Originally Posted by maxcichon

A passively cooled attic will rise to as high as 140 deg F on a sunny 95 deg F day. If you have R40-R60 insulation on the floor of your attic, the AC load can be minimized. If your AC evaporator is in the attic space, it will work harder: the coolant lines run through this space and pick up this latent heat. A properly sized attic fan will keep that same space at 90 deg F if the eave vents are numerous enough. One that is rated for 1170 cfm (7000 cubic feet attic) has a 1/4 hp motor (~ 300 watts). 1500 cfm (9000 cubic feet attic) has a 1/3 hp motor (~500 watts). That is maximum draw/load. They do not actually burn this much. A typical 2 ton AC unit runs at 1800 watts. The math is simple.

I have a 2 story/2 AC unit house and if I don't run an attic fan, the upstairs AC unit runs 8 hours a day continuously. and it is impossible to go up in the attic for more than 1 minute at a time. My last house had the same roof fan after 15 years. My current house is going on 6 years. Works fine.

My neighbor helped me move some stuff into my attic 2 summers ago. he was impressed enough that he had one installed a week later.

Mine is set to 90 deg F.