Powered Attic Fans

[dboat]

I am thinking of adding a couple of powered attic vent "PAV" to the house and garage, one for each. My thinking is that using a little electricity to keep the temp of the attic down, would be a good thing and I would easily get my money back. I just dont think a couple of whirlybirds and some passive vents in the roof, along with the soffit vents are enough. I do have radiant barrier under the roof decking. The garage gets really really hot in the summer. The attic gets hot, but not like the prior house. I was ready to pull the trigger on this until I went web searching and found this thread, about 40 pages of it. I couldnt read the entire thread, but there seemed to be strong consensus that PAV's are a bad thing because they create a negative pressure in your attic and then suck air, not only from the soffits but from the top floor/ceiling of your house. The light fixtures, the light switches, outlets, etc. So now I am confused, do these things really work and are they worth the money? I am convinced now that the solar ones, which are much more expensive, are not worth the money because they are very underpowered and only work when the sun shines on them, rather then when its hot in the attice.

Here is the link for those that chose to read.

http://hvac-talk.com/vbb/showthread.php?t=166795

So, what do you guys think?

Dana

[Silver_2000]

well I read a few of the middle pages of the thread

the "experts" lost my attention when the agreed there was NO reason to ventilate an attic at all...

In my house the AC and ducts are in the attic
The deal is that my house will NOT stay cool without PAV

So all the BS the "experts " are blowing means NOTHING to my situation.

The stuff about sucking AC from the house is likely true IF you have a loose house and crappy insulation. If you have adequate soffit vents the air will go the EASY way

But again - in my house - PAVs are needed

[mikelemoine]

I agree with Doug. If your ducts and fixtures are properly sealed, then you won't have cool/hot air leaks with the fan on. If they are leaking, they need to be fixed anyway.

I had a free energy audit from my power company. The guy brought some foam gaskets to put behind outlets, a thermometer for the freezer and suggested that I caulk the interior attic door. Just enough to seal it, but not so much that you can't open it again.

He said that most homes over 10 years old have some duct leaks that can increase your bill by 20% easily. You can go in the attic with the AC on and use a laser thermometer to look for cold spots, which indicate leaks. They recommended that we have their contractor come out and do a pressure test, they basically pressurize the house with a blower and then look for leakage at the ducts, then re-seal as needed. I haven't done it yet, but my buddy had his mid-80's house done and said that rooms that never got cool enough now feel great. I plan to do ours before Summer, the power company even pays part of the fee (check with your company, they may do so as well).

An interesting way of looking at the attic heat issue is this: Even with the insulating properties of drywall and attic insulation, some heat passes through (IE: or the ice would never melt in a cooler or fridge when the power is out). Now think about the size of the ceiling of your house and the fact that even if it's only a raised to a few degrees above the ambient temperature, it's like a huge radiator warming the air in your home. Cooling the attic will reduce the amount of heat the insulation has to reject, therefore lowering the heat passing through to the home.

When choosing an attic fan, consider things like noise levels and CFM per watt. The fans are relatively quiet, but if you mount one on the gable over your bedroom or living room, the sound may travel through the walls and bug you. You'll also want to make sure it moves enough CFMs to cycle the air in your attic, but not so much that it wastes power or sucks all the insulation out (J/K!). Also important, make sure your attic has sufficient ventilation to allow decent airflow (add vents to the end gables, ridge vents on the peak, etc). Some fans have adjustable thermostats, you might play with that. If your fan waits until the attic hits 150 degrees to kick on, you already have some heat transfer going on. Conversely, if it kicks on too "cool", you might be wasting 100 watts running a fan with little or no savings on AC.

In closing, now is a great time to get up into the attic and check for leaks (water and air) and do projects such as this, before it gets too hot. It's also a good time to take a walk around the house and re-caulk any windows, dryer vents, spigots, etc that might be in need of some maintenance.

[dboat]

My house is a little over 2yrs old but I have lived in it for about 1.5 yrs. Its pretty tight and my attic insulation is about R32-36.. I think I have plenty of soffit vents for the 1500 cfm pav they will install. It will be about 15 feet above the top of the rafters of the second floor. It will be handling about 1800 sq ft of floorspace.. so I think it will be enough without being too much. I am adding another one just for the garage, its not insulated at all and gets freaking blazing hot in the summer.

Dana

[L8 APEX]

I like ridge and soffit vents along with good old fashioned whirly birds. Most installers cover up the soffit vents when they blow it in.

I have not seen powered vents work for long if at all. The heat kills the little motors. Donnie put a solar one on his shed it was cool...

[dboat]

I did some research on the solar pav's. they dont pull as much as as they say they do, they dont run at night when its still pretty hot outside, and they generally cost a bunch more. you are correct in that the electrical ones run hard and in a very inhospitable environment and have a short life.. there is not perfect solution unfortunately. I was leaning solar for a while until some of the data was presented. However, was Donny really happy with the results?

Dana

[L8 APEX]

Not really, it runs but the metal shed is an oven in the summer. I think natural convection and whirly birds are the best system.

[Silver_2000]

Ive had PAV's running in my attic for more than 7 years with no issues.
Replacing the motor is trivial work ...

[Moonshine]

I have not seen powered vents work for long if at all. The heat kills the little motors.

This was my experience. One worked for a year, the second PAV stopped working the second year. Mine operated on a thermostat and were set to come on at 105 degrees.

[99bolt]

I did some research on the solar pav's. they dont pull as much as as they say they do, they dont run at night when its still pretty hot outside, and they generally cost a bunch more. you are correct in that the electrical ones run hard and in a very inhospitable environment and have a short life.. there is not perfect solution unfortunately. I was leaning solar for a while until some of the data was presented. However, was Donny really happy with the results?

Dana

You are correct, even whith direct sunlight, it doesn't move much air. It might be alright if it had a more efficient or bigger solar panel. I thought about wiring in a battery, but it's not worth it. Probably would of been better off just adding some sort of ridge vent.

[Mark #2]

I think natural convection and whirly birds are the best system.
I would agree, my house garage was smoking hot, cleaned the insulation off the soffit vents and added a whirly bird above it, pretty close to ambient temps now. Just need to let the attic leak a little, the problem is that most houses don't have enough soffit vents so you suck from the fixtures, walls, etc, inside the house..

And those coffee cup plates that you put behind external wall sockets work great.

[PUMP]

Do you have gables or is it 100% hip roof? That makes a big difference on wether powered ventilation works well. With a hip roof, the only source for replacement air is the soffit vents and they can handle about 200 cfm per square foot of free area opening. 2 or 3 turbine vents work better than 1 large powered fan. Soffit vents are typically 6"X12" or 6"x16". Free area is in the area of 50% to 60%. If you go powered, use a T-stat set around 110-deg f to control the on off. This will extend the motor life considerably. I have 2400-sf conditioned space with a combination of gabled and hip roof. Ridge vents on the hip roof portions and 2 turbine vents on the gabled portion. R-24 (or better attic insulation). You can usually tell if the soffit vents are blocked by checking the dust accumulation on the screens. No dust means they are blocked or not much air going through them.

[Mark #2]

You can usually tell if the soffit vents are blocked by checking the dust accumulation on the screens. No dust means they are blocked or not much air going through them. Okay, I just inspected mine, guess they were working well, they look like clogged dryer filters now. A lot of street construction in the past year, looks like concrete dust.
Wow, that is something I would have never considered, examining soffit vents, just how OCD do you have to be to even think of this?
But I will be cleaning them soon.;)

[Silver_2000]

Okay, I just inspected mine, guess they were working well, they look like clogged dryer filters now. A lot of street construction in the past year, looks like concrete dust.
Wow, that is something I would have never considered, examining soffit vents, just how OCD do you have to be to even think of this?
But I will be cleaning them soon.;)

Replaced mine last year when I painted the house - They were all pretty clogged .... OCD ? :rll::rll:

[99WhiteBeast]

Okay, I just inspected mine, guess they were working well, they look like clogged dryer filters now. A lot of street construction in the past year, looks like concrete dust.
Wow, that is something I would have never considered, examining soffit vents, just how OCD do you have to be to even think of this?
But I will be cleaning them soon.;)

Are you intending to just remove them and hose them off or blow compressed air through them?

I need to check mine I know I have some insulation blocking some of mine.

I saw a really cool ventless AC unit for the garage on another forum- I'll see what I can find.

[Mark #2]

Are you intending to just remove them and hose them off or blow compressed air through them?

I need to check mine I know I have some insulation blocking some of mine.

I saw a really cool ventless AC unit for the garage on another forum- I'll see what I can find.
The dirt is kind of greasy I was going to scrub them with simple green and a brush in place

[dboat]

If you go powered, use a T-stat set around 110-deg f to control the on off. This will extend the motor life considerably.


Bob, I took your advice. The guy was going to set it at 90 degrees, so we had him change it. He said it will never run at that temp, but we shall see.

Dana

[Moonshine]

He said it will never run at that temp

Won't ever run set at 110? :bs I had mine set to come on at 105 and they ran a LOT, so much so that neither lasted two full years.

[dboat]

Well, I have R32 up in the ceiling and I think that Bob's suggestion at 110 sounded good.. at 90 those things would run all day and night.. believe me, I have been in the attic in the summer and its definitely hotter than 110 in there.

Dana

[PUMP]

With R32 your ceiling U value (rate of heat transfer) is .03 BTU/deg-hr-sf so heat gain through ceiling is pretty small. With that low a U-value, you really only need to worry about keeping the attic around 35-40 degrees above indoor temp during peak cooling. With a higher U-value (U = 1/R - or less insulation means higher U) a smaller temperature differential would be desirable. With ventilation alone, you can not get the attic temperature lower than the outdoor temperature
Heat gain to the attic through the roof is mostly (85% to 90%) radiant. So, when sun is shinning, attic heat gain is mostly from solar radiation while heat removal via ventilation is 100% dependant on air temperature. The optimum ON-OFF temp setting for forced air fan is therefor dependant upon several factors and will vary from house to house. In my experience 110-F is a reasonable starting point and then fine tune to fit any particular house.

[dboat]

Bob, nice post.. I do have radiant barrier on my roof decking too. My next question is what do you do to fine tune it from this point? My hope is that the two fans will truly make a difference. I should note that the fan in the garage will have to work harder since there is no insulation in the garage whatsoever.

Dana

[gbgary]

Bob, nice post.. I do have radiant barrier on my roof decking too. My next question is what do you do to fine tune it from this point? My hope is that the two fans will truly make a difference. I should note that the fan in the garage will have to work harder since there is no insulation in the garage whatsoever.

Dana

you've got cool deck? that stuff is great. i did a start-up once (turning the a/c on for the very first time on a new house) on 105*+ afternoon. the vacant house was hot (windows all open etc). had to go up in the attic to check drains and whatnot (before turning it on) and was really dreading it. i couldn't believe it...i could have worked up there all afternoon it was so nice. your fan may not ever run at 110* if you have cool deck.

[dboat]

you've got cool deck? that stuff is great. i did a start-up once (turning the a/c on for the very first time on a new house) on 105*+ afternoon. the vacant house was hot (windows all open etc). had to go up in the attic to check drains and whatnot (before turning it on) and was really dreading it. i couldn't believe it...i could have worked up there all afternoon it was so nice. your fan may not ever run at 110* if you have cool deck.

Believe me, it'll run.. cool deck makes it better, but it gets really hot up there. But when the guy was going to set the thermostat at 90, I was thinking that thing is going to run 24/7 from April until November..

Dana

[PUMP]

Bob, nice post.. I do have radiant barrier on my roof decking too. My next question is what do you do to fine tune it from this point? My hope is that the two fans will truly make a difference. I should note that the fan in the garage will have to work harder since there is no insulation in the garage whatsoever.

DanaTake temperature reading in attic on a clear day around 3:00PM when outdoor temperature is above 90-F. If the diference between attic temperature and outdoor temp is in the range of 10 to 15 degrees, 110 setpoint is good. If the difference is less than 10-F increase setpoint a few degrees. If the difference is more than 15-F, decrease the setpoint.

[Silver_2000]

attic fans are already on...

[Mark #2]

Yup, mine too, but they are always on, turbines:D
Went into the attic to change the filter, +25 degrees easy. Guess I will be cleaning soffit vents tomorrow as they are pretty blocked with all the concrete construction around here lately.

[dboat]

Take temperature reading in attic on a clear day around 3:00PM when outdoor temperature is above 90-F. If the diference between attic temperature and outdoor temp is in the range of 10 to 15 degrees, 110 setpoint is good. If the difference is less than 10-F increase setpoint a few degrees. If the difference is more than 15-F, decrease the setpoint.

:tu::tu::tu::tu:

[PUMP]

attic fans are already on...With that amount of heat gain to attic you need at least 9" of blown insulation. If you don't have at least that, adding insulation will probably have a 3-year (or better) payback.

[Silver_2000]

With that amount of heat gain to attic you need at least 9" of blown insulation. If you don't have at least that, adding insulation will probably have a 3-year (or better) payback.

I have good insulation
Im thinking about adding the radiant barrier
Looking for cheap effective materials - the bubble stuff they sell at Home deport and lowes is pretty expensive

It wouldnt take long to nail it up to to the bottom of the studs though..

[dboat]

isnt there a spray on radiant barrier?

[Silver_2000]

looks like 1000 sq ft for $150

thats not too bad

http://www.ecofoil.com/Applications/Attic-Insulation

[dboat]

Under rafter or over joist? Gonna do it yourself?
Dana

[Mark #2]

http://www.webstaurantstore.com/24-x-1000-food-service-heavy-duty-aluminum-foil/12224X1HD.htmlhttp:NexTag+Campaign

$76 for 2000 square feet, my rafters are 24" on center.
I have thought about this for a couple of years.

[Beaudee]

I have 2 whirly bird vents,that siver spray on inside of roof,and an electric fan in the atic.I am 2 story 3000s.ft..I also have the cheepest electric bill compared to my neighbors even with a pool running.If you do get a fan get one with a thermostat so you dont half to turn on/off all the time.I think it also moves more air with the 2 whirly birds,my fan is in the center and is thermostat. controlled.I also have solar screens on every window/door in house.

[Moonshine]

isnt there a spray on radiant barrier?

If anyone knows of this I'd be interested. My roof is so high above the attic decking that I'd have to use a ladder or some sort of scaffolding to do a nail on barrier. A spray on barrier would be far easier to apply.

[dboat]

Brian, there is a ceramic additive you can add to paint. Not sure how well it works nor if it can be used on the backside and work or if it needs to be on the outside facing surface.
Dana

[Silver_2000]

in some of the pages I read - there is controversy about the effectiveness of the paint ....


If I were building a house I would make sure the roof sheet has radiant barrier installed

here is some detail

http://www.lrhomesolutions.com/roofing/raddiant_barrier_plywood.htm

[Moonshine]

Yeah, the spray/paint doesn't look too promising. Wish this stuff had been around when I was building the house. It would have been so much easier to install then. But the thought of laying a piece of plywood across ceiling joists so I can set a ladder so I can nail a few feet, then moving the whole rig two feet, and repeating that several hundred times just doesn't sound like a good plan.

[dboat]

Brian, you could get the perforated type and drape it across the floor of your attic as an option.
Dana

[Silver_2000]

there is also controversy on laying the foil on the insulation - due to dust build up and failure of moisture to pass through

[dboat]

Yes, but its hot enough here so that moisture shouldn't be a problem, but you can get perforated foil just in case. The dust issue is a concern and just don't know how much gets in.
Dana

[Beaudee]

If anyone knows of this I'd be interested. My roof is so high above the attic decking that I'd have to use a ladder or some sort of scaffolding to do a nail on barrier. A spray on barrier would be far easier to apply.

I didnt pay for the silver spray stuff(whatever it is) it was done before i bought house.Personally i dont see how it works cost/real effectivness.Whirly vents thems. fan (moving lots of air)is the ticket.:tu:

[Silver_2000]

Yes, but its hot enough here so that moisture shouldn't be a problem, but you can get perforated foil just in case. The dust issue is a concern and just don't know how much gets in.
Dana
Its also for letting moisture out of the house....

concrete slabs + Bathrooms + temp changes = moisture ....

FYI

[dboat]

Got home today and even though 85 outside. Went into the attic and the fans werent running so I guess thats a good sign..

Dana

[PUMP]

Got home today and even though 85 outside. Went into the attic and the fans werent running so I guess thats a good sign..

DanaAssuming that you are not keeping the attic cool with your AC system:eek:, that is a good sign.:tu:

[dboat]

Assuming that you are not keeping the attic cool with your AC system:eek:, that is a good sign.:tu:

I dont think I am.. I had the a/c off since we were out of town. well, actually I had it turned up to 80.. and it wasnt that warm in the house when we got back in.