How does negative pressure ventilation maintain the temperature in the chicken coop

Negative pressure ventilation is the most common method of ventilation in chicken houses during cold weather worldwide. Exhaust fans are used to draw in fresh air, while air intakes not only facilitate the uniform distribution of fresh air throughout the entire chicken house but also help guide cool intake air along the ceiling. As the cool outside air descends through the ceiling, it is warmed and dried by the heat generated by heaters, stoves, and the chickens. The longer the cool intake air moves along the ceiling, the warmer and drier it becomes, and the greater the control that farmers have over the temperature, air quality, humidity, as well as the performance, health, and welfare of the broiler chickens within the house.

01 Measure negative pressure

The level of negative pressure generated by an exhaust fan is typically measured in "inches of water column." If the negative pressure in a chicken coop is 0.10 inches, a tube can be guided from inside the coop to outside, and the end of the tube (outside the coop) can be placed in a glass of water. Water will then be drawn out of the tube by 0.10 inches. When the exhaust fan is turned on during minimum ventilation, the negative pressure generated by the fan will remain constant throughout the entire coop, regardless of its size. Therefore, the location where negative pressure is measured inside the house is not important.

Through negative pressure ventilation, the exhaust fan brings in a precise amount of fresh air, while the air intake guides cold intake air along the ceiling, heating and drying it before it descends to floor level. The negative pressure generated by the exhaust fan does not affect the flock. This is because the negative pressure generated by the exhaust fan is very small compared to the daily changes in external atmospheric pressure. Atmospheric pressure typically varies between 29 and 31 inches of mercury, equivalent to 394-421 inches of water column, a difference of 27 inches. The pressure generated by the exhaust fan during minimum ventilation is usually between 0.08 and 0.12 inches of water column, which is less than 0.04% of the pressure experienced by the flock due to weather changes. The negative pressure generated during minimum ventilation depends on the size of the intake opening corresponding to the exhaust volume. The larger the opening, the lower the negative pressure generated by the exhaust fan. If two 36-inch fans have an opening of 20 square feet, the pressure is about 0.15 inches. If the available opening for the fans is doubled to 40 square feet, the pressure will be reduced to about 0.04 inches.

02 Ensure the incoming air is as warm as possible

The level of negative pressure determines the speed at which air enters through the air intake. The higher the negative pressure level, the faster the air enters through the intake. For example, at a static pressure of 0.05 inches, air will enter through the intake at a speed of approximately 900 feet per minute. If the pressure increases to 0.10 inches, the speed will increase to 1250 feet per minute. Generally speaking, the faster the air enters the house, the longer it will stay along the ceiling, and the higher the temperature will be before it drops to floor level. For instance, increasing the static pressure from 0.05 inches to 0.10 inches (with the same intake opening size) can increase the distance that the cold intake air travels along the ceiling by 30% to 40%. It is important to remember that the negative pressure level is not the only factor that determines how far the incoming air travels along the ceiling. For any given chicken coop, how far the air enters through the intake travels along the ceiling before moving down to floor level also depends primarily on factors such as the size of the intake opening and the temperature difference between inside and outside air. The larger the intake opening, the greater the momentum of the airflow, and the farther the air will travel along the ceiling. Theoretically, increasing the size of the intake opening from 1 inch to 2 inches at a static pressure of 0.10 inches will increase the distance that air travels along the ceiling by 20 to 30%. The challenge for managers is that as the size of the intake opening increases, the static pressure decreases, thereby reducing the speed at which air enters the house. Therefore, increasing the static pressure by reducing the size of the intake opening in the chicken coop may not increase the distance that air travels along the ceiling. Therefore, it may be more effective for managers to increase the static pressure by closing some of the intake openings in cold weather. For example, instead of increasing the static pressure by reducing the size of the intake opening from two inches to one inch, they can close half of the intake openings and maintain a two-inch intake opening at a higher pressure/air speed. The combination of a two-inch intake opening and higher static pressure will help ensure that cool air does not fall to the floor prematurely, thereby creating cool, ventilated conditions on the floor.

03 Impact of temperature difference

Many managers fail to realize that the temperature difference between the air inside and outside a building typically has a greater impact on air travel along the ceiling than the level of negative pressure or the size of the air intake opening. Theoretically, increasing the internal/external temperature difference from 10 to 70 degrees Fahrenheit will reduce the distance that incoming air travels along the ceiling by 70%. For example, if a chicken coop has air entering through an air intake, with the inside at 90 degrees Fahrenheit and the outside at 60 degrees Fahrenheit across the ceiling, but the outside temperature drops to 40 degrees Fahrenheit the next day, if the static pressure and inlet opening remain unchanged, at 0.10 inches and 1.5 inches, respectively, the heavier incoming air quickly falls to the floor on the second day. This is why it is generally recommended to increase the target static pressure as the outside temperature drops.

This picture illustrates that the air entering through the inlet passes through the ceiling at 90 degrees inside and 60 degrees outside

The next morning, in the same poultry shed, the temperature outside dropped to 40 F.

The Optimal Solution to Temperature Difference Impact: Although using negative pressure ventilation systems in cold weather is an effective and widely used method of ventilation in poultry farms, it can sometimes be challenging to manage. This is because there is no one-size-fits-all optimal combination of minimum ventilation fan quantity, inlet opening size, and static pressure that applies to all farms under all internal and external air temperature conditions.
For a chicken coop, a good starting point is to operate an interval timer with a capacity of 1 cubic foot per square foot of floor space (cfm) (20,000 cubic feet per square of house = 20,000 cubic feet of timed fan capacity, i.e., two 36-inch fans), and strive to create a static pressure of approximately 0.10 inches, with the side wall air inlet open about 2 inches. From here, managers can experiment with various combinations of static pressure, air inlet opening size, and quantity to achieve the environment they believe is best for the chickens.