At one end of the greenhouse near the ground is a motorized intake vent and at the other end near the top is a louvered fan, both are wired to a thermostat (here are the parts). I set the temperature at, say, 85 degrees and when it exceeds that, the intake vent opens up and the exhaust fan turns on. This will exchange the entire volume of air in the greenhouse in about a minute, bringing the temperature down and turning off the intake vent and exhaust fan. You also have to have a horizontal air flow fan (HAF) to circulate the air around inside when the vents/fan are not on (important feature if you have a greenhouse sealed with poly). The HAF fan is on a timer and runs during the warmer hours of the day. The ventilation was the most expensive part of the greenhouse, but essential given I wanted to cover it with poly.
Intake vent:
Exhaust fan:
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Love the greenhouse Gallo. I'm jealous.