I installed twin bulbs with a single fan. As soon as you plug it in it powers up the digital thermostat that powers a dimmer switch which controls the bulbs as well. The fan pulls heated air through a channel in the back blowing it back out in the bottom. Air is always circulating. I made three trays that are manually turned from the outside with a window crank handle. I fixed a spring loaded pin through handle to lock it in place too. I still need to trim my egg cartons for holding my eggs. Then try it out. I plan on incubating in this then hatching in my homemade cooler bator.