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In negative pressure ventilation systems, the ventilators are creating a negative pressure by removing air from the house, and as a result fresh air will enter the house through the inlets. In most systems (except in tunnel ventilation systems) the inlets are spread evenly in the side walls, and a limited number of ventilators will be located in the roof or in the side walls.

In a minimum ventilation situation, usually not all the ventilators will be needed to remove enough air for adequate air exchange. As the air enters the house based on the negative pressure and the negative pressure will spread evenly through the house, by itself it doesnt matter where the ventilators are located, as the air will come in evenly through the inlets.

However, when the air enters through the inlets and passes over the birds towards the ventilator, it will pick up the heat from the birds. This means that the air will slowly get warmer when it moves towards the ventilator. This is very clear when we work with tunnel ventilation, where the air towards the ventilators can be as much as 5oC warmer than at the inlet side of the house, but it holds for a system with side wall ventilators or roof ventilators as well.

When the ventilators individually react on a temperature sensor located in the area of the ventilator, the ventilator at the warmest spot will first start to work. This means that the air is coming in through the inlets, moves over the birds towards the ventilator and gets warmed up by the birds. As a result, the air in the ventilator area will be the warmest, the ventilator will start to work more as it is switched on by the higher temperature, and the area of the ventilator wil get warmer. This creates an uneveness in temperature, and the more the ventilator will run the worse it gets.

To create a move even temperature distribution, it is important that the ventilator in the coldest area is switched on when more ventilation is needed, and not the ventilator in the warmest area. In this way, the incoming air will pass over the birds, gets warmed up and moves to the colder area, as there the ventilator is removing the air from the house. In this way the temperature will be more evenly distributed in the house.