Chapter 8 - Introduction

Moisture removal is the primary design consideration for cold weather ventilation. Excess moisture is often related to elevated levels of air contaminants, such as ammonia. The minimum ventilation raterefers to the quantity of fresh outside air which should be moved through the building to absorb and remove moisture and contaminants. Ventilation rates in excess of the minimum level will provide a fresher environment but increase heating costs. Relative humidity (RH) between 50-70% will prevent excessively dusty conditions while staying within a range where pathogens are less viable. Depending on the pathogen, its survival is best at relative humidities below 40% or above 70%.

The strategy for cold weather ventilation moisture removal is to use cold, but relatively dry, outside air as a ‘sponge’ for absorbing moisture within the building. The moisture-laden air is then exhausted from the house. This process depends on heating the cold air to increase its moisture-holding capacity. Warm air holds more water (in pounds of water per pound dry air, the Humidity Ratio) than cool air. This is one of the key principles of winter ventilation (see Chapter 7 for more on relationship between temperature and moisture).

Minimum ventilation rates for poultry depend on several factors:

• Poultry moisture production (which depends on poultry species, bird age, number of birds in the facility, and building indoor temperature)
• Temperature of outside air (when cold outside air is brought into a building and heated, its ability to absorb moisture is nearly doubled for each 20°F increase in temperature)
• Desired inside relative humidity (recommended range is 50-70%)
• Waste handling practice (a deep-pit layer house requires greater minimum ventilation rate than does a floor-litter broiler/ broiler breeder house)
• Drinking water delivery method (spillage and leakage of water requires a greater minimum ventilation rate)
• Litter management (flocks raised on old litter may require a higher ventilation rate for ammonia control)

In current ventilation control systems, air temperature, rather than relative humidity level, is used to control fans and heaters during cold weather conditions. This is primarily due to past difficulties in reliably sensing relative humidity in livestock housing environments. Instrumentation sensor and controller technology continues to evolve so that more reliable and affordable RH sensors and controllers are becoming available. Instruments are available for spot-checking RH level so that a producer can fine tune the ventilation system control strategy.

With some bird management practices, ammonia level or other air quality issues may demand a higher minimum ventilation rate than that needed for moisture removal. For broilers reared on litter, the method of litter handling between flocks can have a substantial impact on air quality and heating fuel use. Many operations practice limited litter removal between flocks. Some practice a period of one to four days of high temperature ammonia ‘cook-off’ prior to placing a new flock. The added fuel cost for this cooking-off period may be offset when there is less need for an elevated ventilation rate for ammonia control during brooding; however, complete replacement of litter (at least in the brooding section) may be a more cost-effective alternative.