Moisture in the air (water vapor) enters a refrigerator each time the door is opened. The water vapor will condense on cold surfaces. The evaporator coil is the coldest surface in the refrigerator. Since the refrigerant in the coil may be twenty degrees below zero, the surface of the coil is well below the freezing point of water. The condensed water vapor quickly freezes into ice. The ice build up on the evaporator will continue to grow thicker and thicker if it is not removed. In units without a fan circulating air, the ice build up will lower the cooling efficiency of the evaporator, but will not prevent the unit from maintaining temperature. A good example is a dorm fridge with no fan. The freezer section is usually the space inside a roll bonded evaporator. The evaporator may form so much ice that much of the storage space is lost, but the temperature of the box will still be adequate.
In units with a separate freezer and fresh-food compartment, the evaporator is normally in the freezer section and an evaporator fan circulates air to the freezer and fresh-food area through duct arrangements. The amount of cold air entering the fresh-food section is controlled by a manual or electronically controlled damper. The evaporator coil in these systems must have airflow from the evaporator fan to work properly and must be defrosted in order to maintain this airflow. Defrost is accomplished by energizing an electric heating element on or near the evaporator coil.
If the defrost circuit fails and ice builds up on the evaporator, airflow from the fan will diminish. Lack of airflow causes more ice to form. Soon the evaporator will be encased in a solid block of ice and no air will circulate. The result will be fresh food temperatures above 50 degrees and a freezer temperature at or near 32 degrees. The unit will run continuously since it cannot reach the set point of the cold control. This continuous running builds even more ice on the evaporator coil.
The way that the defrost cycle is initiated has changed over the years. When energy consumption was not a priority, a clock circuit energized the defrost heater based on time. A clock circuit would switch the unit into defrost every six, eight, or twelve hours (depending on which defrost timer the manufacturer installed) without any consideration of the usage or conditions. Even if the refrigerator was in a low humidity environment and had run very little, the heater still turned on every few hours. This system used a lot of energy.
After government regulations to improve energy efficiency were implemented in the late 1980s and early 1990s, the defrost circuit was redesigned so that a defrost cycle was based on compressor run time. A refrigerator not experiencing a lot of door openings and closings ran less often than one that was heavily used. Therefore some unnecessary defrosts were avoided (the ice build up depends on moisture laden air entering the unit) by initiating defrost based on accumulated compressor run time.
When the government dictated further reductions in energy consumption, manufacturers had to redesign the defrost controls to make them smarter in order to comply. The defrost system with its electric heater uses more energy than any other part of the refrigerator. Therefore it was the best place to look for energy reduction. The solution was adaptive defrost control. The defrost timer was replaced by an electronic control boards containing a microprocessor. Inputs to the control monitored door opening and closings, cumulative compressor run time, duration of last defrost, and in some models the ambient temperature where the unit was located. The algorithms programmed into the microprocessor would adapt the defrost frequency and duration to operating conditions and changes in usage. With adaptive defrost control, a defrost cycle can occur in as little as six hours or as long as seventy-two hours.
Adaptive defrost control does reduce energy consumption, but whether the energy savings is cost effective is another matter. Most do-it-yourself homeowners could troubleshot and repair a defrost system with a defrost timer, but many will call a repairman if electronic controls are not working. A price check on the net for a defrost timer resulted in prices ranging from 18 to 45 dollars. ADC boards seem to start at 65 dollars and most were closer to a hundred dollars. With a service charge and labor, having a technician replace the board will probably be a two to three hundred dollar repair. The money that the consumer saves on electricity will likely have to be spent repairing electronic controls.