Ventilation for Backyard and Small Farm Chickens

 
When we first built our chicken coop, we had concerns about insulating and reducing draughts. This led to us using a building that was already fairly air tight, and we have (by accident and by design) made it even more so. Further reading started to cause concerns for us: had we reduced draughts to the point that we now had inadiquate ventilation. Chickens sweat a lot, they also release large quantities of ammonia in their waste. In order to reduce respiratory ailments, it is important to ensure adequate ventilation of their housing. We know we need to add ventilation to the housing, but how do we do that without creating draughts that will make the animals ill.

In my mind, a draught is defined as a current of moving air, while ventilation is defined as moving air in order to exchange of “old soiled air” with “fresh air”. Given this definition, a serious question arose, where do you draw the line between draughts, and ventilation? How do you reduce one, while maintaining the other? Given draughts and ventilation are effectively the same thing, I am trapped between two contradictory requirements.

Research into this question (both online and through books) led to vague answers: “chickens must not be exposed to draughts”, “chickens require lots of ventilation”, or (my personal favourite) “it is important to reduce draughts, while maintaining sufficient ventilation”. This leads me to believe that it is important to maintain the minimum air current (draught) to support adequate air exchange (ventilation), offering  a suitable balance between the two contradictory requirements. So what is the minimum amount of ventilation required? The only attempt to give answer to this question is that that chickens require “lots” of ventilation, “more than you think”. If it is more than I think, how much is it? Obviously, not a sufficient answer.

To my knowledge, nobody has attempted to really answer this question. This paper is an attempt to resolve these descrepancies in a clear manner.

Definitions

In order to find a suitable balance between draughts and ventilation, it is important identify the exact meaning of these two terms. In the context of livestock maintenance, these two terms refer to a negative and postive effect, it is therefore important to define why these two effects are to be avoided or encourage. By clearly defining these two terms, it is hoped to clearly define the problem, and therefore highlight a solution.

Draughts

In the broadest sense, draughts are re

Ventilation

Fresh Air

Air is a mixture of several chemicals in a gaseous state; some necessary, some neutral, and some are toxic. Necessary chemical components include Oxygen, a chemical consumed by animals in order to produce energy, and Carbon Dioxide, used to regulate respiratory rates. Several gases that can be found in air (such as argon and helium) can be considered completely neutral, in that they have no effect on breathing whatsoever. Other gases, such as Carbon Monoxide, are actively harmful to an animal’s respiratory process.

While some mixture of necessary, neutral, and toxic chemicals are always present in air, in most cases, little thought need be given to the air we breathe. Necessary components are present in sufficient volumes, Neutral components are just there, and Toxic components are in sufficiently low quantities to allow our bodies to manage them. Where there is a cause for concern is when factors lead to the mixture of these components changing their mixture to dangerous levels.

While Oxygen is necessary, and in fact the point to breathing, to high a concentration of O2 can displace all of the CO2. The effect is that the body is not stimulated to cycle the air in the lungs, quickly leading to a lack of oxygen being exchange with the blood stream, causing a shortfall in oxygen in the blood. An animal in a pure oxygen environment will quickly asphyxiate.

Conversely, gases that are harmless and inert, such as CO2, or Argon, in sufficient quantities will displace the Oxygen. This leads to the animal inhaling, but not bringing in sufficient quantities of Oxygen with each breath. Under these conditions, an animal will quickly asphyxiate.

Finally, toxic gases, while always present, do cause physical damage to the body. In most cases, the amount of damage to the body is well within the body’s ability to repair itself. In cases where a larger volume of toxic chemicals make up the air, more damage which accumulates faster than the body can repair it. The more of these chemicals present, the more damage caused. The longer spent in the environment, the more damage is caused. If the body is not given an opportunity to repair the cumulative damage, these effects will result in the death of the animal (with differing effects in different animals).

Several animal life processes result in a change to the chemical make-up of air, and the objective of ventilation is to replace this “consumed”, “polluted”, or “stale” air with “fresh air”, or air in which the balance of chemicals is more appropriate for animal consumption. This leads to three considerations: what pollutants, in what volumes, are the animals releasing; what concentration of pollutants is considered acceptable; and what pollutants, in what volumes, does the source of “fresh air” contain? Or to paraphrase: what is acceptable, how much are we producing, and how much are we bringing in?

Air Exchange Volumes

In asking how much ventilation we require, we are really asking what volume of air needs to be moved in a given time frame. In building engineering this is often defined as the frequency in which a room must have its entire volume of air replaced, this is then broken down into a flow rate defined in cubic metres per minute.

Several variables need to be taken into account in determining the minimum

Ventilation Methods

Good discussion of Ventilation Techniques

http://www.wbdg.org/resources/naturalventilation.php

Difussion

Wind

Qwind = K x A x V, where

Qwind = volume of airflow (m³/h)
A = area of smaller opening (m²)
V = outdoor wind speed (m/h)
K = coefficient of effectiveness

Stack Ventilation

http://www.arca53.dsl.pipex.com/index_files/natvent2.htm

http://www.engineeringtoolbox.com/natural-draught-ventilation-d_122.html

$latex Q_{stack}$ volume of ventilation rate $latex frac{m^3}{s}$
$latex C_d$ 0.65, a discharge coefficient
$latex A$ free area of inlet opening (m²), which equals area of outlet opening $latex m^2$
$latex g$ 9.8 (m/s²). the acceleration due to gravity $latex frac{m}{s^2}$
$latex h$ vertical distance between inlet and outlet midpoints $latex m$
$latex T_i$ average temperature of indoor air (K), note that 27°C = 300 Kelvin $latex K$
$latex T_o$ average temperature of outdoor air $latex K$

$latex Q_{stack} = C_dAsqrt{2ghfrac{T_i-T_o}{T_i}}$1

Mechanical Ventilation

This includes the use of fans.

Case Study

Once an understanding of the mechanics involved is in place, it is possible to apply these principles to our chicken coop on our site.

In an effort to reduce costs, a natural method of ventilation would be preffered, for this reason, an attempt will be made to put a stack ventilation system in place to naturally drive the air through the coop.

During the summer, we are able to leave windows and doors wide open to allow for wind to push air through the building, during the winter, this is not feasible due to low tempuratures, and driving snow. Therefore, most of the focus will be on exchanging the air during the winter, when the temperature difference will be its most extreme. At around -5°C (268°K), the building will have to be shut up to conserve heat. This would be the time when controlled ventilation is most important.

We currently have 4 Cochin Hens, 1 Cochin Rooster, and 7 Comet Hens, for an estimated ??? Kg of chicken. Chickens produce approximately, ??? of moisture, ??? of ammonia, that will need to be removed. Further, chickens produce body heat, representing one of the heat inputs into our coop: 2 Watts/pound.2

Our site consists of a baby barn, converted into a chicken coop with a door leading to the outside for the chickens. The intake can be placed on the north side of the building, which places the intake as far as possible from the birds, and places a wall between them and the incoming air. Unfortunately, this opening would therefore open close to the garage, allowing for pollutants to be drawn in via the intake, though the amount of traffic in the garage would indicate this is not likely to be a significant factor. The outflow should therefore be positioned on the south side of the building, which is the side open to the fields, allowing for maximum exposure to wind which will help draw the air out. The building is ??? meters tall, giving us a starting point for vertical distance between the input and outflow. The entire building has an air volume of ???. Based on the amount of waste produced by the chickens, we will want to exchange all of the air in the building every ??? minutes, and this must be driven by the birds body heat. This translates to a target flow rate of ??? cubic meters per second.

All that remains to be determined is how big of an opening to create to allow for adequate ventilation.

Weather Variables

Data representing temperature and wind around the coop was collected from a nearby weather station. While not an exact match, it should offer a reasonable measure of temperatures and wind effects in our area. Unfortunately, it was not possible to obtain data for an entire year.

Inside Temperature

Vent Size

$latex A = unknown$

$latex Q_{stack} = ?$
$latex h = ?$
$latex C_d = 0.65$
$latex T_i = ?$
$latex T_o = 268K $
$latex g = 9.82m/s^2$

$latex Q_{stack} = C_dAsqrt{2ghfrac{T_i-T_o}{T_i}}$
$latex A = frac{Q_{stack}}{C_dsqrt{2ghfrac{T_i-T_o}{T_i}}}$
$latex A = frac{Q_{stack}}{0.65 sqrt{2 times 9.82 times h times frac{T_i-268}{T_i}}}$

  1. http://www.wbdg.org/resources/naturalventilation.php 

  2. http://www.backyardchickens.com/web/viewblog.php?id=1642-winter-coop-temperatures 

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