Why Does the Compressor Never Shut Off?

Long runs are fine; a warm shelf is not

The compressor (the motor-driven pump that pushes refrigerant around and does the actual cooling) runs until its section reaches the setting, then rests. Sub-Zero’s owner’s guide lists what stretches a run: hot days and a warm room, doors held open or opened often, and a lot of food added at once. Its service manual adds that the units perform best in a room between 60°F and 90°F.

So a compressor humming through a warm afternoon while the thermometer reads right is doing its job. The fault is running hour after hour, overnight too, with the section still warm: heat is getting in faster than the unit can take it out.

The order the causes are ruled out

• Airflow at the condenser (the finned coil behind the grille where heat leaves). Packed dust, or a condenser fan that has stopped, leaves heat in the machine and drags out every run.
• The doors. A gasket that no longer seals the whole way round, or a drawer held off its seat by a package, lets damp kitchen air in day and night.
• Ice on the inside coil. If a defrost fails, frost packs the evaporator (the cold coil behind the back panel) until air can barely pass.
• Refrigerant that has escaped. A sealed system never uses refrigerant up, so a short charge means a leak somewhere.
• A compressor that pumps weakly. It hums like a healthy one and moves less.

What the visit covers

Both sections are measured with a probe thermometer and the run is timed, not guessed at. The condenser, the fans and the door seals come first. The gauges go on last, as Sub-Zero’s service manual orders it, after every electrical and mechanical cause is cleared; the pressure pattern then separates a leak, a blockage in the tubing and a tired compressor.