Ventilation, humidity control
and heat recovery in
swimming pools

Thermal indoor climate determines comfort level.
A swimming pool is a sports facility that requires an enormous amount of energy. A well-designed energy efficient air treatment installation for pools regulates the air quality, the air temperature and the humidity of the indoor climate. Preventing condensation damage to the structure of the building is an important aim.

Furthermore, when finding the right combination of air treatment installation, control technology, and air distribution, an optimum balance needs to be achieved between financial, energy and comfort requirements. An efficiency of 90% does not guarantee that the lowest operating costs will be achieved!

Swimming pool ventilationIn indoor swimming pools, considerable amounts of water are turned into water vapour due to the high space and water temperatures. The surface area of the pool, wet areas, the visitors, the slides and the whirlpools all have their effect.

The extent of the evaporation depends on a number of factors:

  • the water temperature
  • the space temperature
  • the indoor humidity
  • the surface of the pool and wet areas
  • the air velocity across the pool
  • the occupancy of the pool

All these factors create an ever increasing amount of water vapour in the air resulting in a rise in the relative humidity in the pool enclosure. In addition to an unacceptable climate, this will inevitably have negative consequences, such as condensation of water vapour on cold bridges, steamed up windows, corrosion and appearance of mould and damage to the structure of the building. These consequences can only be prevented by reducing the amount of water vapour in the air with the aid of modern dehumidification installations.

Water vapour production

The extent of the water vapour production determines the capacity and energy consumption of the ventilation and / or dehumidification installations responsible for extracting water vapour from the pool enclosure. A modern pool of 300 m2 has an evaporation rate of approximately 60 to 75 litres of water per hour! The combined air temperature and relative humidity determine the absolute moisture content. The preferred relative humidity will be determined based on comfort, the building's structure and energy consumption and ranges between 50 to 65%. A compromise will have to be made between the aforementioned facts. A high relative humidity will, after all, result in a reduction of energy consumption. However, the higher the relative humidity in the pool, the more attention will need to be paid to applying vapour reducing measures and the sooner condensation will occur.

Comfort criteria

In addition to good acoustics, lighting and atmosphere - important aspects for the visitor - a good thermal indoor climate is a prerequisite. A pool that does not meet or insufficiently meets the needs for a pleasant indoor climate will not be very inviting to visitors. This could not only result in a decrease in the number of visitors, but also negatively impact the quality of the building. This is a difficult aspect of pools, as there is a difference in both the activity level and the clothing of the people that use the pool. The comfort zones of swimmers and non-swimmers (fully clothed) do not overlap. The demand for more recreational pools is one of the reasons why the water and air temperature have increased. A water temperature of 28ºC and an air temperature of 30ºC are very common these days. The increase in these temperatures has of course got an impact:

  • increase in energy consumption
  • it has a negative effect on the indoor climate for non-swimmers
  • Increased risk of condensation on windows and other parts of the building.

Apart from choosing the right air treatment installations, it is extremely important that the desired air distribution is determined correctly. In order to avoid 'dead' zones, an air change of at least 4-5 times the content of the pool enclosure is required. The maximum acceptable air velocity in the pool zone is 0.12 m/s, a higher velocity is experienced as draft by wet swimmers. In addition to air flow vents, Interland Techniek also produces plastic air distribution pipes, which (in bright colours) are the perfect choice for a pool, both aesthetically as well as technically.

Efficiency of a system

Obviously an air dehumidification system is chosen that has the highest possible efficiency. In the Netherlands, one usually only talks about the output of the heat exchanger. An air dehumidification system will, however, also have to be assessed on other aspects.

In addition to the output of the air treatment unit, the specific fan power is also very important. Interland Techniek bases itself on total output. The system designer should not let himself be put on the wrong track if the electric fan power becomes much higher to compensate for the internal pressure loss of a heat recovery unit.

Specific fan power

Assessing an air treatment system at an efficiency rate of 90% (or more) will have to be done more scrupulously. An end user benefits from the output of a complete system and not just the heat exchanger as a component.

The EN 13779 provides a definition for the so-called specific fan power with respect to electric energy consumption by the conveyance of air. It describes "the combined amount of electric power consumed by all the fans in the air distribution system divided by the total airflow rate through the building under design load conditions, in J/m3/. Also in the Netherlands, user and adviser, will, in addition to the output, have to determine the specific fan power of the installation in order to gain insight into the level of electric energy consumption.

Various countries such as Denmark and Sweden have already started working according to this new standard. In addition to the output, the specific fan power gives a much better insight into the total efficiency of a system. An important objective when designing an air treatment system is finding an optimum solution for achieving a low specific fan power at the most favourable system output while keeping investments as low as possible. The right combination of these three factors will result in the most favourable operation of an air treatment system!

It is therefore a matter of finding the right balance between these three essential elements.

System selection

Peter van der Velde Interland TechniekThere are air dehumidification systems for private and public swimming pools. Interland Techniek has been a prominent supplier of climate equipment for more than 50 years and has specialised in air treatment systems for swimming pools for 25 years. There is a solution for each type of pool, from private to large public pools. From a Dantherm standard product to a customized solution. With a Dantherm product there is a solution for every swimming pool.

Interland Techniek possesses a lot of know-how about measurement and control technology and air distribution when it comes to designing and producing air handling units. An extensive range of vents allows us to create a draft and condensation free space for every pool. By consciously opting for the right indoor climate, it is possible to create a sustainable building. Energy and maintenance costs will decline and visitor numbers will increase, resulting in the generation of more income.

Public swimming pools

When drawing up a schedule of requirements for a large swimming pool, a number of aspects should be taken into account with respect to the design of the climate Dantherm control system:

  • air temperature
  • water temperature
  • relative humidity
  • number of air changes
  • air velocity in pool enclosure
  • number of visitors
  • ventilation requirements
  • need for energy-saving measures
  • requirements with respect to the building control system

We will discuss three popular Dantherm air dehumidification systems and explain which system should be used when. Each standard concept can be extended, dependent on the requirements of the user, with the type of control of their choice.

1. Ventilation with simple heat recovery
    (cross flow heat exchanger)

2. Condensation and fixed outdoor air volume
3. Condensation and ventilation with heat recovery
    and modulating fresh air volume

1. Ventilation with simple heat recovery

The traditional solution for reducing the amount of water vapour in the indoor air (reduction of relative humidity) is simply heating and ventilating the entire pool enclosure and any adjoining spaces. The fresh air is sucked in and heated. The heated air absorbs moisture present in the pool enclosure and is subsequently conveyed outside. This simple solution often fails to meet expectations these days because of the higher energy costs involved, especially because an indoor pool practically needs to be heated throughout the entire year. This solution could cause problems especially during the summer months when the outdoor air already contains a lot of moisture. Based on the relative humidity indoors, the outdoor air volume will be adjusted. During the winter months, the outdoor air volume will therefore be lower. A plate exchanger will usually be employed for basic heat recovery (only sensible). Latently much valuable energy will be lost (in swimming pools, approx. 50% of energy is latent). This system has been popular since the seventies and is still used frequently because of its simplicity.


  • almost all latent heat is lost (approx. 50 %)
  • outside air needed at night
  • average output heat recovery unit +/- 60 to 65% (almost entirely sensible heat)
  • regulation RH with simple outside air mix control
  • possibility for up to 100% ventilation if desired
  • relatively higher energy consumption compared to systems 2 and 3
  • low investments

2. Condensation and fixed outdoor air volume

When this principle is applied, the outside air volume is kept to a minimum (max. 30%). The indoor air is coming in via a system of ducts. The air that is fed through the evaporator is cooled under the saturation point. The water vapour present in the air condensates on the evaporator and is drained away as water. The air is heated again by the heat of the compressor and the condenser. Before it gets to the condenser, the incoming outdoor air is mixed with the dried air. Subsequently the temperature of the air is of course further increased by the heat emitted by the fan engine.

During the winter months, a built-in heat battery can supply the remaining required heat, while during the summer months any excess heat can be returned to the pool water via a water cooled condenser.

When the water vapour condenses in the unit, a considerable amount of condensation heat is released. This so-called 'latent heat' stays inside the system and is not lost to the outside air as happens in case of dehumidification through ventilation (system A). As a result of the continuous circulation of the indoor air though the machine, almost independent of the outside conditions, the humidity indoors is kept at a preset value and any energy released in the dehumidification process is used effectively for the full 100%. One should also realise that using the condensation principle (or heat pump principle) for dehumidification of a swimming pool guarantees a constant level of humidity throughout the year.


  • full latent heat recovery
  • dehumidification at night by way of condensation principle without ventilation
  • low energy consumption
  • maximum outside air volume 30%
  • simple control RH using on/off adjustment control compressor(s)
  • limited number of settings
  • low investments

3. Condensation and ventilation with heat recovery and modulating outside air volume

A combination of the previous systems is used when visitor numbers are high and maximum heat recovery is desired. The advantages of systems A and B have been combined in this system.


  • the integrated control technology regulates the entire installation. Dependent on requirements and the indoor / outdoor conditions the most effective setting is selected
  • dehumidification at night by means of condensation principle without ventilation
  • possibility of 100% ventilation if desired
  • applicable for large and varying numbers of visitors
  • available as plug-and-play version

Dependent on the size, the structure of the building and the number of visitors, Interland Techniek is able to offer a tailor-made solution for each swimming pool with Dantherm products. The main objectives are creating:

  • a comfortable indoor climate
  • a condensation free structure
  • a draft free installation
  • lowest possible energy consumption
  • favorable operation
  • subsidy provision
  • a maintenance friendly installation
  • and most importantly: a satisfied user.

As soon as the objective and other basic wishes of the customer have been captured in a Schedule of Requirements, it is advisable to continue the preparations in a project team which includes a number of experts. By fully aligning the energy-saving and affordable air treatment system, control technology, and air distribution to the Schedule of Requirements for the pool, an optimum balance between financial, energy and comfort requirements can be achieved.

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