Precooling a reefer trailer, and proving you did it
A reefer load can be lost before the truck leaves the dock: warm walls, a unit left running with the doors open, or a setpoint nobody wrote down. Here is how to precool the trailer properly, and how to keep the record that defends you if the load is rejected.
Written by Rigo Diaz
Precooling in numbers
Precooling a reefer trailer means running the unit at the shipper's setpoint, doors closed, until the walls, floor, and ceiling stop giving off heat, which can take two hours or more in summer. It cools the trailer, not the product, and the unit goes off while you load at an open dock.
Precooling sounds like a driver's chore. From a risk point of view, it is the first entry in the record that decides who pays when a reefer load arrives warm.
I spent more than six years in risk management before I founded Simplex, and I have led it for more than 25 years since. This guide covers how to precool a reefer trailer, what the FDA expects of carriers that agree to handle temperature control, and the records that turn a dispute into a short conversation.
The full briefing, read aloud
7 chapters. Select one to jump to it.
Generated with AI text-to-speech from this article's key insights, in order, rather than reading it word for word. Chapter times come from a transcript of the audio. The article itself was written and reviewed by Rigo Diaz.
Key takeaways
Precooling removes heat from the trailer itself
Run the unit at the setpoint with the doors closed until the body stabilizes. In hot weather that can take two hours or more.
A reefer holds temperature, it does not pull it down
Product should already be at the shipper's temperature when it is loaded. Frozen food should be at 0°F or colder.
Shut the unit off at an open dock
Running it with the doors open pulls in humid air that can ice the coil and block airflow for the whole trip.
The setpoint should come to you in writing
Under the FDA's sanitary transportation rule, the shipper specifies the operating temperature, including the precooling phase when needed.
Your records are your defense
Readings at loading and unloading, or trip temperature data, are how a carrier shows it held the temperature.
In this articleContents
What does precooling a reefer trailer do?
Precooling a reefer trailer removes the heat stored in its walls, ceiling, floor, and doors before cold product goes in. The UF/IFAS revision of USDA's perishable transport handbook warns that this heat, left in place, "can be rapidly conducted to the cargo," especially to cartons loaded near the walls.
It also protects the unit. USDA's handbook notes that in hot weather, precooling reduces the chance of overloading the refrigeration system and prevents the product from warming or thawing from heat in the body.
Precooling the trailer is not precooling the product
A reefer is built to hold temperature, not to pull it down. The handbook is blunt: transit vehicles "are not designed to effectively precool chill or frozen cargoes." Frozen food should be at 0°F or colder when it is loaded, because there is little or no chance to bring it down in transit.
If produce or meat comes to the dock much warmer than the transit temperature, the handbook says to estimate the full heat load, and not to use the trailer if that load exceeds what the unit is rated to handle. Knowing what kind of freight you are hauling tells you how much margin you have.
How long does it take to precool a reefer trailer?
Long enough for the body to stabilize at the setpoint, which USDA says may take two hours or more in hot summer weather. Ambient temperature, the unit's capacity, and the insulation all change the time, so there is no single number.
The recommended procedure is simple: set the thermostat to the desired temperature, close the doors, and run the unit until heat coming through the body has stabilized at the setpoint. Units with microprocessor controls have an automatic precool cycle, but the handbook still says to schedule adequate time.
- Frozen loads: set the thermostat at 0°F and run the unit at least half an hour before loading, or until a temperature gradient is established across the insulation, longer in warm weather.
- Subfreezing weather: for loads such as fresh onions or potatoes, prewarm instead, running the heat cycle early enough to bring the body to the transit temperature.
- Checking the walls: hold a thermometer probe against an inside wall under a dry cloth. An infrared thermometer works only if its emissivity is set for the wall surface.
Should the reefer run while you load?
Not at an open dock. Both handbooks say to shut the unit off when loading from an open dock: with the doors open, it pulls in warm, humid air that can freeze on the coil and block cold-air circulation for the rest of the trip. At an enclosed refrigerated dock, the unit is not needed during loading.
Where you load also changes how cold the empty trailer should be. In humid air, a trailer colder than the outside dew point sweats on the inside, which can wet cartons and contaminate food. UF/IFAS cites this guide from the University of California:
| Where and how you load | Precool the trailer to |
|---|---|
| Refrigerated dock | The shipper's thermostat setpoint |
| Open dock, fast load (15 to 20 minutes) | About 5°F above the setpoint |
| Open dock, slow load (30 minutes or more) | About 5°F below the halfway point between outside air and the setpoint |
For example, at 75°F outside and a 35°F setpoint, the halfway point is 55°F, so a slowly loaded trailer is precooled to about 50°F. The guide says this avoids moisture and frost on the inner surfaces and heavy defrost cycling afterward.
How to precool a reefer trailer before pickup
This routine combines USDA's preloading and loading checklists with the precool guidance above.
Step 1: Get the setpoint in writing
Confirm the operating temperature, any precool instructions, and whether the unit should run continuously or in start-stop mode. If the rate confirmation is vague, ask before you dispatch, not at the gate.
Step 2: Inspect the trailer and the unit
Check that the unit runs and the thermostat is calibrated, the air chute and bulkhead are in place, door seals close tightly, walls have no cracks or holes, floor drains are open, and the floor grooves are clean. The inside should be clean, dry, and odor free. Fold it into your pre-trip inspection.
Step 3: Precool with the doors closed
Set the target for the dock you are going to, close the doors, and give it the time it needs. Double-check the setting; UF/IFAS puts that instruction in capital letters for a reason.
Step 4: Load with the unit off at an open dock
Record product temperatures as the load goes on. Leave at least 4 inches between the load and the rear doors for return air, keep cartons off the air chute, and secure the load.
Step 5: Close, restart, and record
Confirm the setpoint and mode, place any temperature recorders where the shipper specifies, and write down the seal number. If product went on warmer than the specified temperature, note it on the paperwork before you sign.
What does the FDA require of carriers?
The FDA's sanitary transportation rule (21 CFR Part 1, Subpart O) splits the work. The shipper, which can be a freight broker, gives the carrier the operating temperature in writing, including the precooling phase when needed. The loader verifies the trailer was properly precooled before loading, and the receiver checks for temperature abuse on arrival.
When your written agreement with the shipper makes you responsible for sanitary conditions, in whole or in part, the carrier duties in 21 CFR 1.908(e) apply as the agreement assigns them:
- Precool each mechanically refrigerated compartment as the shipper specifies before offering the trailer
- After the trip, give the receiver the operating temperature on request, and show the shipper or receiver, on request, that you held it
- Keep written procedures for cleaning and inspecting equipment and for temperature control
- Train personnel on hiring and as needed, and record the date, type of training, and who was trained
Procedures are kept for 12 months after they stop being used, and training records for 12 months after the person stops doing that work. The FDA offers a free one-hour training module for carrier personnel.
How do you prove you held the temperature?
With records made at the time. The rule says a carrier can show it maintained the operating temperature by any means agreeable to the carrier and shipper, such as ambient readings at loading and unloading, or time and temperature data from the trip.
That data comes from telematics that read the unit's controller and sensors, or from portable recorders placed in the load. Either way, the strongest file has the written setpoint, the precool and product readings at the dock, the seal number, and the trip data.
The stakes are real. If anyone in the chain becomes aware of a possible material failure of temperature control, the food cannot be sold unless a qualified individual decides it is still safe. If a receiver rejects a load, document it the same way you would document damaged cargo.
Then check your policy before you need it. Cargo policies can exclude refrigeration breakdown, and our guide to what motor truck cargo insurance covers explains the gaps to close before you haul reefer freight.
How Simplex helps you protect temperature-controlled loads
Precooling protects the load. The right coverage and well-kept records protect your business when something still goes wrong.
Cargo coverage for reefer freight
Cargo policies can exclude refrigeration breakdown. We quote motor truck cargo insurance and can place reefer breakdown coverage with cargo programs.
Insurance built around what you haul
As a broker, we place commercial truck insurance from auto liability to physical damage and cargo, so your limits match your freight.
Training and records in one place
Our compliance programs include assigned driver trainings with reminders, monitoring, and file management, so the records you rely on are easy to find.
Frequently asked questions
How long does it take to precool a reefer trailer?+
Should the reefer unit run while the trailer is being loaded?+
Does precooling the trailer cool down the product?+
Is a carrier legally required to precool a reefer trailer?+
How cold can a reefer trailer get?+
Where this information comes from
The requirements below come from the FDA's sanitary transportation rule and FDA guidance, and the practices from USDA's perishable transport handbook and its 2019 university revision, all checked on October 19, 2026. Follow your shipper's written instructions and your unit manufacturer's manual where they differ.
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1
eCFR — 21 CFR 1.908
Shipper, loader, receiver, and carrier duties, including precooling as the shipper specifies and showing temperature was held.
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2
eCFR — 21 CFR 1.904
Definitions of operating temperature, shipper (including freight brokers), and the $500,000 non-covered business threshold.
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3
eCFR — 21 CFR 1.910 and 1.912
Carrier training and what training records must show; 12-month retention for procedures and training records.
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4
FDA — Sanitary Transportation of Human and Animal Food
Who the rule covers, its exemptions, and the three published waivers.
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5
FDA — Training for carriers
FDA's free one-hour training module for carrier personnel.
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6
USDA AMS — Handbook 669, Protecting Perishable Foods During Transport by Truck
The recommended precooling procedure, the open-dock caution, and the preloading and loading checklists.
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7
UF/IFAS — HS1328, Protecting Perishable Foods During Transport by Truck or Rail (2019)
Revision of Handbook 669: precool targets by dock type, frozen-load precooling, wall readings, and temperature recording.
Written by
Rigo Diaz
Founder & CEO, Simplex Group
Founder and CEO of Simplex Group, with more than 25 years leading the company and helping trucking entrepreneurs navigate compliance, insurance, and the day-to-day demands of running a carrier. Before founding Simplex, he spent more than six years in risk management, experience that continues to shape how he approaches insurance, compliance, and carrier risk today.
Published · Updated
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A cold trailer and a written record protect the same load
Precooling is a few hours of fuel and patience. The record of it, from the setpoint in writing to the readings at delivery, is what decides who pays when a receiver says the load arrived warm.
If you want a second look at your cargo coverage for temperature-controlled freight, talk to our team.