In an electric-defrost study scenario, the heaters can switch off while the evaporator fans stay off. That pause is purposeful: the coil needs to cool before the fans move air through the box again. A Heatcraft unit-cooler manual gives a concrete example of how one combined defrost-termination and fan-delay switch produces this sequence. Its temperatures and wire colours are model-specific, not universal 313A exam values.
What are the two jobs of this control?
In Heatcraft's unit-cooler installation manual, the combined control is a single-pole, double-throw thermostat. One contact tells the defrost timer to terminate heating after the coil warms sufficiently. The other permits the evaporator fans to restart only after the coil cools again. They answer different questions: “Is defrost finished?” and “Is the coil cool enough to move air?”
That is different from the low-pressure switch, which senses refrigerant pressure, and from the high-pressure controls, which have different loads and switch actions. Identify the sensed variable and the controlled circuit before reasoning from a label.
What happens as the coil warms and cools?
For the particular Heatcraft control described in that manual, when coil temperature rises above approximately 55 °F, the defrost-termination contact closes and the fan contact opens. The timer can leave defrost and stop heating, while the fans remain off. As the coil cools below approximately 35 °F, the fan contact closes and the termination contact opens. The fans may then run if the rest of the circuit calls for them.
A paper timeline makes the gap visible: heating → warm-coil termination → refrigeration resumes with fans held off → cool-coil fan restart. The manual says a warm box may cause the fan to recycle during initial pull-down. Do not diagnose a bad fan motor from “fans off just after defrost” alone. These are example thresholds from one fixed control; use the equipment's own documentation for any real unit.
Why not restart the fans as soon as heating stops?
Heatcraft says the fan delay prevents warm air from being blown into the refrigerated space while the coil cools. The control's two temperature events provide a simple distinction: termination stops the heating interval; fan delay holds airflow until the coil has cooled. A timer's maximum defrost duration is another separate function and does not prove the coil reached its termination temperature.
If a written problem says “defrost ended but fans are still off,” ask for the coil temperature and contact state before blaming the fan circuit. If it says “fans stay off after the coil is cold,” other parts of the fan circuit and the exact model would matter. This is a reasoning exercise, not a field diagnostic procedure.
How should a 313A candidate study the sequence safely?
Draw two contacts sharing a common and mark which is closed at a warm coil and at a cool coil. Then narrate the change through the timer, compressor and evaporator fan without memorizing the example setpoints. Manufacturer designs vary, including electronic controls that do not use this mechanical thermostat. For live equipment, follow the model wiring diagram and authorized hazardous-energy procedures; CCOHS lockout guidance explains the safety framework.
For exam scope, start with the Ontario 313A exam information and current-standard note. Red Seal still identifies its previous occupational standard as the current exam basis, checked 2026-09-30. The free sample is original, unofficial draft practice pending certified-mechanic review, not a prediction of your result.
Sources
- Heatcraft: Unit Coolers Installation and Operations ManualFan Delay & Defrost Termination Control and sequence; checked 2026-09-30
- CCOHS: Lockout/Tag outHazardous-energy controls for authorized work; checked 2026-09-30
- Red Seal: Refrigeration and Air Conditioning MechanicCurrent versus newer exam-standard status; checked 2026-09-30
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.