When to add a fan and when passive vents are enough
The conditions that push a coop past passive ventilation, what kind of fan actually helps, and where keepers waste money.
Passive vents handle the majority of backyard climates. The cases that genuinely need a fan are a smaller list than the marketing suggests.
A fan starts to earn its keep when daytime summer highs regularly exceed 85°F, when humidity stays above 70% for weeks, or when the coop sits in a wind shadow that kills natural airflow. Pole barns and coops attached to a house are the common geometry that benefits.
If you are buying one, buy it once and buy it right. A four to six inch DC fan with a thermostat or simple timer moves enough air for flocks under 20. Mount it on the outlet side near the ridge to push warm air out, not on the inlet side to push warm air in. Pulling beats pushing in this geometry.
The wasted spend is usually the opposite: a too-large household exhaust fan that runs constantly, dries out the bedding, and drops winter temperatures faster than the flock can tolerate. Most fan installations also need a manual seasonal cutoff. Run it in summer, stop it in winter.
Run the calculator with hot-summer or humid-coastal climate selected to see what the passive-vs-fan threshold looks like for your flock and floor area.