Have you ever noticed a little door inside the bigger door at the back of a semi trucks trailer? The tiny door on a refrigerated trailer is called the vent door. It is screened and, along with a door at the front of the semi truck‘s trailer, is used to air the truck out whenever the refrigeration unit is turned off.

Why do refrigerated trucks have a little vent door?

When a produce truck‘s refrigeration unit is turned on, the unit cycles the air inside the trailer. But when the unit is switched off, the moisture inside the sealed truck can lead to mold and mildew, which might poison future loads. Drivers use a pair of vent doors at the front and rear of the trailer to air it out.

Semi trucks designed for hauling produce do not always need their refrigeration unit running. Sometimes they are hauling crops such as root vegetables that do not need to be refrigerated. Other times they are carrying other cargo on a “backhaul” to return to their home base. And on occasion, semi trucks are even returning home empty.

Do you know how your refrigerator begins to smell funky if you go a long time without cleaning it out? A refrigerated truck runs the same risk. If a semi truck is cruising down the road with its refrigeration unit switched off, a sealed trailer is a recipe for mildew or even mold.

How do truck drivers use their vent doors?

Vent doors are engineered to create as much airflow as reasonably possible. Therefore, a semi truck trailer will have one vent on the front, upper, driver’s side of the trailer. It will have a second vent on the rear, lower, passenger’s side. The driver simply opens both screened vents, then drives down the road.

If a semi truck driver is hauling a crop such as onions that do not need refrigeration but do need fresh air, these vent doors should do the trick. If the driver of a refrigerated semi truck is driving with dry goods or empty, the vent doors allow them to air out and dry their trailer.

There are many rumors surrounding semi truck trailer vent doors

When Jalopnik interviewed truck drivers and semi truck trailer manufacturers to learn the purpose of vent doors, the publication discovered a lot of misinformation. Some truck drivers told the publication that the small doors in the back of a refrigerated trailer are called “fruit doors” and were originally engineered to slide a box of fruit into the truck without letting all the cold air out.

Others told Jalopnik that the tiny doors are called “inspection doors.” These folks claim that regulators who are curious about what is inside the truck can open the tiny door to check–without breaking the food supplier’s seal on the larger doors.

In addition, some trailer manufacturers seemed hesitant to tell Jalopnik the true use of vent doors, as if they are some form of proprietary technology. But they are truly engineered for venting out refrigerator trucks, despite whatever rumors you might hear to the contrary.

Next, learn why some semi truck wheels don’t touch the ground or see a manufacturer explain the true use of vent doors in the video below: