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.
You hit it pal. Passive is fine but active saves money in the long run. Hot air hangs in the attic and radiates into the lower levels, even with insulation, so your AC unit works harder in the evening trying tocatch up.
Meanwhile your roofing cooks as well as the framing. Remember that hot air, and susequently hot lumber, absorbes more moisture. Heat and moisture are the two worst enemies of your frame, composite shingle roof. You are in SC. It gets very humid there, yes?
Re did my neighbor's 32 year old roof last fall. He had very inadequte passive venting despite three noisy turbines halfway up the pitch. The rafters, at the ridgeline, had cooked so brittle that you could break them with your hand.
My attic fan has worked for 32 years. I replaced a belt once because it thumped.
Set your attic thermostat at 90 degrees and see how much it lowers your electric bill. Add a roof ridge vent as soon as you can because there will always be dead air trapped at the peak, even with a fan.
And, as usual, Woody is right. Your intakes need to be sufficient and keep the venting on in winter. It is acually more important because of condensation on the super cold surfaces in the occasional wam winter day.
roadster with a stick