Yes - there was an attic fire here based on a faulty attic fan. Probably not common but an extra consideration.

I find it amazing how common they are here even on new construction. I wonder if they are helpful if you have a very well sealed ceiling but generally the thinking is they are still not helpful.

They are phenomenal! You must make sure you have the proper ventilation in your roof. They are to be used when the outside temperature is lower than inside. They work by pulling in cool air through doors or windows and pushing up and out the warmer air inside through your attic ventilation. You need to live in an area where evenings offer this scenario. You wouldn't run it in the middle of the day like I think you're describing. We use ours all the time up here in northern Ohio.

"I've read that the current thinking on attic fans is that they simply pull the cold, air-conditioned air out of the house through gaps in the ceiling. And you pay for the electricity to for it to do that."

Only if there is not enough air flow INTO the attic.
The typical passive ventilation area is NOT designed to operate with a fan.

If there is not enough air flow into the attic from outside the fan CAN depressurize the attic and pull conditioned air from the house.

"They are to be used when the outside temperature is lower than inside. They work by pulling in cool air through doors or windows and pushing up and out the warmer air inside through your attic ventilation."

That is a whole house fan, NOT an attic vent fan.

Attic vent fans attempt to pull outside air into the attic and remove the superheated air in the attic.
Since the heat gain across the ceiling into the interior is directly driven by temperature difference (delta T), keeping the attic at a lower outside temperature than solar loading on the roof heats the attic to is worthwhile.

The biggest weakness is usually inadequate air flow into the attic from undersized soffit vents.
They are often sized for passive air circulation, not for use with a fan.

You are correct, my bad.

So how do I know if an attic vent fan is appropriate in my case?

Turn off the fan for a week and see if you notice a change in amount of time your AC cycles on and off. If you see little change, then you can assume the fan is not helping. If you notice the AC is running more often, then it would seem the fan is helping to reduce the cooling load.

mike-home - not exactly an easy thing to do and obviously the sun load would have to be stable as would the temperature and humidity. Oh yeah - constant number of occupants in the house, constant dryer runs and constant oven cook time.

jb - what kind of attic insulation do you have/type and how much? Is your ceiling sealed? - you might look at a light and see if there is any covering/caulk or foam. Some older houses might actually have very few ceiling penetrations - ie no lights, smoke detectors etc. If so - then the pulling of conditioned air is minimized.

I assume the OP has access to the attic and can reach the point where the fan is connect to the AC power. Most likely the fan is connected to the AC power lines via two wire nuts. If this is the case, then remove the wire nut on the hot side and pull the wires apart. You can shut off the circuit breaker first if you are uncomfortable doing this with a live circuit.

I highly recommend having a simple wall switch to allow you to turn off the fan when you do not want it operating. I flip the switch on for my attic fan in mid-May and turn it off in mid-September. This avoids having the fan running on any unusually warm days in April or October.

I think the biggest benefit comes in the evening when the sun goes down. The attic fan helps exhaust the trapped hot air in the attic by pulling in the cooler evening air. This would be the time I would note how much the AC is operating.

There are days in my house when the day time high has 95 degrees and I can hear the fan still running at 10:30pm. Two hours after the sun has gone down it is still above 100 degrees in the attic.

My home had an unused attic fan when I purchased it. I asked the previous owner if it was working. He said yes, but it did not work as well when turned on as when left off. I think the soffit vents must be adaquate and that is very hard if the fan draws a lot of air. He said his was drawing air down the exit vents at the peak. Around here all new construction uses very many passive air vents near the peak of the roof. I have seen 14 vents on homes that were not 2000 square feet.

Mike - shutting it off is easy. Figuring out whether it helps is hard.

I agree. I can't tell you if my attic fan is a net positive or negative.

Wait a minute! You have an attic fan in the Ceiling? No way do you want to run it with the AC going. If it's in the Attic, that's something else.

I had a attic fan in the ceiling of my first house and it allow us to delay till later in the year turning on the AC. It pulled air in thru the windows and doors and blew it out into the attic where the air escaped thru attic vents. Lots of homes had these before AC came along. I had one put in our house when it was built cause I knew it would save a lot of money.

But, an attic fan in the attic helps to keep the attic from getting so blistering hot. It sucks air in through louvers somewhere in the attic (usually eve vents) and exhausts it outside at the opposite end of the house. That will keep the attic much cooler in the summer time.

do a site search lots of years of posts on this topic in faq's.
my advice is to not use it unless you have a perfect air barrier between attic and conditioned space and no ductwork in attic.
best of luck