Smart Humidity Control for Laundry Rooms

June 1, 2006 By: Jörg Fetz, Sensirion Sensors

An RH sensor-based line-drying technology will save your silks and holds potential for other appliance applications as well.

The pressure is constantly increasing to bring ever-smarter products, including white goods, to market more and more quickly while keeping production costs down. New household appliances should have as small an effect on the environment as possible—reducing energy consumption is an excellent case in point. These requirements call for innovative solutions that help both manufacturer and consumer save money while enjoying the benefits of new technology.

One household appliance that meets these challenges is a family of laundry-room air dryers from ESCO Schönmann (, equipped with controllers from WMAG ( These controllers incorporate CMOSens RH sensors from Sensirion. Domestic drying rooms are more prevelant in Europe than in the U.S., where they are for the most part limited to commercial establishments.

ESCO Schönmann also manufactures air dehumidifiers, which operate according to the same principle used in its clothes dryers. The dehumidifiers turn on automatically when the RH exceeds 65%, preventing mold formation as well as damage to structural materials. The devices are also used to increase the comfort zone in living spaces, defined in part by a certain range of temperature and RH.


Laundry-room air drying is an especially gentle way to dry clothing such as silks and knitwear that should not be placed in a hot-air tumble dryer. In drying rooms, the laundry is hung on lines and dried indirectly by dehumidifying the air. In ESCO Schönmann's dryers, the air is drawn in by the device and led past the cooling surfaces of a condenser; the condensed moisture is carried away in a liquid state. The dry air is reheated and blown out like a fan under the hanging laundry, removing moisture as it passes by (Figure 1).

Figure 1. Air drying is kinder to many fabrics than the harsh environment of a tumble dryer
Figure 1. Air drying is kinder to many fabrics than the harsh environment of a tumble dryer

As-Needed vs. Time-Controlled

The precursors of today's laundry-room air dryers were time-controlled. The user set a certain time during which the air was dehumidified. When the time had elapsed, the dryer stopped whether or not the laundry was dry. This meant that the user had to check once or twice to make sure the laundry was actually dry. Variables that determine fluctuating drying times include how much laundry is on the lines, how good the air circulation is, whether or not a window or door was was left open, and what sort of clothing is on the line. A pair of jeans dries much more slowly than does a silk shirt. The operation was labor intensive because it required a fair degree of human intervention.

Another disadvantage of the time-controlled method is the unnecessary use of energy. Because it is inconvenient to repeatedly enter the drying room, the operator typically sets a very long drying time.

For 10–15 kg of laundry, a laundry-room air dryer has an average power consumption of 1.2 kW. So optimized drying times can also save money.

Following Energy Guidelines

Guidelines are being more closely defined as to the amount of energy a device may consume. For example, in Switzerland there is "Working out a measurement method for laundry-room air dryers," (Contract No. 65322, Project No. 25464). This document also states that the dryer must turn off when the laundry is dry to save on energy consumption.

Because the RH in the drying room is a measure of the degree to which the laundry is dry, an optimized control method will use that value as a regulating parameter. The dryer will then operate only as long as it needs to. After the operator presses the start button, no human attention is required. The new controller from WMAG implements an RH measurement method and controls the dryer according to the following algorithm (Figure 2):

Figure 2. Time profile of RH during the drying process
Figure 2. Time profile of RH during the drying process

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