A unit running with the door open draws warm, humid outside air into the body, burns fuel and speeds up frost on the evaporator; a unit shut at every stop has to work extra to return to target once the door closes. Both sides have a point, so the decision follows stop length and door state. Thermo King's door switch turns the unit off when the door opens and restarts it when the door closes, and is offered to cut fuel use and heat gain into the load.
| Stop | Unit | Why |
|---|---|---|
| Very short, door closes at once | May stay on | Recovery is short anyway |
| A few minutes, door open | Shut down | Warm air is not drawn in, fuel is not wasted |
| Long wait, door closed | Stays on | The body warms |
| Long wait, door open | Shut down | The unit cannot cool the street |
If the load sits in insulated containers, warming of the body air while the door is open does not pass straight to the product, so shutting the unit at stops creates no risk. Do not leave the rule to each driver's habit: once the sentence 'if the door will stay open longer than this many minutes, the unit is shut off' is posted in the vehicle, practice becomes uniform and measurable.
When comparing fuel, remember that a fuel card shows purchases, not total consumption; report unit fuel only when it is measured separately.
