I have seen some of the people that SELL the fans claim only $3 to $4 a month to run one.

However...
That is not quite the bald face lie that it seems to be because they "cheat" by averaging the cost to year around, 12 months. I also expect that is for a smallish fan and in a cheap electrical cost area.

I honestly don't know the specifics, but when I had my roof replaced earlier this year, I had the contractor install a solar powered vent (at the suggestion of a buddy). How it works (other than by solar ) , I have no idea...

About 3-4 years ago I had a new roof put on with ridge vents. At the same time I got the bright idea that if I bought a couple of cheap fans at Walmart and placed them in the attic it would help circulate the air, keeping the attic cooler and therefore helping the A/C to keep the house cooler.

I have a large vent on the front of the house over the garage and I placed a fan there blowing out. And then placed another fan further back in the attic blowing towards the other fan.

Over the next few weeks the A/C ran constantly. And I mean ALL THE TIME. The house was 4-5 degrees warmer than normal and would not cool down. It made no sense as the new roof was lighter than the old roof.

I cannot recall the exact sequence, if I asked here on SigForum first, or if a quick flicker of power tripped the fans and they did not turn back on and I noticed a difference and then asked here on SigForum, but the response was fairly unanimous: Turn off those fans you idiot, you are sucking cool air out of your house through ceiling's electrical fixtures and circulating warm air through your attic making your A/C work harder.

Over the next week or so I experimented by leaving the fans off for a day, turning them back on, leaving them off, etc and sure enough, those fans killed the cooling ability of my A/C.

When I had roof work done, I did not install any fans, but did go with a radiant barrier. Made a huge difference. I did live in a house with no A/C but did have a giant whole house fan. When the temps dropped in the evening, open the windows and kick that fan on. House was cooled in about 15 minutes.
Noisy (like a C130 noisy) but a great performer.

Looks like I'm going to turn mine off when I get home tonight and see if I notice anything different.

I'm in Tucson where it gets HOT! I discovered the fan behind one of the gable vents when I painted some reflective coating top of the porch roof. Tracked it to the switch and discovered it wasn't working. Replaced the thermostat and it's pretty much been running ever since. Time to revisit.

quote:

Originally posted by Nev:
I discovered the fan behind one of the gable vents ...

If you have gable vents, rather than soffit vents and either ridge vents or cans, then your attic uses cross-ventilation, rather than convection. A fan at one end, with a thermostat, helps overcome a disadvantage of that design: What happens when there's not enough wind, from the right direction, to move air through the space.

So you did the right thing in fixing it, IMO, and probably want to leave it operational.