A refrigerator consists of a thermally treated compartment and a heat pump that conveys heat from the core of the fridge to its outside environment so that the inside of the fridge is cooled to a warmth below the room temperature. Refrigeration is a necessary food storage technique in advanced countries. The lower temperature lowers the propagation rate of bacteria, so the refrigerator decreases the rate of spoilage. A refrigerator keeps a temperature a few degrees beyond the freezing point of water. The excellent temperature range for perishable food accommodation is 3 to 5 °C (37 to 41 °F). A comparable device that keeps the temperature under the freezing point of water is called a freezer. The refrigerator replenished the icebox, which had been a popular household machine for almost a century and a half.
Freezer units are employed in households and in industry and commerce. Food stored at or under −18 °C (0 °F) is reliable indefinitely. Largest household freezers keep temperatures from −23 to −18 °C (−9 to 0 °F), although some freezer-only units can produce −34 °C (−29 °F) and below. Refrigerator freezers usually do not achieve lower than −23 °C (−9 °F), since the equivalent coolant loop works both compartments: Lowering the freezer section temperature extremely causes difficulties in maintaining above-freezing temperature in the refrigerator compartment. Domestic freezers can be included as a separate compartment in a refrigerator or can be a separate machine. Domestic freezers maybe both upright units following a refrigerator, or containers. Various modern upright freezers appear with an ice dispenser formed into their door. Some upscale models combine thermostat displays and controls, and sometimes flatscreen televisions as well.