Some Kenmore refrigerator breakdowns require a complex repair. It obviously demands relevant expenses. However, it will surely cost less than a new appliance. A hissing sound should alert the owner about needed troubleshooting. The sound of hissing in a refrigerator is caused by the activation of something called a thermostatic expansion valve. If coolant was constantly flowing through the copper coils in the rear of the fridge it would get too cold and you'd both waste power and everything inside would freeze. So to maintain the proper temperature there is a valve which only allows coolant through at the appropriate times. The valve itself does this by using the same basic principle as a thermometer. That is, when a substance gets cold it contracts and when it gets hot it expands. So when the fluid in the valve gets warm enough the fluid in it expands slightly and just barely unblocks the port allowing a small amount of coolant to hiss out. Then, when it gets cold again, the valve closes maintaining the refrigerator's temperature. During the auto defrost cycle, the freezer temp is allowed to rise to 32F (0C) to allow the ice to melt. This process is sped up by turning on the heater. As you don't want the food to even begin to thaw. The whole process takes about 20 minutes and when the temp in the freezer reaches 32F (up from 0F or 0C up from -18C) the defrost thermostat operates and signals the control board to stop the defrost cycle, which then turns off the heater and restarts the compressor and evaporator fan again to cool the temps back down to the set temps. This is when the hissing may occur as during normal operation (not defrost cycle) the evaporator unit is a lot colder. This cycle occurs once every 8-11 hours depending on the manufacturer. Nevertheless, the hissing sound could be related to the broken evaporator filter inside or malfunctioning coil filter. A professional repair technician will help to track the issue and eliminate it easily.
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