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Required ventilating rates it depends?

Kevin Janni
Professor and Extension Engineer
February 11, 2012

A teacher once told a class that a quick answer to many apparently simple problems the instructor would pose in class is "it depends" because good answers usually depend on the situation, conditions, goals and objectives associated with the problem. The instructor was making the point that the students would need to do some thinking, analysis and explaining before they could give a complete answer.

So the answer to the simple question, what is the required ventilating rate (cubic feet per minute) depends on the situation, conditions, goals and objectives. So as the manager, what are you supposed to do? It depends. What is the situation? What is the goal?

The goal of ventilation is to provide plenty of fresh, draft-free air at animal level to remove moisture, gases, airborne pathogens and/or excess heat. The amount of air exchange, in terms of cubic feet per minute (CFM), needed to meet the goal changes throughout the day, with weather throughout the year, and as animals age or numbers change.

Sometimes dairy managers have additional goals. In cold weather, managers sometimes want to prevent water pipes and waterers from freezing up or keep manure from freezing to the floor so they reduce ventilating rates to retain cow heat. During blizzards, managers usually want to keep snow from blowing into the freestalls or pens. These are reasonable goals. And one way to attempt to achieve these goals is to reduce ventilating rates temporarily. "Temporarily" is the key word, it is important to remember to restore the ventilating rate to proper levels once weather conditions permit.

Many conditions and factors impact the ventilating rate needed. Some are:

Some factors change in minutes, hours and over days while others may not change much at all. So it depends.

The recommended rates in Table 1 are minimum design rates that ventilation system designers use to size equipment based on number of animals, animal size and age, and weather (i.e., cold, mild and hot) for warm dairy barns. Designers and suppliers use these or other rates to size fans, inlets and fresh air tubes in mechanically ventilated dairy barns. Table 1 gives the rates for warm dairy barns. I use the same rates for cold barns, too, even though more air exchange is required to remove moisture from cold barns during cold weather because cold air does not absorb much moisture.

Some people prefer to use an air changes per hour (ACH) method to calculate minimum ventilating rates. The new 8th edition of the MWPS-7 Dairy Freestall Housing and Equipment Handbook, expected out in 2012, describes how to calculate a cold weather ACH ventilating rate by dividing the room volume (cubic feet) by 15 and a hot weather ACH rate by dividing the room volume by 1.5. Ventilation system designers are encouraged to calculate ventilating rates both ways and use the larger rate.

The ACH method gives excessive ventilating rates in rooms with high ceilings. Crowding calves in barns where the air exchange rate was calculated using the ACH method can have insufficient air exchange. The CFM per head method will generally give a lower ventilating rate and should be considered a minimum. Higher ventilating rates are fine in cold barns as long as the rates do not create drafts at calf level.

One reason for using a higher design ventilating rate is that many ventilating systems have undersized components (ex., inlets or fans) which do not allow the system to provide the design rate, so oversizing reduces chances that an undersized component will cause insufficient air exchange. Dirty fans, corroded louvers and slipping belts can reduce airflow by 50% or more. Undersized or partially plugged inlets reduce airflow rates too.

Design ventilating rates are used to size fans, inlets and tubes. Once the system is installed, managers need to observe the animals, bedding and building conditions to decide whether to increase or decrease the ventilating rate, adjust inlets or make other management changes to provide plenty of fresh draft-free air to the animals.

If you are interested in information about pre-weaned calf ventilation, you will want to attend the 2012 University of Minnesota Extension Virtual Dairy Day on March 7, 2012. Ventilating pre-weaned calf facilities will be one of the presentations. For information about the schedule and presentation sites, visit the University of Minnesota Dairy Extension website.


Table 1. Cumulative ventilating rates (cubic feet per minute per animal) for dairy animals in warm mechanically ventilated barns (MWPS-7, 2012).


Cold weather

Mild weather

Hot weather

Calves (0 to 2 months)




Heifers (2 to 12 months)




Heifers (12 to 24 months)




Cows (1400 lb)




MWPS-7 Dairy Housing and Equipment Handbook. 2012 MWPS, Ames, IA.


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