In cold weather, the temptation to shut the fans to save fuel is strong. But a house that receives no fresh air quickly accumulates moisture, ammonia and carbon dioxide, and the litter becomes wet. Minimum ventilation reconciles these two conflicting needs: the least air required for air quality, without chilling the birds.
This article covers how minimum ventilation is run and set in the cold season: the fan timer cycle, humidity and ammonia control, inlet settings and heater–fan coordination. Calculating exhaust fan capacity is covered in the article Calculating house ventilation capacity. Technical figures are quoted from the Ross Broiler Management Handbook (2025) and the Cobb guide.
According to the Cobb guide, a minimum ventilation system manages air quality and humidity with fans running on a cycle timer, and is always active regardless of the outside temperature. Cobb lists three main functions: supplying the oxygen the birds need, controlling humidity and maintaining litter quality.
The Ross handbook specifies when it operates: when the house temperature is at or below the set point (the birds’ comfort temperature), or up to 2 °C above it, depending on bird age. When the temperature rises above this limit, the controller should leave the timer and start transitional ventilation (temperature control). Ross stresses that, whatever the outside conditions, the house must always have some ventilation, and that minimum ventilation should run on a timer, not a thermostat.
The basis of minimum ventilation is the cycle timer: fans are on for part of a fixed period and off for the rest. The rules in the Cobb guide:
Cobb’s sample table for a 5-minute timer (in seconds):
| Age (days) | On | Off |
|---|---|---|
| 1 | 60 (20%) | 240 |
| 5 | 75 | 225 |
| 8 | 90 | 210 |
| 14 | 120 | 180 |
| 22 | 150 | 150 |
| 30 | 180 (60%) | 120 |
This table is a starting point, not a fixed instruction. Ross also gives a method for calculating on-time in Appendix 7 (total ventilation requirement divided by the capacity of the timer fans, multiplied by the cycle length) and immediately stresses that this is a theoretical estimate, and that actual fan and timer settings must be based on the real conditions in the house, air quality and bird behaviour. According to Ross, the minimum on-time should be long enough for incoming air to reach the highest point of the roof and begin to descend; this time is determined with a smoke test.
Ross also warns about variable-speed fans: if a fan runs at very low speed, the negative pressure in the house falls and the inlet opening would have to be less than 3 cm for air to be thrown to the roof, which is not recommended.
The Ross handbook makes a key point: minimum ventilation is more about controlling relative humidity than supplying fresh air. According to Ross, if a house is ventilated only to the birds’ theoretical requirement, humidity usually rises and the litter becomes wet; but if it is ventilated enough to control humidity, the birds’ fresh air needs are more than met. Rising humidity in the house is often the first sign of under-ventilation.
Humidity limits in the sources:
| Topic | Value | Source |
|---|---|---|
| House relative humidity | Below 70%; below 60 to 65% where possible | Cobb |
| Humidity at chick arrival | 50 to 65% | Ross |
| Air speed at bird level | Usually below 0.15 m/s (Ross); below 0.3 m/s (Cobb) | Ross and Cobb |
Ross explains why bringing cold air in under the roof helps keep the house dry: as cold air warms, its relative humidity falls and it absorbs more moisture from the house and litter. Ross also notes that high humidity raises the birds’ perceived temperature and low humidity lowers it; so before changing the house temperature, the correctness of the minimum ventilation rate should be checked first, because fluctuations in humidity may be caused by incorrect ventilation.
Wet litter and insufficient ventilation raise ammonia. The limits in the guides:
| Gas | Target limit | Consequence above the limit (Ross) |
|---|---|---|
| Ammonia (NH3) | Below 10 ppm (Ross and Cobb) | Detectable by smell from 20 ppm; above 10 ppm damages the lung surface; above 20 ppm increases susceptibility to respiratory disease; above 25 ppm may reduce growth |
| Carbon dioxide (CO2) | Below 3,000 ppm (Ross and Cobb) | Above 3,500 ppm causes ascites |
According to Cobb, if CO2 exceeds 3,000 ppm or oxygen falls below 19.6%, the ventilation rate should be increased. For re-used litter, Cobb also requires ammonia below 20 ppm before chick arrival.
A practical note from Ross: since ammonia can only be smelled from 20 ppm upwards, smelling the air is not enough. Ross recommends quantitative assessment with instruments measuring humidity, CO2, CO and NH3.
In winter, the inlet determines where cold air goes. Key operating rules:
Dimensions, mounting height, deflectors and a pressure table by house width are covered in the article Poultry house inlet windows. The available models are presented in the poultry inlets category.
The biggest winter mistake is “competition” between heater and fan: the heater warms the air and the fan removes that same heat. Ross gives clear rules:
If the house is heated with direct-fired jet heaters, minimum ventilation becomes even more important, because according to Ross combustion inside the house adds moisture, CO2 and CO. Heater placement relative to the inlets and combustion safety points are covered in the article Poultry jet heaters.
According to Ross, the best way to assess the settings is to observe bird comfort and behaviour and air quality. On entering the house, without disturbing the birds, the following are checked:
Cobb recommends that if chicks are inactive or restless, an increase in timer on-time should be considered; improving air quality at bird level can improve activity and distribution.
It is ventilation that, using timer-controlled fans, brings into the house the least air required to supply oxygen, control humidity and maintain litter quality, and is always active regardless of the outside temperature (Cobb). According to Ross, it operates when the house temperature is below the set point or up to 2 degrees above it.
Cobb prefers a 5-minute cycle (never more than 10 minutes), with a minimum of about 60 seconds on. As the birds grow or air quality deteriorates, on-time is increased in steps of 10 to 15 seconds while the cycle length stays the same.
According to Cobb, below 70% and below 60 to 65% where possible. Ross regards rising humidity as often the first sign of under-ventilation.
Because minimum ventilation is for air quality and humidity, not temperature. Ross states that during this period the heater should run whenever the temperature is below the set point, even if the timer fans are on, and that both should be coordinated by a single controller.
To have minimum ventilation settings reviewed, send the following information: house dimensions, the number and size of exhaust fans and inlets, the type of controller and heater, and the current problem (humidity, ammonia smell or wet litter). A Toyooran Sanat Pouya specialist will recommend the equipment required. See also the ventilation and inlet equipment.
Send the house specifications and the current problem.
Request a ventilation consultation
Topic: Ventilation
Category: Engineering technical guide
Tags: Poultry house minimum ventilation, Poultry house ventilation in winter, Winter poultry house ventilation, Poultry house fan timer, Poultry house humidity control
Published: 2026-10-22T10:15:02+03:30