Domestic freezers can be a separate compartment in a refrigerator, or can be a separate appliance. Household refrigerators usually have a separate compartment where the heat pump is used to pump even colder temperatures to the contents. Most household freezers maintain temperatures from . Some freezer-only units can achieve and lower. Most household refrigerators generally do not achieve a temperature lower than , because it is difficult to control the temperature for two different compartments. This is because both compartments shares the same coolant loop. 