What is the best tempature to set the attic fan to come on at ?

 

It depends entirely upon where you live.

Where are you?

Setting an attic fan to turn on in Fairbanks at 90F is a waste of time as is setting your fan to turn on for your attic at 60F in Phoenix.

 

It depends on how hot the attic is. We set them around 100o to turn on.

 

We are in Oklahoma City. In July the average high is in the 90's with lows being in the 70's. House built in 1999, Attic is insulated to R30, 1600sq ft conditioned area with another 400 sq ft for attic space over the garage. Fan is a 4.3A 120V or about 500W. A/C is 3 tons, probably about a 10 SEER. Look like the "break even" point (Where the fan cumsumes the same amount of power as it saves in A/C costs) is about 5,000 BTU. It seems as if the current thermostat has about a 1 degree differential causing the fan to cycle every 2-3 minutes in the evening as the attic cools off. I was thinking of going with either a 120 on/110 off or a 110 on/90 off fixed temp "snap disc" thermostat (like the one used to control the fan in a furnace) to prevent short cycling of the fan. Using a wider differential thermostat would also solve the issue of the fan coming on in the fall and spring when the A/C is not running.

Which thermostat do you think would get me closest to my 5,000 BTU goal ?

 

For what you want to do and play with a little Id go for a real fan control. Dont go for that fixed klixon one. Honeywell , White Rodgers Johnson Also Cam-Stat. just get a fan control for a furnace and put it up there. one white rodgers has one range 50o to 265o with differential up to 15o.
Look around and ask for a furnace fan control.

ED