Air will move from one place to the other as a result of a difference in pressure between the two places. This means that if we want to ventilate a poultry house or a hatchery, we have to create a pressure difference between the inside and outside.

The most common system for ventilation is ventilation by negative pressure. This means that the pressure in the house is lower than the outside, forcing the air to come into the house by the inlets or any other opening.

To create this negative pressure, we have to pump air out of the house. This will create the desired negative pressure, and as a result air will enter the house. Pumping the air out of the house is normally done by a (number of) ventilators. The total fan capacity should be in accordance with the maximum amount of air we need to move per unit of time. This depends on a number of factors such as:
- maximum number and size of birds.
- type of birds, or actually the growth or feed consumption of the birds.
- reason for ventilation, either air replacement or cooling by air velocity.

Once the fan(s) start running, air is moved out of the building. The total size of the openings in the house will determine how much air can come in, and therefore the resulting negative pressure.

Negative pressure has two major points to keep in mind.
- How higher the negative pressure is, how greater the speed of the air will be when entering the house. This means that if the negative pressure is low, the air jet entering the house will not likely make it to the middle of the house, therefore creating a dead spot without air refresment. Too high negative pressure, and resulting high air velocities, might result in undesired cooling of the birds by air velocity (draft).
- How higher the negative pressure is, how smaller the amount of air moved by a fan, although the fan is running at full speed. This means that not the fan speed determines the amount of air refreshement, but the combination of fan speed and negative pressure. Each fan has a specific diagram, that shows the fan capacity in relation to the negative pressure. When designing a ventilation system, make sure that the fan capacity is calculated with an adequate level of negative pressure.

To have the ventilation system operating in the expected way, it is important that the air comes in through the inlets, and not through all kinds of cracks and leaks in the house. To check this, close all the inlets, let one ventilator run, and measure the pressure difference between inside and outside. A significant negative pressure should be achieved.