is very dry, and the dehumidification capacity will be sufficient for this.

For an extensive walkthrough on determining conditions and calculating humidity loads, sign up to receive our comprehensive “Selection Guide – Dehumidification of Swimming Pools” here

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