In an automatic pump-down circuit, the thermostat does not necessarily stop the compressor at the same instant it closes the liquid-line solenoid. The compressor keeps running until suction pressure reaches the low-pressure control's cut-out. Following that order is more useful than memorizing a switch label in isolation.
What happens when the room thermostat is satisfied?
In Danfoss's documented circuit, the thermostat opens and the liquid-line solenoid closes. Refrigerant feed to the evaporator stops, but the compressor continues to draw from the low side. Suction pressure falls until the low-pressure control opens at its specified cut-out; then the compressor contactor drops out. This is the automatic control sequence in that circuit, not instructions for a manual service pump-down or permission to bypass a safety.
Draw three boxes—thermostat, solenoid and low-pressure switch—and put “solenoid closes” before “compressor stops.” The cut-in and cut-out guide explains the pressure switch itself; this article follows the complete circuit.
Why does the compressor restart after a new cooling call?
When temperature rises, the thermostat closes and energizes the solenoid in Danfoss's example. Liquid feeds the evaporator again, suction pressure rises, and the low-pressure control closes at cut-in. That permits the contactor to start the compressor. The solenoid opens before the pressure switch's restart action. Actual systems can add timers, interlocks and safety controls, so read the equipment wiring diagram before predicting a field sequence.
What does a low-pressure trip alone tell you?
A switch opening on low pressure tells you its pressure condition was met; it does not identify why. In pump-down operation it can be a normal stop after the solenoid closes. Danfoss also describes low-pressure switches used as protective cut-outs and warns that a protective trip does not automatically prove low charge. In a study scenario, ask whether the thermostat was satisfied, whether the solenoid had power, and whether the pressure change followed the specified sequence before naming a fault.
For actual service, follow the manufacturer's diagram and authorized isolation procedure; CCOHS lockout guidance covers hazardous and stored energy. Review the current 313A exam outline, then use the free original, unofficial sample for a separate concept check. Its draft questions still await certified-mechanic review.
Sources
- Danfoss: Sequence of operation, field wiringThermostat, solenoid, pressure switch and contactor order on page 20; checked 2026-10-07
- Danfoss: Pressure controls for heat pumpsPump-down versus protective low-pressure control; checked 2026-10-07
- CCOHS: Lockout/Tag outAuthorized hazardous-energy isolation; checked 2026-10-07
- Red Seal: Refrigeration and Air Conditioning MechanicCurrent exam-standard status; checked 2026-10-07
Examen Studio is independent and not affiliated with the Red Seal Program or Skilled Trades Ontario. This article is a study aid; on the job, follow current codes, regulations and manufacturer instructions.