Do power attic fans help?

[printmanjackson]

Do power attic fans do a better job of venting the heat out? I have heard some people say they are a waist of money because the motors won't last long running in the heated condition they run in. I have 3 in my attic and all 3 motors are done. I want to replace them with either a higher quality moter or take them out completly and use something else. I live in Tennessee so our arttics here get pretty hot.

[lentz]

They seam to help a little. I have to change them out about every three years. A ridge vent works much better. You do have to have opening in the boxing or in the eaves for the air to circulate.

[adrianf]

http://www.fsec.ucf.edu/en/publicati...sec-gp-171-00/

Go to conclusions for summary and this is with a solar/battery fan.
HTH

[grumpy20716]

If home has ridge vent system we will not install or repair fans.The fan will not allow the ridge vent to work as designed

[deejoe]

IF an attic space has the proper amount of soffit and ridge (or other high ,near the peak ) vents, then no other type of venting is ever needed.
Gravity air flow, entering the soffits and exiting high near or right at the peak will do the trick.
No electrical powered or high priced turbine fans are required.

[platchford]

If you have a ridge vent then don't bother replacing the fans. If you don't have a ridge vent then you may consider adding a ridge vent rather than replacing several fans.

[mark beiser]

Powered attic vents, aka PAVs, pull conditioned air up out of the house through light fixtures, wiring penetrations through the top plates in walls, the attic access, and any other path it may find.
The more passive ventilation from outside, the less air it will pull up out of the house, but it will always be pulling air out of the house.

I have been in a number of homes where the difference between not cooling to the set point on a 85-95º day, and maintaining the set point up through our 100º outdoor design condition, was simply to disconnect the power to the PAV.

See the 2nd part of my signature.

[dan sw fl]

Go to conclusions for summary and this is with a solar/battery fan.

~ 9 year pay back is not attractive to nearly all Americans.

PV powered fans need to be cheaper and electricity rates need to increase.

And with updated code requirements for AHUs and ducts in a conditioned space and high ceiling R value,
the future impact of attic fans will be nil.


" Photovoltaic Attic Ventilators

Two PV attic ventilators were installed on the house's asphalt shingle roof on August 6th, 1997.(3) Both were installed near the peak of the A-frame roof with one on the east face and another on the west face as shown in Figure 2. The fans are designed to provided between 600 and 800 cfm of attic ventilation at peak solar irradiance (1000 W/m2) depending on the free soffit ventilation area. The ventilators consist of a 19.5" x 16" mounting with a 10 Watt thin-film PV module. A five bladed radial fan and a direct coupled DC motor provides attic air exhaust. The units were purchased for $300 each; installation would typically add another $100 - $150 per unit. "

" ... Comparing periods with similar weather conditions, the test revealed that the PV vent fans have the potential to reduce measured peak summer attic air temperatures by over 20oF. However, the impact over the cooling season is fairly modest with well insulated attics. Measured space cooling reduction was approximately 6% - worth about 460 kWh annually at the test home. "

[adrianf]

"9 year pay back is not attractive to nearly all Americans."

Agreed. I haven't been a FAN of pav(s) for many years now.

[Joe Mascitti]

Just wanted to throw this out here...I was a firm believer in PAV's until reading along on this forum. I have done a lot of work over the winter in the attic. Adding insulation...making sure all soffet vents were not blocked. Also, my roof has many ridges, all have ridge vents. Taking the advice from people on this forum, I have added an attic tent, caulked all the penetrations through the top plates and disconnected my PAV. Today it was 96* and sunny...my attic temp reached only 110* Now it is currently 88* outside, my attic temp is also 88*. My upstairs Temp was 76* throughout the day, although my upstairs AC runs contantly from 1pm till 10pm, the humidity is very low. Every pro I have spoken with saids this is normal for a properly sized unit. I have 2 high efficency units(Ruud, 3 ton upstairs 2 ton downstairs) and cannot complain about my electric bill which runs about $50-$60 more than my electric bill in the winter. With my PAV, I could not achieve a temp below 78*. Thanks to all on this forum who know what they are talking about.

Ask yourself this. Can a sealed attic with a PAV achieve only a 14* temp difference? Or is a well ventilated attic better (and no $$$ wasted on running the fan) Ditch the PAV

[AlphaRat]

Just wanted to throw this out here...I was a firm believer in PAV's until reading along on this forum. I have done a lot of work over the winter in the attic. Adding insulation...making sure all soffet vents were not blocked. Also, my roof has many ridges, all have ridge vents. Taking the advice from people on this forum, I have added an attic tent, caulked all the penetrations through the top plates and disconnected my PAV. Today it was 96* and sunny...my attic temp reached only 110* Now it is currently 88* outside, my attic temp is also 88*. My upstairs Temp was 76* throughout the day, although my upstairs AC runs contantly from 1pm till 10pm, the humidity is very low. Every pro I have spoken with saids this is normal for a properly sized unit. I have 2 high efficency units(Ruud, 3 ton upstairs 2 ton downstairs) and cannot complain about my electric bill which runs about $50-$60 more than my electric bill in the winter. With my PAV, I could not achieve a temp below 78*. Thanks to all on this forum who know what they are talking about.

Ask yourself this. Can a sealed attic with a PAV achieve only a 14* temp difference? Or is a well ventilated attic better (and no $$$ wasted on running the fan) Ditch the PAV

Joe, I am curious what your attic temperature was running before you did the work. I have been told by some A/C guys that my attic is the hottest they have ever been in.... Not sure about that, but it was 120 deg this morning at 10AM when the outside temp was about 80 deg. I have been up there before when it was 130-140 and that was on a day when the outside temperature was in the low 90's..... It has been over 100 here past several days, but i have not had chance to check attic temperature.

I have soffit vents (not sure if they are all open but would presume they are since there has not been any new insulation blown up there since house was built in '90's). I also have the little rectangular vents on top - I think they call them air hawks (???) - but they are not powered vents or turbines, just vents. I have about 5 of those - I think my attic is around 2000 sqft.....

NO ridge vents and NO PAV's... About 9" of the pink rolled insulation R-30 and I also saw some R-19

I really don't think my house stays that comfortable and the A/C runs freaking constantly..... Just got a new Trane 4 ton 14 seer last year and think it is a tad undersized (about 2500 sq ft).......

Curious your thoughts AND ANYONE ELSE'S !!!!! on what are best things to do to help the situation.

[Joe Mascitti]

97* yesterday..my attic peaked at 113.7*...I cannot remember exactly before, but even with the PAV I think I was in the 120's...When I had my roof re-done a few years ago after a big hail storm. I asked the roofer to be sure to put a Full ridge vent on every ridge. Prior only the "Main" roof had one, and it was about 1/2 the length of the ridge line....

[Joe Mascitti]

6:30 here...outside 94.1* inside 77.3* Attic 103.6*

Sunny

Dewpoint 71*

Heat Index 101*

I am taking my attic measurement 8ft about the floor...

[MiniBoy]

Hi all ..

I have a little different situation .. while suggesting the addiion of a fan to one of two 24 X 36 in. gable vents, I discovered that while there are NO soffit vents, a 30 ft. continuous ridge vent was added when the roof was redone 6 or 7 years ago. It is now quite obvious that a PAV is not a very good idea, but I'm wondering if passive flow through ridge vent alone ( east / west breeze ) and through the gable vents ( north / south breeze ) will be sufficient to keep the summer attic temp reasonably low, or should soffit vents be added ? The attic floor area is 720 sf, and I found 2 layers of 3-1/2 in. unfaced fiberglass bat insulation under the floor, but no vapor barrier. There is 1 - 13 in. Diam. NuTone bath vent in the ceiling which needs to be sealed to the cieling drywall, and the attic folding stairway needs to be sealed, and I may suggest replacing it, as well as building an insulated, gasketed and hinged cover from the attic side side. A/C air handler is in the attic and all duct work is in good condition and tight, although additional insulation may be in order.

Any thoughts will be much appreciated,

Russ

[Random Guy]

In addition, you need to close off the gable vents. The soffit vents allow cooler outside air to enter and the hot attic air to rise out of the ridge vents. Having the gable vents could allow all of the air leaving the ridge vent to come in the gable vents, sort of like having a supply vent too close to a return.

[MiniBoy]

In addition, you need to close off the gable vents. The soffit vents allow cooler outside air to enter and the hot attic air to rise out of the ridge vents. Having the gable vents could allow all of the air leaving the ridge vent to come in the gable vents, sort of like having a supply vent too close to a return.

Hi Random Guy;

I certainly follow your thinking .. this is why I scrapped the gable vent mounted PAV idea in the first place, as it would be pulling air through the ridge vent, and when the PAV CFM became satisfied, the opposite gable vent might not have been supplying any outside air, leaving that end of the attic basically unvented and as hot as usual !

I'll go back and check to see what the insulation in the soffits situation looks like and consider venting them. Would a 6 X 10 vent in every other rafter bay be sufficient, or a smaller one in every bay ?? Come to think of it, those rafters might be on 2 ft. centers .. gotta go back and investigate ..

Thanks for your input,

Russ

[Shophound]

Hi Random Guy;

I certainly follow your thinking .. this is why I scrapped the gable vent mounted PAV idea in the first place, as it would be pulling air through the ridge vent, and when the PAV CFM became satisfied, the opposite gable vent might not have been supplying any outside air, leaving that end of the attic basically unvented and as hot as usual !

I'll go back and check to see what the insulation in the soffits situation looks like and consider venting them. Would a 6 X 10 vent in every other rafter bay be sufficient, or a smaller one in every bay ?? Come to think of it, those rafters might be on 2 ft. centers .. gotta go back and investigate ..

Thanks for your input,

Russ

If there is blocking between the rafters where rafter and blocking meet the top plate of the outside wall, and there are no holes drilled in this blocking, you can punch your soffit full of holes...it won't ventilate to the ridge vent. However, often there is not blocking between rafters at this point...only way to tell is to either remove a section of soffit face board, or get in the attic and crawl all the way to where rafters dive down to the attic floor (not fun...been there, done that!) at the exterior wall.

I can't see a gable end vent taking in ridge vent exhaust unless that same exhaust cools to the point where it sinks down, and the gable vent has a sufficient negative pressure going through it to draw that same air in. Even so, the air had to cool down before it got there...it is likely near ambient conditions.

[MiniBoy]

If there is blocking between the rafters where rafter and blocking meet the top plate of the outside wall, and there are no holes drilled in this blocking, you can punch your soffit full of holes...it won't ventilate to the ridge vent. However, often there is not blocking between rafters at this point...only way to tell is to either remove a section of soffit face board, or get in the attic and crawl all the way to where rafters dive down to the attic floor (not fun...been there, done that!) at the exterior wall.

I can't see a gable end vent taking in ridge vent exhaust unless that same exhaust cools to the point where it sinks down, and the gable vent has a sufficient negative pressure going through it to draw that same air in. Even so, the air had to cool down before it got there...it is likely near ambient conditions.

Shophound:

Thanks for the reply ..

Being from the "Old School", and reading a couple of these threads, my thinking has changed regarding the PAV route. The intent of this job was to lower the attic temp in the summer, to reduce the A/C load. I have moved away from the PAV theory to the passive thermoconvection, as it won't use more energy and require fan and control maintenance. I have a couple of issues on this job .. cutting in soffit vents seem inevitable, but I am not clear on whether the gable vents could not be left to be passively functional, rather than block them up, as one contributor has suggested. They are at least 2 ft. from the ridgeline, and it would seem that the soffit to ridge vent "chimney effect" would convect a goodly amount of the hottest "source heat" from under the roof deck between the rafters out the ridge vent, while the gable vents would allow a certain passive cross flow of cooler outside air closer to the floor, on a breezy day at least. However, I have to recall a phrase from another poster on this thread .. and this is not a direct quote .. if air did what we would like it to do, or hope it would do, life would be easy . So, I am mindful that what we hope and want to happen is not necessarily what does happen ! For certain, the idea of sucking conditioned air from the living space to the attic is self defeating, to say the least !!

I need to go back and do a little more investigating .. my recollection of the job indicates no blocking between the rafters at the top of the eaves walls, as well as the bat insulation appears to end at the top of those walls, and the rafters do NOT have birdsmouth cuts at the top of the wall, rather sit on top of the wall, so there should be a couple of inches of space between the top of the insulation and the bottom of the roof sheathing .. but this has to be confirmed the first of the week.

Thanks for your taking the time to post .. appreciate your input .. it's 1:00 A.M. here in Massachusetts .. goin' ni-nites ..

Russ

[rnevil]

I live in Lousiana and during the summer my AC runs constantly on hot days. This year I had the AC system checked and all checked OK except for the fact that my attic is very hot (over 130) even though I have ridge vents and soffit vents. I think that my soffits vents area is sufficient but suspect that the ridge vent area is lacking because my home has four hip roofs(short ridges). I was considering installing a power vent (recomended by one AC contractor)but after reading here I have the feeling that installing wind turbines may be a better option. I have seen some homes in my area with a combination of ridge vents and wind turbines. I am also not sure how many turbines I should install. Can there be to many? Any thoughts and help would be appreciated?

[Znobyrd]

Mevil, sounds like you have alot of vents, good. I would definitely install the attic fan. I installed just one 1350 cfm gable fan for my 1250 square foot attic on the west end of the house. It has a thermostat, so it will turn itself on at 90 degrees, and off at 90 degrees. Alot of air movement up there. It works great, and the cooling bill showed it.