First, the details. I live in Central Florida. Concrete block house. Asphalt shingle roof. Three 6' roof vents spaced out around the roof. Vaulted ceilings inside the house. Flat roof in garage with attic storage. Blown in insulation in the attic above the living area. Central A/C system.

We had a huge live oak shading half the roof for half the day that had to be removed, so in preparation for that, I bought a couple of 20' box fan and placed them in the attic. One is placed just off the garage above the living area, pulling air out of the attic space above the living area. The other is at the very end of the garage up against a large vent, larger than the fan, blowing the hot attic air to the outside.

My thought process in this was that the air in my attic in the summer was already north of 110-120 degrees (this is guess, I do not have a temperature gauge up there), and with the loss of the shade tree, would probably climb even higher. I assumed the fans would pull in the outside air through the roof vents, and even though that ambient air was 90+ degrees, that was cooler than the stagnant air in the attic, and this cooler attic would help maintain the cooling costs due to the loss of the shade.

But that isn't what happened, within 24 hours of turing on the fans, the A/C was working overtime. It could not maintain a temperature of below 77 degrees and in the late afternoon heat of the day, would hit 82-84 degrees and then even when the sun would set, would take forever to cool down, not getting back down into the 70's until after midnight.

I had the A/C system checked, it was functioning perfectly.

Then, trying to eliminate changes, I turned off the fans. And within 24 hours, our house was back to normal, 76-77 all day long and 71 degrees at night.

I left things alone for a week, then I turned the fans back on, and within 24 hours, the temperature (and humidity) were back up and the A/C system could not keep up no matter how much it ran.

I left the fans on for a few days and when I turned them off, the the temperature was back to normal.

So, what possible correlation could there be in trying to pull hot air out of the attic and making the A/C work overtime and not be able to keep up? My only thought is that the fans are not pulling air from the outside through the roof vents, but are somehow instead pulling cold air from the house. But I just cannot see how these fans could pull air from the living space.

You don't need fans in your attic.
Ideally.
You need ridge vents, vents in your eaves and at least R33~35 insulation between your living space and the ceiling (attic floor).
Add radiant barrier to the under-roof of the attic as a bonus if possible.

Ideally, air enters the attic through soffit vents and exits through roof/ridge vents.

Do you have soffit vents? If so, how many?

The reason I ask is that it sounds like inadequate air intake for the fans.

quote:

Originally posted by holdem:
I turned the fans back on, and within 24 hours, the temperature (and humidity) were back up and the A/C system could not keep up no matter how much it ran.

I have had an attic fan in two houses, and they never did what you're experiencing. I'm not a HVAC pro - but there are some here in the forum and I'd bet one or more will chime in shortly.

In the meantime, if I had to guess, maybe you have a lot of air leakage in the walls/doors/windows of the house, and maybe the fans are oversized? That combination would end up sucking a lot of outside air through the cracks. If it's hot/humid outside, that much outside air would drive an AC system nuts.

Could well be something else.

you could have leaks in the AC conduit, also if you have lamps in the ceiling they are portals into the house with the air you are moving around in the attic.

When you run them check the ac outlets temp, check around recessed lamps and returns, see if you have fully cold air or if you have any leak through.

You should check the temp at the same register with the fans off.

Right now we have massive humidity with the tropical storm moving over the area, I saw close to 88% yesterday, so running the fans you are pulling in fresh humid hot air...

We have a ceiling fan I installed in the garage, during the colder months I'll open the inside door and turn on the fan, having it in the garage keeps the noise out of the house and opening doors/windows pulls through nice cool air.

I found it doesn't help at all during the summer.

Light fixtures, electrical outlets, switches, vent fans, air handler closet and various seams that are not air tight will flow a great deal of air into the attic with the fans providing vacuum.

It may not seem like much, but all of that cool/dry air is replaced by warm/moist air then flowing in through windows, doors and other areas that are not airtight.

As others state, the ridge vents, and/or soffit and gable vents are adequate to remove the greatest amount of heat buildup.

And the older a house is, the more likely any attic fan will increase the loss of conditioned air.

Interesting problem. I’m no HVAC person but i’v studied heat transfer and pressure differentials between rooms. My theory is you have fans blowing air into the attic and that increases the pressure. Where else will the hot air in the attic go? I’m assuming a good portion of the hot air being displaced go through the cracks between the attic and the house below and you have hot attic air being forced into your living space.

I wonder what would happen if you reverse the fans so it’s blowing hot attic air out intermittently? I know it should make your house cooler but I don’t know if that will make you air conditioner work less. It depends if the air being pulled from the living space is hotter than the air that will be pulled in.

May have created negative pressure sucking in warm outside air into house through openings in outlets, around doors/windows. Also if you left your attic access open some hot air will come through there, even though it's supposed to only rise. This will be worse when opening and closing doors a lot.

Yeah - as others said - you're probably pulling conditioned air into the attic space and thus outside air into the conditioned space.

More insulation instead of more fans...

I have a large attic fan in the middle of my upstairs hallway mounted above a 3’x3’ self opening and closing louvred vent. We only use it in the fall and spring when the humidity is low, opening downstairs windows to draw in the cool fresh air which drops the indoor temperature to the same level as outside. About ten minutes run time does it. Some have referred to it as a “whole house fan”. I have R38 insulation in my attic. That’s how it was explained to me to use it by the person I bought the house from as I was unfamiliar with the concept.

you're pulling conditioned air from your living space up into the attic with the fans. if you're properly ventilated, you don't need fans. power vent fans are actually not allowed per energy code now in a lot of jurisdictions.

Many good answers here.  I used to be BPI certified. Sounds like your house and a/c duct work is a leaking mess.  If you have a gas water heater or gravity flue furnace in the house, your issue could be a little scary when running a single attic ventilator. If you are interested in having it fixed or at least checked.  I fully recommend finding a company that does whole home energy audits and is BPI certified.  Which will be a blower door test with pressure pan testing.  Most older homes are a leaking mess.  Most newer homes are so tight that they need outside make-up air added to the house.  If you're NOT going to have a home energy audit done and you do have gas appliances. Please have a high quality carbon monoxide detector in the house.  BPI = Building Performance  Institute Inc.

Yellow Jacket--I'd like to know more about this. We have a thermostatically controlled fan in the attic, it's very noisy, and I'd love to get rid of it.
What is the basis for not allowing power vent fans in attics in new energy code?

quote:

Originally posted by holdem:
But I just cannot see how these fans could pull air from the living space.

There are probably dozens of what are called "air bypasses" between your living space and the attic, so that's exactly what's happening. Some of them may be immediately obvious, some not so much.

People have lots of misapprehensions about how attics work. Now, I am not an HVAC or structural engineer, but I do pay attention to those who are.

When we had our home re-roofed a couple years ago we were given a brief education on how such things work. In attic ventilation more is not necessarily better. For a given attic space there is an ideal balance to be struck in air movement, and that is achieved with a calculated balance of intake and exhaust ventilation.

Could be your attic space was already optimal. I would have waited to see if your energy usage went up, after losing the tree, before messing with it.

Fans like that are ONLY good when the air OUTSIDE is cooler than the air INSIDE (the living space) and you want to get the warmer INSIDE air out.

My theory and I'm sticking to it, so there!

quote:

Originally posted by sjtill:
Yellow Jacket--I'd like to know more about this. We have a thermostatically controlled fan in the attic, it's very noisy, and I'd love to get rid of it.
What is the basis for not allowing power vent fans in attics in new energy code?

Not certified in anything, but a attic fan can create negative pressure in the attic if you don't have proper soffit venting. If your fan moves 1400 cubic feet, you need the ability to replace 1401 cubic feet of air or else it will find its own way in.

A properly vented roof requires no power ventilation and in most cases is a complete waste of energy. Buy insulation and ventilation, pay for it by skipping the wasted power cost of running the attic fan.

quote:

Originally posted by sjtill:
Yellow Jacket--I'd like to know more about this. We have a thermostatically controlled fan in the attic, it's very noisy, and I'd love to get rid of it.
What is the basis for not allowing power vent fans in attics in new energy code?

it's just the electricity consumption. they actually can be installed if they are run from a secondary power source (solar panels.) they are, as you know, tied to thermostats and at hot times of the year, or depending on your t-stat setting, could run almost uninterrupted for days and weeks.

I know this is the case in georgia. of course, if they are already installed, they are fine. they are not illegal to have, just can't install new ones.

a little light reading:
https://www.energyvanguard.com...-Georgia-Energy-Code
https://www.energyvanguard.com...ators-Are-a-Bad-Idea

basically, as has been said in this thread, power attic fans keep your attic cooler by sucking your conditioned air out of your living space and up into your attic.

Too much fan for the amount of supply air from the soffit vents.
Try putting a smaller pulley on the motor and a larger pulley on the fan shaft. That will reduce the volume of air it moves. That should also make it run quieter.