A Fan takes a Variable amount of MJ ranging from 1 to 8 to move the temperature of the Alveary it is structured into down one. Example Normal/Normal to Cold/Normal.
Each tier of temperature you want to raise (or lower) your Alveary requires another Heater (or fan) structured into the Alveary structure. For example from Cold/Normal you need one heater receiving 6-8MJ to reach Normal/Normal. Then moving from Normal/Normal to Warm/Normal you need a second heater connected to the structure with an additional 6-8MJ. Note that a single heater may not be able to receive 12-16MJ to accomplish this, one heater may be requiered per tier of temperature.
Some areas appear to have limitations on the rise and fall of temperatures. Using only 1MJ on a fan to bring the nether Hellish/Arid brings it down to Warm/Arid however unlike above for some reason a single fan was able to bring it down from Hellish/Arid Two tiers lower to Normal/Arid. And unlike a test on overworld whos cooling limitation was Cold/Normal in the nether it was possible using enough MJ and fans to lower it to Icy/Arid. This shows a possible large variance in the biome on how it effects the uses of heaters and fans.