Mechanism of automatic humidity control

Estimated reading time: 14 min

In rooms with water connection such as bathroom, kitchen and WC it is not unusual to measure increased humidity levels. Bedrooms and other rooms several people stay in at the same time experience more water vapour, simply due to breathing. Once the air in the room is saturated with water components, the steam creates condensation on cool surfaces. Water accumulation at home is not only a visual problem, instead fungi quickly start growing at room temperature. In the long run they are quite harmful to humans as well as building materials. In order to create a pleasant atmosphere and prevent mould growth ventilation systems with automatic humidity sensors and control are very helpful.

In this guide we will explain to you how automatic humidity control works and what the differences between the many devices on the market are.


  • When does mould start to grow and why
  • Solutions/devices on the market
  • What you should pay attention to in the location
  • What is a hygrometer
  • How does a humidity sensor work

When does mould start to grow and why

Starting with a constant relative humidity of the indoor air above 65% mould spores feel very comfortable – and start preparing for growth without any restraints in this favourable climate. To a minor degree spores are naturally part of air – but in dry, cold air they simply do not find any breeding ground. However, the ideal damp and warm climate is created quickly, a household with 4 people generates up to twelve litres water each day by breathing, sweating and everyday chores alone! That includes activities such as showering, doing laundry, mopping and cooking. Indoor plants, pets and furniture furthermore contribute to the humidity levels.

What makes mould so dangerous is the risk of allergies and illness for humans and its ability to disintegrated building material. Spots that are particularly vulnerable, e.g. corners and window sills, are cooler that the rest of the house. This is where warm air condenses and creates damp fogging and drops of water. However even without the formation of visible condensation water (in 100% relative humidity) there can be an infestation if humidity in this spot exceeds 80 percent. Joints and back sides of furniture in regularly damp rooms, like bathroom and kitchen, are often affected.

Signs of mould infestation:

  • musty, earthy smell
  • visible mould stains
  • discoloured furry growth in damp spots
  • permanent headache and nausea
  • coughing, ongoing respiratory problems and diseases
  • reddened eyes, constant fatigue without any reason

A quick mould test from the drugstore or specialist shop will deliver more insight! More about mould growth and the most common species in our Blog

Water aggregation states

  • Relative humidity is stated in percent and indicates the water vapour contents in the air. Usually manufacturers refer to this value when giving specifications about humidity control.

  • Absolute humidity is stated in gram. This value describes the density (concentration) for one gram water in a cubic metre of air.

  • Warm air stores more water vapour than cold air (under consistent pressure). With a temperature underneath the dew point condensation water is produced, above this point water stays gaseous.

In cold basements ventilation only helps in certain situations. Ventilation during daytime when the atmosphere outside is warmer than inside even more humidity can enter the rooms. This is due to the fact that warm air can bind more water than cold air. A warm breeze from the outside then transports additional, unnecessary water into the cellar. Once the gaseous mixture starts to cool down, condensation water is generated. That is why it is recommended to air out a celler only during night time or in the early morning when the outdoor air has not been warmed up by sunshine. Another possibility would be to ventilate during the day if the air is very dry, humidity levels can be measured with a hygrometer.

Can mould be removed retroactively with dehumidifiers too?
Not directly, no. Dehumidifying measures only help to prevent spores from spreading even further. Dehumidifiers are more of a precautionary measure! If the infrested spots are rather small, cleaning with high-percentage alcohol or chlorine is enough to remove the damage. In case of a large-scale infestation the cause (e.g. structural damages, incorrect ventilation practices) must be eliminated.

Solutions/devices on the market

Our range includes portable dehumidifiers and fans for permantently fixed installation in damp locations. Apart from the portability of the devices there are differences in airflow and integrated features:

Kitchen & Bathroom Fans

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In damp rooms (bathroom, kitchen, toilet, washing room) we recommend a permantently installed device, e.g. bathroom fans for simple wall, ceiling or window installation. In enclosed damp rooms without separate windows this generally the best and easiest option.

Read more about this topic in bathrooms and toilets without windows.

The operation of those devices is controlled by fully automatic humidity sensors, and is adjustable to react to humidity levels of 60, 70 or 80 %. Further options are timers or the connection to light switches, flush or motion sensors. The time interval of the delay can be adjusted to 20 minutes in most cases. That means the fan extracts damp air for as long as 20 minutes after you took a shower. The benefits of automated humidity control are obvious: Residents are not required to constantly control or manually turn on the device. Even when you are absent for longer periods or the room is used very little, e.g. a pantry, automatic humidity control ensures a mould-free climate.

Fans for small rooms extract damp air and directly transport it to the outside or into a duct system. However it is still necessary to let in fresh air regularly, for example by opening doors or windows.


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If you wish to relocate the device according to which room you are using, a portable dehumidifier might be the right choice for you. There different technical developments to extract water from the air. One method would be to cool the absorbed room on the inside of the device (possible between 5° to 40° room temperature). That way the air can bind less water than warm air, and releases humidity as condensation into a collection tank. The emitted, drier air is then brought to room temperature by the device.

Another solution would be the absorption of humidity with hygroscopic chemicals, similar to household sponges or salt. This method is not as efficient and requires regular dehumidification or exchange of the pellets.

Dehumidifiers bind or reduce water contents of the air in a catch tank. The dried air is then transported back into the room again.

Using automated humidity control at home

  • damp rooms (e.g kitchen, washing room, WC)
  • laundry room
  • bedroom and living room
  • cellar, pantry
  • greenhouse

Possible applications in the industrial sector

  • hospital, laboratory
  • food storage and walk-in refrigerators
  • plant breeding, greenhouse
  • production processes under humid conditions
  • switch cabinets, PC fans

What you should pay attention to in the location

No matter if we are talking about dehumidifiers or permanently installed fans, both types are based on the same mechanism: As soon as an increased humidity level is recorded, the automatic control is activated. The indoor air is supposed to be dried or transported away, that is evident. Depending on the place of installation the additional features required for the device may vary.

What your device should be able to do (depending on its place of installation)

  • regular dehumidification of air, e.g. with timers
  • fully automatic operation with sensors or hygrostat
  • constantly keeping humidity at a healthy level
  • standby function, also night mode if required
  • quiet and smooth operation
  • qualified for use in damp locations
  • waterproof materials, protection of the motor
  • simple mounting, maintenance, cleaning
  • low operating and maintenance costs
  • programmable humidity control (60, 70, 80, 90% RH)
  • appealing and user-friendly design
  • connection to drain pipe if required
  • defrosting function / operation not dependent on the weather
  • mobility, application in different locations
  • sufficient airflow for the place of installation
  • variable rotation speed with speed control
  • for bathroom/WC: adjustable time-delay function if required

In bedrooms for example it is important that ventilation feature and, if applicable, existing lighting should not disturb the sleep. The tank of the device should be big enough to collect water for as long as eight hours. A whisper-quiet operation is also essential in nurseries or offices.

In humid rooms such as kitchen, bathroom and WC it is important that the technical components are protected by IP protection classes or at least have the manufacturer’s indication to be “Suitable for damp locations“ to ensure trouble-free operation. Not all fans are secured from short circuits and rust!

Individual humidity control made easy!

The size of the appliances is often a decisive factor in smaller and frequently used rooms, for example in a bathroom. Therefore it is advantageous to mount the device into a wall or the ceiling. Window fans are designed to save as much space as possible and are super easy to install.

Changing the water tank (in dehumidifiers) or letting out humid air (in case of fans) should go off without a hitch. Ideally, the devices that are easy to use and to clean. If filters need to be replaced, they should be easy to access and to cause unnecessarily high costs. In the best case the device consumes only little energy and operates in a sustainable way.

It is also beneficial if a standby feature is included that automatically activates the appliance once a certain composition of the air is detected. In fact, it is just as important to prevent air that is too dry. The optimum of water content in the air is between 50-60% so that neither mucosa in the nose nor the throat become irritated. Otherwise viruses will have a walk-over.

For application in cellar and drying on construction sites products are recommendable that are connected with hose for constant drainage. Fans should be equipped with two hygrometers indoors and outdoors that active operation under favourable weather conditions.

What is a hygrometer?

A hygrometer measures humidity. In this process the temperature of the air is compared to the humidity levels and the water contents are derived from the values in percent relative humidity.

Similar to a thermometer that measures temperature by using heat-sensitive substances e.g. mercury or – non-toxic – gallium, indium and pewter, the hygrometer measures humidity with water-sensitive materials.
Did you know that rather bizarre and odd methods are still common practice? One of those examples is the use of human hair. Invented in 1783, this technology is said to trace back to the alluring hair of a blonde woman! Hair from horses and sheep have also been used, as they pose a very reliable and cost-efficient determination method!
Natural hair expands in saturated air (100% relative humidity) by about 2.5% of its length compared to completely dry air! This change in length is enough to move the sensitive indicator through a lever system inside the hygrometer.

There are other common materials used in measuring, e.g. sensitive synthetic materials or metals. They are not necessarily more efficient than a hair hygrometer, occasionally they are actually even less accurate! There are also chemicals that change colour and dew point measurement systems.

hygrostat is a device that does not only determine relative humidity, but also regulates it to a desired value. The adjustable removal of the lever/switch to the water-sensitive material allows a controlled operation of the device starting at 60, 70 or 80 % relative humidity. The technology behind it is simple: The more change of length is required to reach the lever, the later the device is switched on.

Not only dehumidifiers count as hygrostats: Their close relatives, the humidifiers are also part of this category. Humidifiers are usually used during winter when the radiator and boiler cause air to become very dry. As you can see, neither too much nor too little water in your breathing air feels particularly pleasant!

How does a humidity sensor work

Another method for measuring humidity levels is employed in electric hygrostats. In case of capacitive measurement small sensors are used that similar to a computer chip: For example, their bottom layer might be made of glass that is immune to humidity. In the center there is a thin, reactive coating between two electrodes. The coating is usually made of polymeric synthetic material, ceramic, cellulose acetate, etc.
Water vapour in the air makes its way to this coating and, depending on its concentration, affects the coating’s electric capacity as a conductor. Simply put, this method measures the electric capacity of a thin, centered coating. The humidity sensor connects the automatic start of the fan and its intensity to an electrically evaluated, changed humidity.

Fans that are controlled by sensors are available in different designs with different technical specifications. The impedance hygrometer measures the resistance of a material under changed humidity levels, for example. However, for now we shall leave it at that with all the trade language, we would rather introduce some of our top-quality fans with automatic humidity control!

Fans with sensors for damp locations in our product range

Picture sources:
Product pictures ©
Watter aggretation states © snapgalleria,
Hygrometer © manfredxy,