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Originally posted by parabellum:

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Originally posted by tatortodd:
Unnecessary if you have good ventilation to begin with (e.g. ridge vent and unobstructed soffit vents).

We've got that stuff.

If you're properly vented, then you don't need it. Convection will circulate the air from outside through soffit vents and out ridge vent. Combining a ridge vent with an attic fan actually prevents proper air flow as the fan will suck extremely hot air from the top of your roof through the ridge vent (opposite of how the air should flow) causing eddies in the air flow. Plus the already mentioned costs of the fan motor and air infiltration from the conditioned space. The one thing no one has mentioned is when you suck conditioned air into the attic, that air must be made up from somewhere, so your conditioned spaces replace that air through poorly sealed parts of the house and when you go in or out your doors.

There is no good argument for them if you have adequate ridge and soffit vents.

Sorry, I guess I didn't fully understand exactly what Para had in his house.

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Originally posted by mrvmax:
In my area they were causing attic fires so I disconnected mine (I had a coworker who's brother was a contractor and he had made a lot of repairs after fires). I put radiant barrier in my attic and that lowered the temp dramatically. Now the ridge vents will allow air flow through the attic and it does not get excessively hot, no fan is needed.

The radiant barrier is one of the big differences between the first time I lived in Houston and now (the other is higher SEER heat pump). First time was a 2005 house (first owner) and now is a 2016 house (also first owner). This house is 1000 sq ft bigger but my electric bills are half what they used to be the first time I lived here.

The first time I lived in Houston, several of my coworkers retrofitted radiant barriers and said it made a big difference. I looked pretty hard at it, but got transferred to Alaska before pulling the trigger.

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Originally posted by parabellum:

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Originally posted by cparktd:
It's just a fan, can't use a huge amount of electricity.

One argument is it pulls cool air out of the living space... bull feathers... unless you have a really crappy poorly sealed house.

So, then, what is your explanation for what I've outlined in this thread?

There are lots of variables possible. One thing we always looked for was to make sure the fan had plenty of make up air available to it in the attic so it really should not create enough suction to pull much if any air from the living space. Just relating my experience, I have done the math and my fan consumes about 50 cents a day in the peak of summer.

I have installed a ton of them on request but never tried to sell anyone on one.

I thought the idea odd myself- a fan that size, running up the electric bill. I'm going by what the technician said. He also said that in new homes, these things are usually not installed.

I have one and can't say if it helps with the cost of cooling my 79 year old house, but I do know it reduces the attic temperature from cook you in an instant to cook you in 15 minutes.

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Originally posted by parabellum:
I thought the idea odd myself- a fan that size, running up the electric bill. I'm going by what the technician said. He also said that in new homes, these things are usually not installed.

Assuming 12 hours a day of use at 4A for 30 days at 12¢ per KWh it costs about $20 a month. Maybe add an extra $10-$20 per month in additional A/C usage per month. No way it would exceed $40 per month to operate.

I disconnected my attic fan, as I have adequate ventilation.
As Yellowjacket and Skins mentioned, you can easily cause a negative pressure in the attic. This will draw air from the living space, thus creating a negative pressure that space. Guess where the makeup air is coming from? All the cracks and cranies in the exterior walls. This is unconditioned outside air that the AC needs to take care of. Installing a large oversized fan in the attic is not what most believe is a logical choice to remove all that hot attic air.
Disclosure, I am not a professional engineer, bit I did sleep at a Holiday Inn Express lastnight.

Not long ago I was looking into a solar powered fan setup for my attic. When I got crunching the numbers it became apparent that they weren't moving enough air to make any difference. It didn't seem like an investment that would ever pay off, and this for a fan that wouldn't be connected to my service panel.

If you've ever been to any of the big commercial chicken coops especially in the South, and seen the amount of airflow they push through those things, you'd realize just how much air you have to move to have a significant affect in your attic temps.

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Originally posted by hvactech:
Disclosure, I am not a professional engineer, bit I did sleep at a Holiday Inn Express lastnight.

We shall give your comments the extra weight your forum username indicates appropriate for the subject at hand.

Based upon the comments in the two pages of this thread, it's shaping up to be an issue of inefficiency in the whole system, rather than the power consumption of the fan.

I used to design scroll and reciprocating AC compressors for the nation's largest compressor company. I am not a system designer, nor do I claim any installation expertise.

1. I had no fans in the attic. It was hot. Damn hot. I had only gable vents and eave vents.
2. I installed a gable fan opposite the pull-down door and in-attic 2nd floor HVAC. No joy. Still hot.
3. I had two roof-top fans installed 1/3rd of the way from each eave. I set the temperature to a reasonably hot level. Now the attic is tolerable.

The items in the attic are not disintegrating. The HVAC system is not trying to operate in 140 F air to create 65 F cool air. It is much easier to cool the upstairs. The fans come on infrequently, and have barely discernible noise. My electric bill was "level" so I can't comment on that. But, I am very happy that I created good attic ventilation using the PAVs. It is possible so to do.

Yes, I do have some conditioned air being blown out of the attic. But, compared to before, it is amazingly good now. I may put in a pull-down door hood to improve the situation, but we are up and down there so much that would be a hindrance.

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I set the temperature to a reasonably hot level. Now the attic is tolerable.

What is your personal benefit to the attic not being hot? Attics are supposed to be hot.

I've replaced 100's of attic fan motors because people for some reason believe there is a benefit to cooling their attic by 10°. I always told people there's no point to it, but I will happily do any service you want that is not dangerous.

I read somewhere that you needed to change the attic air once per hour. So you figured the volume of air, cubic feet, times the efficiency of the fan. Turns out that I used four 7-inch Rotron fans in the port holes at the top of my roof.

I used a thermostat to cut them on at a certain temperature and a remote LED to show when they were on. I have about 24 inches of insulation in the attic. It is suppose to help your roof last longer as well.

I could see problems if your fan is too big and the air flow is disturbed.

Dang, good timing on this thread. My powered attic fan crapped out a few years ago and I have not bothered to get my old fat sorry ass up into the attic to deal with it ... but was just now thinking about it after this friggin' heat wave.

Getting the feeling that it is best to let the sorry thing waste away and just unhook the power supply to it ... guess can use that power for a light or maybe for a security camera out that upper part of the house. Hmmmm.

Bottom line, glad I learned something from this thread. :P

I have family that has a properly sized one in their attic. Newer home (Last 15 years) that is well insulated. Not sure about the power consumption. No AC and it definitely helps keep the house cooler than without. So a thumbs up for a PAV in their case.

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Originally posted by old rugged cross:
I have family that has a properly sized one in their attic. Newer home (Last 15 years) that is well insulated. Not sure about the power consumption. No AC and it definitely helps keep the house cooler than without. So a thumbs up for a PAV in their case.

If it's in an area that gets cooler nights, especially if not super humid. Whole house fan. Cools entire house in 20 minutes during the night. Button up the house during the day, repeat process = super cheap A/C alternative. Great for areas where A/C isn't required, but nice to have a few months a year.

If your PAV was so big that it exceeded the intake air flow of your ridge vents, yes it would start sucking air from air conditioned space or wherever it could. A small attic powered vent fan (sized properly does well). Personally I think more attic insulation is a much better expenditure and you only pay once and it usually pays for itself within a few months. Like anything, someone experienced has to look at your entire attic picture to figure out which works best to save electricity.

Using an attic fan can reduce 2nd floor temperatures by 10°-20°. You’re only needing to pull that accumulated 125° air out of the attic. A whole house fan is a different appliance; that will ventilate the house. There’d be a louvered grille in the hall ceiling; any room wanting a breeze, you simply crack open the window.

Sounds like you have about 20x the cfm capacity required. If they “disconnected” the fan, it was probably wired to a temperature/ humidity sensor relay what’cha’ma’call-it. Some fans have a speed switch. Running the fan at a lower speed during more mild weather could produce residential comfort without closing up the windows and operating the AC system.

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Do any of you guys have any experience with this kind of thing?

Homes in GA have ridge vents. A PAV will work against what a ridge vent provides. I learned this the same way a long time ago. The builder installed the ridgevent incorrectly and our home was hot. We installed an attic fan. Years later when we were having the home re-roofed, the roofer explained why we were having the issues (upstairs was hot) and removed the attic fan and correctly installed the ridgevent.

AC is intended to be a nearly closed system so as not to pull hot air from the outside in and cool it.

You are correct about the initial heat coming from a duct when the AC kicks on. It's pushing out the static air that has been heated by sitting in the vents.

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Originally posted by Skins2881:

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I set the temperature to a reasonably hot level. Now the attic is tolerable.

What is your personal benefit to the attic not being hot? Attics are supposed to be hot.

I've replaced 100's of attic fan motors because people for some reason believe there is a benefit to cooling their attic by 10°. I always told people there's no point to it, but I will happily do any service you want that is not dangerous.

It's a bigger problem in the South. If the attic is too hot, you can cook the asphalt and asphalt shingles on your roof deck and shorten the roof's life. If you see a house with shingles that are curled at the edges, it's from inadequate attic ventilation and over temperatures. Also most everyone's A/C ducting is in the attic, while it's generally insulated, it's generally only insulated to around R6, so it too will cause the a/c to work more. Plus some people use the attic as storage.

Depending on how a house is vented, insulation, vents, and so forth, a small fan on a thermostat can both reduce attic heat, and electricity usage. As long as the attic is properly vented to begin with, my preference is to have more fiberglass attic insulation blown into the attic.

The modern trend here on new houses, good ones anyways, is they spray foam insulate the entire bottom of the roof deck and the entire attic is sealed (not vented) and stays cool like the rest of the house.

My house was built CBS in 1966 with eave 4x16" eave vents every 4' along the underhang in South Florida, but nothing at the top of the roof. Modern code made the roofer install 3 10x10" roof vents near the peak of the roof when they re-roofed it to allow the hot air at the top to escape and pull cool air from the eave vents.