moisture on the cooling coils. To be effective as a dehumidifier, an air conditioner must be designed or modified so that most or all of the water that condenses is drained away in liquid form, rather than re-evaporated. Even if condensate is drained, a modified air conditioner is still less efficient than a single-purpose appliance with a design optimized for dehumidification. Dehumidifiers are designed to pass air directly over the cooling coils and then the heating coils in a single efficient pass through the device. In addition, most air conditioners are controlled by a thermostat which senses temperature, rather than a humidistat that senses humidity and is typically used to control a dehumidifier. A thermostat is not designed for the control of humidity, and controls it poorly if at all. Ice buildup Under certain conditions of temperature and humidity, ice can form on a refrigeration dehumidifier's evaporator coils. The ice buildup can impede airflow and eventually form a solid block encasing the coils. This buildup prevents the dehumidifier from operating effectively, and can cause water damage if condensed water drips off the accumulated ice and not into the collection tray. In extreme cases, the ice can deform or distort mechanical elements, causing permanent damage. Better-quality dehumidifiers may have a frost or ice sensor. These will turn off the machine and allow the ice-covered coils to warm and defrost. Once defrosted, the machine usually will automatically restart. Most ice sensors are simple thermal switches and do not directly sense the presence or absence of ice buildup. An alternative design senses the impeded airflow and shuts off the cooling coils in a similar manner. Certain malfunctions of dehumidifiers, such as partial loss of refrigerant, can cause repeated icing of the coils. This condition requires repair or replacement of the equipment. Thermoelectric dehumidifiers Thermoelectric dehumidifiers use a Peltier heat pump to cool a surface and condense water vapor from the air. The design is simpler and has the benefit of being quieter compared to a dehumidifier with a mechanical compressor. However, because of its relatively poor Coefficient of Performance, this design is mainly used for small dehumidifiers. Ice buildup may be a problem, similar to problems with refrigeration dehumidifiers. Absorption/desiccant dehumidification This process uses a special humidity-absorbing material called a desiccant, which is exposed to the air to be conditioned. The humidity-saturated material is then moved to a different location, where it is