Even though many people would consider air circulation and humidity control as totally separate functions, they are closely intertwined.
You may have the proper sized openings to create the proper amount of air changes.
You could still have hot or cold spots in your greenhouse if you do not have proper circulation.
The same can be said about removing humidity.
For a ventilation system to have optimum efficiency and benefit, there must be balance. Having proper air circulation allows you to achieve that balance.
Horizontal air flow (HAF) fans typically come with a cage around the blades, a hanger bracket and a cord with plug. This allows them to be attached or suspended from the frame at the proper location. The motors are rarely more than 1/3 hp.
HAF fans always are installed in pairs and blow in opposite directions. A short greenhouse will have one in the front right corner and in the back left corner. A longer greenhouse will still have one in the front right and back left but also two half way down the length. The one on the right will blow in the same direction as the front right. The one on the left will be blowing in the same direction as the back left.
HAF fans should never be mounted in such a way that allows them to be blowing directly at plants. This would create an uneven drying. Some people will aim the fans slightly in the direction of the cover to ensure maximum air flow along the cover to maintain dry covers.
These fans run continuously to ensure that the temperature and humidity are spread evenly throughout. It also ensures that your thermostat or humidistat are reacting to air or moisture that is representative of what is going on in the greenhouse.
Ventilation is a difficult area of greenhouse production to get perfect. The more attention you pay to the details and modify what you are doing, the greater your production. The tricky part is that with all the variables, no two years will be the same. Carefully consider all your options and the situation you’re dealing with and you will have success!