The difference in temperature. The difference in air temperatures outside the building and inside the room should be at least 13 ° C. The difference is set in order to determine with higher accuracy, because in the warm season the ventilation works with less performance, and in severe heat, reverse traction is possible at all. Let me explain if the temperature is higher on the street than in the room, the air outside will fall under the strength of its weight, and the one that simply cannot push it out inside the building, and will fall down (physics).
