I’ve stood on many loading docks, and I know that moment of panic: you walk past the trailer, put your hand near the rear doors, and feel warm air. You check the display, and it says "Defrost." Your first thought is, "Why is my reefer blowing warm air in defrost mode? Is it cooking my frozen load?"
Before you call for emergency roadside assistance, take a breath. In many cases, heat generation is a necessary part of the process, but blowing that heat onto your cargo is not. Let's dig into the technical manuals from Thermo King and Carrier to understand what is actually happening inside your box.
The Mechanics of Defrost: It’s Supposed to Get Hot
First, we need to understand how your unit gets rid of ice. Under normal operation, frost accumulates on the evaporator coil, blocking airflow. To clear this, the unit must introduce heat.
• Hot Gas Defrost (Common in Thermo King): The unit uses high-temperature refrigerant vapor. This hot gas passes through the evaporator coil to melt the frost from the inside out.
• Electric Heat Defrost (Common in Carrier): The controller turns off the compressor and energizes electric heaters located underneath the evaporator coil to melt the ice.
Here is the critical rule: During a standard defrost cycle, the controller is programmed to de-energize (turn off) the evaporator fans.
If the fans are off, you shouldn't feel a strong blast of air. You might feel radiant heat coming from the unit because the coil heaters can reach temperatures up to 54°C (130°F) before the termination thermostat (HTT) cuts them off. This radiant heat is normal; forceful blowing of hot air is not.
Are You Sure It’s Defrost? (Check for "Heat Mode")
This is the most common misunderstanding I see. Modern reefer units, like the Thermo King SLXi or Carrier PrimeLINE, are designed to hold a specific temperature, not just "make things cold."
If you are hauling fresh produce (say, at +4°C) and the ambient outside temperature drops, or if the system overshoots and makes the cargo too cold, the unit will switch to Heat Mode.
• The Logic: If the return air temperature drops 0.5°C (0.9°F) below your setpoint, the controller switches to heating,.
• The Difference: Unlike Defrost mode, in Heat Mode, the evaporator fans stay ON to circulate the warm air and protect your fresh produce from freezing.
So, if your display says "Heat" (often indicated by an orange light) and the fans are blowing, the unit is doing exactly what it's supposed to do.
When Mechanical Faults Cause Warm Air
If your display explicitly says "Defrost" (often an orange light or specific icon), but the fans are still blasting warm air onto your frozen cargo, you have a mechanical failure.
1. The Evaporator Fan Contactor is Stuck
During defrost, the controller sends a signal to open the fan contactor (TN or TE) to stop the fans. If this contactor is welded shut or the relay fails, the fans will keep running while the heating elements are glowing hot. This is dangerous for frozen loads and requires immediate repair.
2. Damper Failure
Even if the fans stop, a physical damper usually closes to isolate the warm evaporator section from the cold cargo area. If this damper is stuck open or damaged, heat can naturally convect into the trailer. Regular inspection of the damper door is a critical part of your pre-trip inspection.
The Root Cause: Why Is Your Unit Defrosting So Much?
If your unit seems to be in defrost mode constantly (blowing warm air cycles every hour), the problem is likely the box, not the engine. Frequent defrosting means moisture is getting in.
• Air Leaks: Research shows that when a vehicle travels at 90 km/h, the pressure difference between the inside and outside of the van can reach 250 Pa. If your door seals are weak, this pressure forces massive amounts of humid air into the box.
• Insulation Degradation: As insulation ages, its air leakage coefficient increases. A standard reefer should have an air leakage coefficient (L) of ≤0.15 to 0.25 depending on its class. Higher leakage means more moisture, more ice on the coil, and more frequent warm defrost cycles.
Next Step
If you are asking "Why is my reefer blowing warm air in defrost mode?", check the following immediately:
1. Look at the Display: Is it actually in "Heat Mode" (fans ON is normal) or "Defrost Mode" (fans should be OFF)?
2. Listen: If the display says Defrost but you hear the evaporator fans roaring inside, pull over. You may need to manually stop the unit to save the load.
3. Inspect Seals: Check your rear door gaskets. If you can see light coming through, you are letting in the moisture that causes the problem.
If the fans are running during a verified Defrost cycle, schedule a service immediately to replace the fan contactor.