Regarding defrost, which control is used to block out the compressor circuit?

• A. Low ambient control
• B. High pressure switch
• C. Thermostat
• D. Fan control

Answer: (A)

Defrost requires a brief interruption of the refrigerant circuit so the outdoor coil can be cleared of frost and heated effectively. The control that blocks the compressor circuit during this defrost sequence is the low ambient control. It interlocks with the outdoor unit to prevent the compressor from running when conditions call for defrost (typically at low outdoor temperatures or during the defrost cycle), ensuring the outdoor coil can thaw without continued cooling action pulling heat away.

The other controls don’t specifically manage the compressor during defrost. A high pressure switch protects against unsafe pressure but doesn’t regulate defrost timing. A thermostat controls indoor temperature and wouldn’t block the outdoor compressor for defrost. A fan control only affects the outdoor fan, not the compressor itself.