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Which dehumidification method for which purpose?
Selection factor room temperature
The average air temperature in the room to be kept dry is the most important decision criterion for selecting a suitable dehumidifier.
∞ power below 8 °C
In unheated basements, weekend homes or cooler rooms with an average room temperature below 8 °C during the wintertime, we recommend using a desiccant dehumidifier. Their functional principle makes it possible to keep environments permanently and effectively dry even at low temperatures.
Even if the temperature occasionally rises to 12 °C, these devices still work satisfactorily. Above 12 °C, however, their energy balance becomes worse and using them is no longer reasonable from an economic point of view.
All-rounders from 5 to 35 °C
As of an average room temperature of 8 °C, it is generally possible to use refrigerant dryers for dehumidification.
If the temperatures permanently drop below 15 °C during the winter months, it is absolutely necessary to use a device with a hot gas defrost system.
These all-rounders can be used variably throughout a very large temperature range, while using refrigerant dryers with an air circulation defrost system for dehumidification is only reasonable at average temperatures above 15 °C – see also chapter 2 – “Room climate limits of the different methods”.
Selection factor operating costs
When it comes to the cost-benefit effect and dehumidification performance in relation to power consumption, the compressor-operated condenser dryer can very clearly claim victory in almost all fields of application.
Peltier condenser dryers, on the other hand, are less expensive to buy and more energy-saving at first sight, but they are also characterized by a significantly lower dehumidification performance and an approx. 400 % higher power consumption for each litre of condensate that is dehumidified.
The power consumption of desiccant dehumidifiers can be up to 100 % higher in direct comparison with compression refrigerant dryers. However, the operating costs rather take a back seat when choosing a desiccant dehumidifier, since certain application requirements can only be realized using this type of dehumidifier.
Selection factor effective radius
Compressor refrigerant dryers – great for any room size
The larger the room to be kept dry, the more favourable becomes the use of a compressor refrigerant dryer. This device group offers the largest range of differently powerful fan/condenser combinations for private users.
To keep large rooms dry, the dehumidifier must be supplied with large amounts of damp air, which requires a powerful fan. And to dehumidify these air volumes effectively, the device’s condenser must be equally powerful.
Therefore, when selecting a device, do not only pay attention to the room size suitability specified by the manufacturer, but also check these values for reasonability based on the air flow rate, power consumption and dehumidification specifications. A simple rule of thumb: There is no such thing as “many litres for only a few watts”, even if this is something often insinuated by many providers. ;-)
Peltier dehumidifiers – specialists for small volumes
Peltier dehumidifiers are no conventional room air dehumidifiers, for they were not designed to dehumidify entire rooms but rather to keep specific areas dry. The compact construction and silent operation predestine this device class to be used in closets and shoe cabinets, pantries or in some cases also in small, windowless bathrooms without much ingress of moisture (no shower), since Peltier dehumidifiers are on principle only suited for environments without any additional external moisture ingress (see section “Infiltration” below).
Such desiccants are mainly used to protect moisture-sensitive goods during transport and storage. Everyone knows these little bags that can be found in shipped goods such as electronic items, handbags, suitcases, shoes or medicinal products. Granule bags are very well suited for keeping such goods in confined containers dry.
But there are also larger bags combined with a receptacle available on the market as “dehumidifiers”. Granules are, however, unsuitable for this purpose for several reasons.
On the one hand, their effect is limited to only a few cubic metres of ambient air and can only be noticed in areas without any additional external ingress of moisture (see section “Infiltration” below).
Furthermore, granule dehumidifiers are very expensive in relation to their drying performance since they are a single-use solution requiring the user to regularly buy new granule bags. Above that, granules remain silent and simply stop working once they are saturated with water. No prior warning, no “tank full” indication, simply no more drying ;-)
Desiccant dehumidifiers – professional technology for small, cold rooms
Particularly in cool basements, unheated or only temporarily heated interior spaces, there is virtually no alternative to this device class.
Powerful compressor refrigerant dryers may also still operate quite effectively in environments with temperatures of at least 12 °C, but if the average room temperature constantly lies below 8 °C, desiccant dehumidifiers are able to reach the dehumidification performance required for dry keeping in a more efficient way.
Selection factor functional range
Finally, an equally important criterion for selecting the suitable dehumidifier may also be its range of functions.
Devices of the same performance class sometimes differ with regard to their comfort equipment that may be important for the user’s specific application purpose.
A 24 h timer should be integrated for periodic, automatic dehumidification, and an automatic switch-off function is required if you want the device to switch off after a certain period of time.
Furthermore, many devices offer hygrostat-controlled automatic operation, which is, however, set via a unitless rotary switch in some devices while other devices allow the user to enter the target humidity level as a real value.
The site of operation, too, may be decisive. If you want to use the device in unattended continuous operation, e.g. in a weekend or holiday home, the dehumidifier should be equipped with a drainage connection so that the condensate is discharged into a drain instead of a water tank that has to be emptied regularly.
Last but not least: Infiltration
What sounds like a term from a spy novel does not refer to the infiltration of hostile subjects, but of damp ambient air. For with regard to dehumidifier capacity calculation, “infiltration” refers to the external ingress of additional moisture into the room to be dehumidified.
Therefore, the infiltration factor plays an important role when calculating the dehumidification capacity required. After all, not only the air inside the room contains humidity. Additional moisture penetrates into the room from outside, for example due to the condition of the building’s insulation, through door gaps, or when doors, windows etc. are opened.
For instance, if you want to dehumidify a room with a temperature of 20 °C from a relative humidity of 80 % to 60 %, the water content must be reduced from 13.8 g/m³ (80 % RH) to 10.4 g/m³ (60 % RH), i.e. by 3.4 g per cubic metre of air.
This amounts to 340 g or ml in a room with 100 cubic metres of air, right? No. The moisture introduced from outside must also be taken into account.
If we assume an outdoor climate of 25 °C and 70 % RH, the water content of the outdoor air is 16.2 g/m³, i.e. 5.8 g more than inside. Casually speaking, this moisture would like to mix with the indoor air, which, however, happens only partially as the room is closed and well insulated. This is where the infiltration factor comes into play, which is 0.3 for well insulated rooms, for instance.
Hence, every hour 5.8 g/m³ x 100 m³ x infiltration factor 0.3 l/h = 174 g/h (0.174 l) would be introduced from outside, which corresponds to a daily water volume of 4.176 litres in 24 hours (0.174 l x 24) to be dehumidified.
“Moisture factory human being”
But additional moisture is also introduced from inside. As in the case of infiltration, this factor additionally increases the moisture load. A single pot plant adds about 150 ml of moisture to the room air every day. This is already more than a typical Peltier device can dehumidify on average within 24 hours. However, the moisture load factor becomes even more influential if persons are present.
During sleep, every human being produces approx. 50 ml of moisture per hour that is given off to the room air through the skin. This value increases to 70 ml during easy sedentary activities and exceeds 100 ml during household activities. The moisture ingress must therefore be taken into consideration when selecting a dehumidifier.
It goes without saying that you cannot use a Peltier device to keep a bedroom dry if two sleeping persons alone add 800 ml of moisture to the room air during eight hours of sleep, while a Peltier dehumidifier in practice has a dehumidification capacity of only maximally 300 ml in 24 hours, for instance. In the morning, the air would be more humid than on the day before.
Considering that up to 2 litres of water are emitted into the ambient air when cooking food and even 2.5 litres when taking a shower, it soon becomes clear that all applications involving an additional moisture load are a hopeless endeavour for Peltier devices or granules!
Therefore, when planning your requirements, always include power reserves for the additional moisture load. The easiest way is to rely on Trotec’s application recommendations for the respective device, for here all typical parameters of use are already taken into account.
Quick overview of application possibilities sorted by dehumidifier type
|Peltier (electrical)||Compressor||Granules||Desiccant wheel|
|Air circulation||Hot gas|
|Keeping very small closed areas dry (< 10 m³) without infiltration (moisture input)||Recommended||Possible||Possible||Recommended||Possible|
|Keeping rooms with temperatures from 0 to 8 °C dry||–||–||–||–||Recommended|
|Keeping rooms with temperatures from 5 to 35 °C dry||–||–||Recommended||–||Possible|
|Keeping rooms with temperatures from 15 to 35 °C dry||–||Recommended||Recommended||–||Possible|
|Water damage restoration||–||–||*||–||*|
|* commercial types only, comfort dehumidifiers are not suited|
Practical knowledge concerning dehumidifiers – summary of all chapters
Chapter 1: Basic knowledge regarding humidity – relative is everything
Chapter 2: Overview of dehumidification methods – condensation and desiccation
Chapter 2.1: Condenser dryers with compressor technology
Chapter 2.2: Condenser dryers with Peltier technology
Chapter 2.3: Desiccant dehumidifiers
Chapter 3: Which dehumidification method for which purpose?