Page 3 of 40 FirstFirst 1234567891013 ... LastLast
Results 27 to 39 of 513
  1. #27
    Join Date
    Aug 2004
    Location
    North Richland Hills, Texas
    Posts
    14,914
    Quote Originally Posted by Brian GC View Post
    I believe a thousand small holes that add up to one sq. ft. will have more resistance than a single hole of the same area. The perimeter surface area of the holes would be much greater, slowing the air flow. An example of this would be water flow within pipes. Ten small pipes that add up to the area of one large pipe would definitely have a slower flow rate. Also, distance from the FAV would reduce flow as well.

    Brian
    True, but when you are talking about air leaks through holes, the difference isn't that great. My intent was to point out that while there may not be a huge hole between the attic and conditioned space, if you add up all the small leaks, they equal a big hole.

    In the average house, built before it was common to seal the wiring holes for wire penetrations through the top plates of walls, it is not uncommon for those holes to represent half, or more, of the leakage area in the house.

    Adding a mechanical fan to that will exaggerate the already existing problem. It is usually rather impractical to go back in an existing home and seal all of those holes, and depending on the house design, you may not even be able to get to many of them.

    There are exceptions, but as a general rule, PAV's are a bad idea.
    If more government is the answer, then it's a really stupid question.

  2. #28
    Join Date
    Apr 2002
    Posts
    11,808
    Quote Originally Posted by mark beiser View Post
    1000 small holes, that collectively add up to 1 square foot, will collectively allow the same quantity of air through that a single 1 square foot hole would.

    Pretty much every bit of independent 3rd party research I have seen on the subject comes to the conclusion that PAVs are not worth the investment in the up front cost, maintenance and operating cost, even in attics that are very well sealed off from the conditioned space.

    Since I have gotten into measuring house leakage, I have had the opportunity to actually measure exactly how much air PAV's were pulling out of a number of homes.
    The worst one I found was a 6,000ish square foot house with three 1600 CFM PAVs very little passive ventilation, and a large number of non air tight recessed light fixtures and poorly sealed thermal bypasses. With all of the fans that roofers installed last spring, about 2800 cfm was being pulled out of the house. the main part of the downstairs couldn't maintain a 78 set point on a 85 day, with an AC system that was about 20% oversized for keeping it 75 on a 100 day with normal infiltration.

    The best one I have tested was pulling about 150 cfm out of a 1800 square foot house. They had a 1200 cfm PAV, with 2 gable vents and lots of soffit vents, 8' ceilings with no recessed light fixtures or attic access within the conditioned space. The PAV was causing a little over half an air change per hour, mostly from pulling air in through electrical outlets and light switches through the wire penetrations in the top plates of the walls.
    its a very cliamte specific answer mark and hard to explain to some

    attic fans are a disaster waiting to happen down here

  3. #29
    Join Date
    May 2004
    Location
    south louisiana
    Posts
    3,187
    attic vent fans are a disaster here in the south also.

    and btw mark..did you buy into the comfort institute package?
    Great stuff isn't it?
    We only had one guy locally that attended the seminar that bought the pkg.
    Only he didn't buy a blower door or duct blaster..just used the trademark until he got himself in trouble...
    imagine that...

    Once you start actually testing these things it is hard to understand why these types of devices are still being installed.
    I've found diehard fans of attic vent fans in the a/c industry that swear by them..
    I generally leave that battle up to the home owner to battle out with the ac company.
    The cure of the part should not be attempted without the cure of the whole. ~Plato

  4. #30
    Join Date
    Feb 2008
    Location
    Tennessee
    Posts
    91
    so the question now is how does a HO know when they have a problem and how big is the problem? Is there anyway to tell just how much these power attic fans are costing us? Is it the only solution to turn them off completly? If this is as big as you say it is Mark then someone needs to start telling people about it. To educate people about a problem starts when something as simple as people start talking about it.

  5. #31
    Join Date
    Mar 2008
    Location
    Long Beach, CA
    Posts
    3,384
    I think PAVs are overkill. They consume electricity, move too much air, the motors wear out, they’re more difficult to install, and are noisy. I recommend turbine fans. They are a less expensive, easy to install, last longer, and are a more consistent, quiet, cost free means of exhaust. They also pose less draw on conditioned air.

    As for holes in ceilings, I would imagine the eave and gable vents equal at least 50 - 100 times the area as ceiling holes. The air exhausted through a more passive turbine fan would easily be replaced through eave vents, making ceiling hole loss negligible, IMO.

    Attic heat should be dealt with to maximize the affectivness of the AC.

    Having better sealed windows and doors will reduce this leakage effect with or without a PAV.

    Brian

  6. #32
    Join Date
    Mar 2008
    Location
    Baltimore MD and Ridgebury PA
    Posts
    542
    Quote Originally Posted by mark beiser View Post
    True, but when you are talking about air leaks through holes, the difference isn't that great. My intent was to point out that while there may not be a huge hole between the attic and conditioned space, if you add up all the small leaks, they equal a big hole.

    In the average house, built before it was common to seal the wiring holes for wire penetrations through the top plates of walls, it is not uncommon for those holes to represent half, or more, of the leakage area in the house.

    Adding a mechanical fan to that will exaggerate the already existing problem. It is usually rather impractical to go back in an existing home and seal all of those holes, and depending on the house design, you may not even be able to get to many of them.

    There are exceptions, but as a general rule, PAV's are a bad idea.
    I wish I had this information about three years ago when I installed a PAV in my attic to reduce the amount of heat up there thinking it would reduce my cooling mode. I guess I made a big hole in my roof for nothing... now I may end up adding a ridge vent to replace the PAV and simply disconnecting the PAV or setting the temperature control ridiculously high.

  7. #33
    Join Date
    Apr 2008
    Location
    Webster, MA USA
    Posts
    36
    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

  8. #34
    Join Date
    Aug 2007
    Location
    Atlanta, Ga
    Posts
    213
    All I can add is this:

    I have ridge vents and a PAV...last summer temps were as high as 105 here in Atlanta. When I took advice from several people on this forum and shut off my PAV, my attic temp only rose slightly and my 2nd floor, stayed more comfortable. I think that sums it up. Even without the PAV, at 105 outside temp, my attic was 115-120 which to me is not bad. Yes, I have a thermometer in the attic and it is accurate... Thanks to all on the forum for the good advice. I have to say, that EVERY ridge on my roofline has ridge vents in addition to 3 gable vents.

  9. #35
    Join Date
    Mar 2008
    Location
    Atlanta, GA
    Posts
    149

    You need soffit vents.

    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.
    Attached Images Attached Images  

  10. #36
    Join Date
    Apr 2008
    Location
    Webster, MA USA
    Posts
    36
    Quote Originally Posted by Joe Mascitti View Post
    All I can add is this:

    I have ridge vents and a PAV...last summer temps were as high as 105 here in Atlanta. When I took advice from several people on this forum and shut off my PAV, my attic temp only rose slightly and my 2nd floor, stayed more comfortable. I think that sums it up. Even without the PAV, at 105 outside temp, my attic was 115-120 which to me is not bad. Yes, I have a thermometer in the attic and it is accurate... Thanks to all on the forum for the good advice. I have to say, that EVERY ridge on my roofline has ridge vents in addition to 3 gable vents.
    Joe .. thanks for the reply.

    I certainly agree, there is a plethora of great info on this forum !

    I wouldn't think that your 10 or 15 degrees above ambient outdoor temp would be too terrible ? Ideally, the attic temp would like to be equal to outside ambient, but that probably doesn't happen often on a blazing hot 105 degree day ??

    You don't mention soffit vents, just ridge and gable vents, as the situation I am working with .. is that the case ?

    Thanks again for taking the time and interest to post,

    Russ

  11. #37
    Join Date
    Apr 2008
    Location
    Webster, MA USA
    Posts
    36
    Quote Originally Posted by Random Guy View Post
    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

  12. #38
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,338
    Quote Originally Posted by MiniBoy View Post
    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.

  13. #39
    Join Date
    Apr 2008
    Location
    Webster, MA USA
    Posts
    36
    Quote Originally Posted by shophound View Post
    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

Page 3 of 40 FirstFirst 1234567891013 ... LastLast

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
Comfortech Show Promo Image

Related Forums

Plumbing Talks | Contractor Magazine
Forums | Electrical Construction & Maintenance (EC&M) Magazine
Comfortech365 Virtual Event