These units are easy to install under cabinets, in closets or anywhere with electricity. Typically in separate-condenser dryers, cool air is used to cool down the process air from the inside drum and to condense the vapor. In combined washer dryer units, however, cold water is used instead. The water flows in the opposite direction to the air, allowing the air to cool and to release its moisture, which is pumped out along with the water used to cool the air. These machines normally take longer than regular dryers, because the combo unit has a smaller drum, so there is less volume to allow air circulation and the drum itself must be dried immediately after a wash cycle. This water-fed drying system consumes water for both the washing and drying phases of operation, and may not be suitable for areas where water is scarce. In recent years alternative methods of cooling the condenser have emerged. Since condensing dryers discharge waste heat inside buildings, they may increase air conditioning heat loads in summer or provide useful heat in winter. Energy and water usage must be evaluated for the entire system over the entire year to ensure a valid comparison. Washer dryer combinations are normally built with a number of sensors and systems to automate much of the washing and drying cycles. Some of the higher-end model washer dryer combos have sensors that monitor water level, suds levels, temperature levels, and garment dryness. The information gathered by these sensors are used to control the spin speed, cycle settings, draining systems, and other functions. Some washer dryer combination units have child lock features to help keep children and the machines themselves safe. These locks typically prevent the door from opening when the unit is full of water or if the internal temperature is above a certain limit. The amount a combo washer dryer unit can hold varies depending on the type of unit. Most hold a slightly smaller amount compared to full size machines. LG makes the only full size washer dryer combos available in the US. Like the equivalent difference between washers and dryers, the washing capacity is greater than the drying capacity on most combo units. Airflow through the drum is a necessary part of clothes drying, so the difference is greater (these units can generally dry only 1/2 to 3/4 of their maximum wash capacity). AEG-Electrolux debuted the first heat pump washer-dryer combo unit at IFA Berlin, September 2013 (AEG Okokombi). Commercialization of this product started in April, 2014, and it is currently available in the whole of Europe.