Solar attic fan
 

I am planning to have a solar attic fan installed this year. I was curious if any of you here have them and what's your feedback about them. My attic has the usual soffit and ridge vents, last fall i installed an attic tent and was very pleased with the heat savings compared to 2008.

Ravi

Thanks,
ravi

I think solar attic fans are a great way to go if you can't get enough natural convection through normal ventilation. However, if you can do it naturally, all the better/easier/less costly.

We had a new roof put this year and even though we have the soffit and the ridge vents a couple of the plywoods for the roof had some discoloration. I think that happens because of the moisture build up. I guess i will try the attic fan, plus i can get some tax rebate on it too for next year.

Yeah, if you are getting moisture you probably need more ventilation. You really don't want that new roof rotting out on you.

I saw some solar attic fans online at..
Solar Attic Fans

And I think similar products can be had at Lowes or HomeDepot these days.

We have an old AC powered gable fan installed, but the thermostat seems to have
died after 10 or 15 years of use.

So, I'm also thinking of adding some extra ventilation to my attic this summer.
But first, I'm going to stick a wireless temperature probe up there for a few hot days.
If the new ridge vents are working well, I may not need any extra air flow..

I had a 200W 120V gable fan that I replaced with a solar fan from Home Depot. I figure I'm saving about $0.25 a day when it's in operation, or about $30 a year. It cost about $250 for the fan and the panels, so payback is about 8 years.

I'm also thinking about installing a solar-powered attic vent fan.

I saw one at an energy fair a couple of years ago, and it really seemed to move the air. Makes sense too that the heat from the sun ends up cooling your attic.

We just went through a big heat wave, so the main thing I am looking at is cycling more air through the attic, to keep it cooler, to run the air-conditioning less.

Anyone else have more direct experience with this?

I was also thinking that maybe I could put my wireless indoor/outdoor transmitting thermostat in my attic to get some temperature info for before and after.

I have one of those cheapo remote temperature sensors hanging up near the peak of my attic.
I've seen it hit 120F recently up there, when the outdoor temp was in the high 80s, low 90s.

I've checked out my old attic fan and it still works. Not sure how much power it uses,
but I'm sure that it's not real efficient. It's on the east wall.

When we aren't using the house AC, we keep the attic stair (pull-down)
open a few inches to allow the house air to be pulled up into the attic.
Acts a little like one of those 'Whole House' fans.
But, when we are using AC cooling (like today), most of the air input into
the attic comes from the peak vent on the west wall.


It starts up just a little above 100F and tends to stay on until late in the afternoon.
I don't like the hysteresis of it's sensor, since it stays on for so long.
I think the attic temp drops down to around 90F before the fan shuts off..

I'm trying a Timer experiment now. AC power goes off for 30 minutes and
comes back on for 30 minutes, all day long.
So, during the heat of the day, it's going to running on a 50% duty cycle.
Should save us 50% on fan power use.. :rolleyes:

If hot air rises, will a cooler attic really have an effect on the lower living area?

I'm also looking for ways to run my a/c less.

The way I see it, it's all about "Delta-T", that is, the DIFFERENCE in temperature.

The bigger the difference on one side of the wall than the other, the more heat tries to push through to the other side to even out the temperature.

Insulation slows down this process, which is why we insulate our houses, to keep warmer longer in the winter, and cooler in the summer.

I recently put the remote reading sensor from my wireless indoor/outdoor thermometer in the attic of my house. It's been hot lately, and we've been running the air conditioning, set to 80 degrees F.

In the attic in the middle of the afternoon, it's been 120-125 degrees. So, that's a 40-45 degree difference than in the house. If it were winter, we might have the furnace set to 70 and the outside temperature might be 25 or 30. That's that SAME 40-45 degree difference!

Basically, the AC is working as hard in the summer, as the furnace is in the winter when it's below freezing! Imagine if you had a way to warm up the outside in the winter - your furnace would have to run that much less because of it.

That's kind of what you are doing if you can cool your attic. If you can drop the temperature there, it's that much less heat trying to beat its way into your house!

Anything you can do to reduce the Delta-T (including raising the thermostat in the summer) reduces how much energy is used to heat and cool the house. It just seems like a good attic fan is a simple, cheap, way of doing that.


(The summer/winter comparason isn't perfect. In the winter, heat loss is sort of everywhere - walls, attic, foundation, etc. In the summer, the attic is the hottest thing, but you also have to consider solar gain, and that the walls of the house are unevenly heated as well)

http://i46.photobucket.com/albums/f1...0slot/peak.jpg

There are tar-like stains on the outside of my house, around the vents.
That's caused by out-gassing from the (1956) wood, and maybe roofing tar paper.
I have climb a ladder with pressure-washer sprayer to clean those stains.

The hot air in my attic accumulates in the peak, and had little effect on the ceilings of the living space.
But, as the day wears on and the peak hits 120f, the air down at the insulation becomes very warm.
I think the hot air just soaks into the insulation, warming the wood and the sheet rock under it.
Radian heat from the roof bombards the insulation directly.
The insulation and the ceiling it covers, warms to the room temperature, of the attic..

Before long, the living space starts to warm up. I can reach up and feel the warmth of the ceiling.
Radian heat is raining down.

When possible, we pull down the attic stairs and let cooler air updraft into the attic.

But, when we have the AC running and the house is sealed, there is very
little natural outflow of air.

Picture a can with two holes in the top, sitting in the sun. At first the air expands with the heat and some air flows out.
But once that's over, stagnate hot air will just sit in the can.

So, installing a fan in the east side vent (see pic above), sucks hot air out,
and pulls cooler outdoor air into the attic from the west side vent.
That air flow keeps the 120f air off the attic floor, and helps keep my living room ceiling cooler.
So, my Sanyo ASHPs don't have to work as hard..

Hi,
This is probably all be good logic, but Soutface.org, which is generally a pretty good outfit has some reservations about powered attic vent fans: http://www.southface.org/factsheets/...s%2000-771.pdf

They mention two things
The power used by an AC powered vent fan might be more than the saving. Obviouosly this would not apply to a solar powered one.

If the living space ceiling is not sealed well, the powered vent fan might lower the pressure in the attic enough to pull conditioned air from the living space up into the attic. This seems like a valid concern to me as a lot of homes don't have well sealed ceilings.



Gary

That would only happen IF you have a very poorly sealed ceiling AND really insufficient attic venting.

Air flow takes the path of least resistance. Houses like mine will have vents on the underside of the roof overhang. That's how air gets into the attic. Air going out the ridge vent, end vent, or powered attic vent, comes IN through these vents on the bottom of the overhang.

Obviously, start with the "low-hanging fruit". As a general disclaimer on all home projects - start with the easy and cheap stuff - caulking, weatherstripping, and insulation.

If your powered attic vent is working so hard that it's sucking air out of conditioned space, you have worse problems!

My single story ranch has vents just like that on the north and south side. I like the idea of your cross ventilation.

What type of fan are you using at your vent, how is it powered, and switched on/off?

Hi,
Not sure that's true.
I've read elsewhere that air movement from the living space to the attic space is the single largest source of infiltration/ex filtration in a lot of homes.
I'll try to find that reference.

I know when I added insulation to my attic I first pushed the old insulation out of the way and sealed every penetration for wires, plumbing, vents, can lights etc. -- there were lots of them :) All those little holes add up, and even if most of the flow out the fan is coming from the attic vents it seems like it would not take a lot of flow of conditioned living space air to offset the benefits of the attic fan?

Maybe someone can find some measurements on real attics?

Gary

A couple of careful studies on this:

FSEC-GP-171-00

Home Energy Magazine - Ventilation :: Drawbacks Of Powered Attic Ventilators

The FSEC study found a good reduction in attic temp (20F) and about a 6% saving in AC costs. But, the AC ducts in this house went through the attic and that may have been a big contributor to the AC saving. They found a payback period of 20 years, but they paid a lot for their PV powered attic fans.

The HomeEnergy on looked at 8 homes and found that powered attic vents did depressurize the attics and caused significant flow from the living space to the attic. Not sure what kind of ventilators they were using in this one, and they may have been more powerful than the typical solar ones. They also found some other problems associated with the power ventilators.

There are a bunch of refs out there saying that AC powered attic vents don't payoff.

Gary

Hi Ravi,
Just wondering what an "attic tent" is -- is that a radiant barrier?

Gary

It's a simple fan with it's own sensor that turns it on at around 110 deg F.
Looks a bit like this one.
http://hci.frontstepsmedial.netdna-c...an-300x300.jpg

When I first installed it, I was standing on the ground about 18 feet below it,
and I could feel the blast of very hot air coming down on my head!!

It was installed during the 1980s and I'm amazed that it's still working after all these years..

One thing I found out when my old roofing shingles were replaced, the tar paper underneath was so brittle,
it crumbled like saltine crackers. My Roofer told me that excess heat ages shingles a lot faster.
He won't install new shingles without installing a ridge vent, because of heat damage.

I just measure the temperature in my attic, midway between the roof and the rafters it's 138 degrees F. The outside temperature is 110 degrees, in the shade. Does that warrant additional attic ventilation?

Wow, it's been cooking there since May 1st. Very Texas like...

Yeah, I would consider a ventilated attic and maybe a reflective barrier to bounce back the radiated heat from the roof..

Weather Station History | Weather Underground

Yeah, it's been a super hot summer, and no relief in sight.

I think I might try one of those gable fans. Thanks.

Gary said,
There are a bunch of refs out there saying that AC powered attic vents don't payoff.

I think of my AC powered attic fan kinda like a very low cost heat pump.
It started up around noon today, but we never turned on the AC until about 3 PM.
By 6:30 PM it was cool enough to shut off the Sanyos again.
Without the attic fan, the house heats up earlier and stays hotter longer.

When we did turn the Sanyos on this afternoon, the house cooled down a couple of degrees, very quickly..

Right now, 7:45 PM, the ceilings are at 76.6F and the attic sensor is at 93.1F.

It's 76 indoors and 71.6 outdoors (and dropping fast).. :)



In my case, I don't mind paying a few extra bucks for power,
when it allows us to enjoy more hours of mild summer weather.
Not really expecting the fan to pay for itself in 10 years.. :p

It's going to be too hot to work up there..
Get some work lights and wait until about 1AM to go up there..
Or, take some big fans up there to keep a little cool when installing it..

Hi X,
Sounds like the way you manage the time that its on makes it work well for you.

I think there are a lot of variables -- how well the living space ceiling is sealed against airflow up into the attic, whether there is also a radiant barrier, how much power the fan uses, whether its a not so expensive DIY install or a more expensive install, ... I should not have made a blanket statement.

I got one of the solar powered attic vent fans when they were closing them out at Costco for $80. Its now installed near the outlet of my solar air heating collector in the shop to keep the hot air from stagnating up at the ceiling in the winter. It does a really nice job of keeping the air mixed up -- no controls needed it just runs automatically when its sunny :)

Gary

Whirlybirds
 

These Whirlybird type vents work fantastic and they take no power. My ac would lose ground when it was 95 plus outside. This year I installed 4 of these on my roof. The ac not only keeps the temp now, but actually cycles on and off rather than running continuously late in the afternoon.

I've seen a few of those around and wondered how they spin, is it wind or the hot air updraft?

Some people don't like them..
Attic Ventillation. Roof vents vs Whirly birds.

Actually both ways work , it's amazing how many cfm's they can pump out even with no wind.

If the wind turns them and the wind pressure on one side doesn't interfere
with drafting air up and out, that's cool..
But, around here there isn't a lot of wind on many of the hotter days..
Average wind speed here over the last 7 days is 0.5 mph
WOBURN WEATHER CENTER - Weekly Conditions
During the day, it's about 2 MPH sometimes.. In the winter, we get a lot more wind..
And that's when I don't mind some hot air in the attic.

However, if it's turning when the wind isn't blowing..?. That means the hot air
that's drafting up out of the attic is doing work.
Which means the air flow is being restricted, slowed down.

IMHO, a long ridge vent on the apex has got to be the best way to vent.
But it needs some air input in the soffit area to work best.
http://www.cornerhardware.com/howto/images/ht076_2.jpg
Sadly, my house didn't get any over-hang/soffit area.

My ridge vent spacer material is a plastic mesh cloth (like a pot scrubber).
I'm pretty sure it's the worse type to use, if you want good air flow.

The roofers has installed a real nice open-air-flow vent,
http://www.americanprideroofing.com/...ridge-vent.jpg
but they didn't have the proper size cap shingles to cover it.. Had to replace it with mesh junk.. :(

Don’t Waste Money on Powered Attic Fans
Barry Westbrook, P.E.

Most of us were taught from an early age that we can reduce our summer time cooling costs and extend the life of our asphalt shingles by sucking the hot air out of the attic. This is a wasteful, damaging, and dangerous myth that needs to be debunked.
Ironically, powered attic fans will almost always increase the cost of cooling the home. In many cases, they will also elevate radon gas levels in the living spaces of the home and create moisture management problems during hot weather. Finally, powered attic fans will not extend the life of the shingles. Why is this true?
Building scientists at Oak Ridge research facilities have conclusively proven that the amount of ventilation in attics does not significantly affect the cooling requirements of the building nor does it affect the life of the asphalt shingles. These findings are in direct contradiction of the long-held beliefs about the need for attic ventilation.
It turns out that actively ventilating the attic has almost no effect on the surface temperature of the asphalt shingle. Also, most of the heat transmission from the roof is radiant energy coming directly from the roof decking to the ceiling of the home. Although the air in the attic also gets hot, the air is not heating the ceiling. The ceiling is heating the air. Ironically, in most homes, powered attic fans literally suck the conditioned air out of the living space and increase the cost of cooling.
Considering these facts, is there a need for attic ventilation at all? For most attics, the answer is yes. During the winter months, the roof surface becomes cold. Without some ventilation, moist air from the living spaces will leak into the attic and form condensation on the roof decking whenever the temperature drops below the dew point of the air. Condensation on the roof decking can promote mold growth and wood rot. However, only passive venting is needed to prevent wintertime condensation in the attic. Powered attic fans are almost always installed due to concerns about hot weather.
Some attics need no ventilation at all. Many new homes and some existing homes are retrofitted with what is termed as a "closed attic assembly". In a closed attic, spray foam insulation is installed directly to the underside of the roof decking creating an “igloo” effect. This insulation thermally isolates the roof assembly from the attic. This prevents the radiant heating of the house in the summer time and prevents moisture condensation on the roof decking in the winter.

Closed Attic Assembly with Spray Foam Insulation

In summary, although there is a need for ventilation in most attics, the installation of powered attic fans are counterproductive. Homeowners looking to reduce summertime energy costs should invest in additional insulation or install radiant barriers instead.


Questions and comments can be submitted to the following address:
Barry Westbrook
℅ DocAir
4014 Flagstone Ct
Franklin, Tn 37069
615-373-2498

What if the ducts and/or air handler are located in the attic space?

I'm a bit doubtful the fan causing so much suction inside the house, that it pulls up radon..
But, I can see where some leaks/ventilation configurations might cause negative pressure, all the way down to the basement.

Currently, my Radon alarm meters are displaying 2.6 ppm, a couple of months back, the readings were 2.5 or less..

I'm going up and unplug my little attic fan, and keep an eye on those radon alarms.

XRinger,

I can assure you that it will. That is how I first got interested in this subject. We had a client in Bowling Green, Kentucky where they started smelling gasoline vapors in the basement. The ultimate source was a leaking underground storage tank. The proximate cause was two powered attic fans that were litterally pullling huge volumes of soil gas into the building. Subsequent air sampling showed high radon levels as well as gasoline constituents.

Of course, if the ceiling/attic interface is air tight, this could not happen. However, it still doesnt justify the expense of a powered attic fan.

Barry Westbrook
℅ DocAir
4014 Flagstone Ct
Franklin, Tn 37069
615-373-2498

I'm not sure what the level of vacuum would be in the basement.
It likely kicks up pretty high when the oil burner fires up, or if the dryer is running!

We have an attic pull-down stairs in the back hall/basement stairway.
It's sealed, but the way it's constructed, doesn't allow for a really good seal.

What also worries me, is the fans in the kitchen & bathroom.
Those pull a suction on the living areas.

One of the projects on my to-do-list is taking care of our radon problem.

Seems like the best way to keep your attic cooler is installing PV.. :D

Absolutely. The top of your attic may be 150 degrees and the bottom 120 degrees, but if your air conditioning ducts are in the 120 degree area, its still heating up the air you just cooled!