The greenhouse needed to have some air movement both to keep the plants from becoming leggy, and to move prevent hot air being trapped in the top of the greenhouse. The calculation of the cubic feet of air that needs to be circulated, I ordered a 3-speed 16" oscillating fan. The fan I purchased is a Hurricane 16" wall mounted fan. Installed it on the north wall next to the small vent window, and above the gas heater. Running the fan on low made an immediate improvement in the air distribution of the greenhouse.