HAF fans make greenhouse heating and cooling more effective by evening out temperatures throughout the structure. This is done through gentle but high volume air circulation, which eliminates hot and cold spots. The objective is a smooth, flowing mass of air circulating throughout your greenhouse.
As a guideline, one fan is required every 50 feet. A 12in variable speed fan is recommended for greenhouses that need more control with air flow, and a 20in fan is recommended for larger greenhouses and for maximum air circulation.