Smart Humidity Control for Laundry Rooms

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

When the laundry is placed on the lines (1), the RH in the room quickly increases to 90%–95%. The hygrostat of the controller is set to 50% RH at the factory, so as soon as the RH rises above 50%, the dryer turns on automatically. Depending on the amount of laundry and the characteristics of the fabric, the room RH decreases at different rates (2). When the actual RH falls below the set value of the hygrostat (3), the dryer switches off.

For the next two hours the dryer remains in standby mode, i.e., it turns on again if the air humidity increases over the set value. This can and will happen if the dry clothing is not removed promptly because the floor, ceiling, and walls release moisture that was previously absorbed from the humid air. The set value at (3) is user-selectable:

  • 1. Set value <50% RH 1/2 → the laundry will be drier
  • 2. Set value >50% RH → the laundry will be less dry

The RH Measurement Evolution

WMAG's first humidity-controlled dryers were based on a mechanical measurement principle. A plastic band expanded or contracted depending on the RH. This change in length was converted into a switching pulse that turned the dehumidifier on or off. The technique was expensive and imprecise, and had problems with contamination. Furthermore, the sensors had a large hysteresis that caused drying times to be overly long with a concomitant over-consumption of energy.

The second generation of controllers incorporated electronic humidity sensors with analog output signals that had to be evaluated by a complex circuit. For both the first and second generations it was up to the manufacturer of the controller to perform the calibration. This required a sizeable outlay for precise reference instruments and climate chambers.

The third, and current generation of combined humidity and temperature sensors, the SHT11, offers several advantages over its previous counterparts.

For starters, they have a digital output and integrated calibration. They are based on CMOSens technology, a combination of sensor element and evaluation electronics (Figure 3) Everything is integrated on one single CMOS chip—the sensor elements, evaluation electronics, calibration data, and digital interface—a feature that significantly reduces development time.

Figure 3. Block diagram of the CMOSens chip and its components
Figure 3. Block diagram of the CMOSens chip and its components

The chips are fabricated by means of a common CMOS semiconductor process. The sensor elements are capacitive, and therefore immersible (well-suited to high-humidity areas), robust, and characterized by long-term stability.

Tests at 85% RH and 85°C and a dwell time of more than 1200 hr. caused a reversible drift of only +2% RH. Accuracy is ±3.5% RH.

This very compact solution (Figure 4) provides the controller manufacturer with certain benefits not offered by conventional RH sensors:

  • 1. No calibration required
  • 2. No additional external components except for a pull-up resistor
  • 3. No additional circuitry for temperature measurement
  • 4. Short development time
Figure 4. A photo of the chip shown in Figure 3
Figure 4. A photo of the chip shown in Figure 3

It is important to note that at temperatures below 15°C, frost forms on the condenser of the dryer so it must be periodically defrosted to maintain its energy efficiency. In addition to RH, the temperature can be read out over the digital interface of the SHT11.

More Markets

The home appliance industry stands to benefit further from collaborative development work of the sort illustrated by ESCO Schönmann, WMAG, and Sensirion. The drying-room application's energy savings and ease of operation translate into potential uses in other devices including tumble dryers, dishwashers (for the final drying cycle), steamer ovens, and even microwave ovens, which could be made smart enough to keep from dessicating your croissant.

CMOSens is a registered trademark of Sensirion.

Jörg Fetz, MSEE , can be reached at Sensirion, Staefa, Switzerland; +41 (0)44 306 40 07, [email protected],