Sealed systems such as domestic refrigerators and freezers need to be treated differently, since there may not be provision for fitting gauges unless line tap valves are used.
If the temperature control is by thermostat only and there is no low pressure safety control, continuous running of the unit will result if the system:
1. Is short of refrigerant or has a restriction in the liquid line.
2. Is operating in a high ambient temperature or with restricted air flow over the condenser.
3. Has a faulty fixture door seal, imposing a high evaporator load.
4. Has excessive frost build-up on the evaporator.
5. Is operating with a defrost heater in circuit (has a defective timer), giving a high evaporator load.
