Every time the door is opened part of the room’s cold air flows out and warm, humid air comes in to replace it. This air has to be both cooled (sensible heat) and have its moisture condensed and frozen (latent heat). Every cubic metre of outside air at 30 °C and 50% humidity brings about 70 kJ of heat to a 0 °C room and more than 110 kJ to a −20 °C room; it also leaves behind more than 10 grams of water per cubic metre. This water becomes frost on the evaporator; a door opened often means frequent defrost.
In preliminary calculations air change is estimated with a "number of air changes per day" that depends on the volume of the room. In a small room the door is large in relation to the volume, so the number is high: in normal use about 17 changes a day are taken for a 30 m³ room, 9 for a 100 m³ room, 4 for a 500 m³ room and 2.5–3 for a 1,000 m³ room; in heavy use these values are doubled, and for frozen rooms they are reduced a little. In large distribution warehouses the calculation is based on the door size, the time it stays open and the density difference between the inside and outside air.
In most stores this load is larger than supposed: at a door with heavy traffic it can be between a quarter and a third of the total cooling load. The ways of reducing it are well known: strip curtain, air curtain, high-speed door, automatic closing, a door-open alarm and an anteroom at 0/+5 °C in front of frozen rooms. Two doors left open opposite each other is the worst case; the draught passes right through the room. If the humidity of the anteroom is kept low, less water enters the frozen room too, and the evaporator frosts up later.
The second meaning is deliberate ventilation. Fruit and vegetables produce carbon dioxide through respiration and ethylene as they ripen; in a closed room the carbon dioxide must not exceed 0.5–1%. Rooms that are not opened for long periods are supplied with a few volumes of fresh air a day by a timed fan and damper; in ripening rooms ventilation after ethylene is part of the programme. If the fresh air is taken in when the outside air is cold (at night) its load is smaller; in large plants ventilation with heat recovery is used. The cooling load calculator takes door usage intensity as an input and shows the air infiltration load as a separate item.