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  1. #79
    Join Date
    Jan 2007
    Location
    Charm City--the city that bleeds
    Posts
    2,779
    Quote Originally Posted by platchford View Post
    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.
    Depends on your house. I had a two-story rowhouse in 21228, and the PAV made a significant temperature difference on the top floor. The AH is in the bsmt, so very few holes are available for air infiltration from 2nd floor to attic.
    We lived without a PAV for a couple years there, and noticed a difference in temp immediately after install (the very next day, hot summer day).
    But I suppose if you had a different style house such as a ranch it might not be as cost-effective, or even make a sensible temp difference.
    It's great to be alive and pumping oxygen!

  2. #80
    Join Date
    Mar 2008
    Location
    Jackson, NJ
    Posts
    176
    Quote Originally Posted by The Doctor View Post
    Depends on your house. I had a two-story rowhouse in 21228, and the PAV made a significant temperature difference on the top floor. The AH is in the bsmt, so very few holes are available for air infiltration from 2nd floor to attic.
    We lived without a PAV for a couple years there, and noticed a difference in temp immediately after install (the very next day, hot summer day).
    But I suppose if you had a different style house such as a ranch it might not be as cost-effective, or even make a sensible temp difference.
    I have a ridge vent, soffits, and 2 gables. I had the builder install the attic fan thinking it would help. This thread is pretty much saying it was a wasted effort, however like yourself my air handler is in the basement (and I have a two floor house). The only attic penetration I have is for the light fixture boxes (1 in each bedroom) and 2 bathroom exhaust fans. I know the builder incorporates something he calls 'Super Seal' Package, which is basically foaming all the holes he makes. I still wonder if my PAV is going to be worthless or whether I might gain something from it yet.

  3. #81
    Join Date
    Mar 2008
    Location
    Long Beach, CA
    Posts
    3,147
    Quote Originally Posted by The Doctor View Post
    But I suppose if you had a different style house such as a ranch it might not be as cost-effective, or even make a sensible temp difference.
    It seems if a PAV cools the upper floor of a two story house it would have the same positive affect on a ranch. Moreover, a ranch attic covers the whole house and will affect the whole house more directly.

    If the slight suction of a properly vented PAV can draw water heater gasses back into the house, so could a bath and kitchen fan in that house. Testing that house would be simple.

    More people here seem to have had positive results from PAVs than not. Those who object to them seem to base there conclusions mainly on theory.

    Slow and consistent removal of overheated air in the attic is agreed to be a good thing.

    The only apparent objection here to the PAV is the rate of flow. So it seems the most obvious answer would be to use a properly vented lower cfm PAV .

    Brian

  4. #82
    Join Date
    Feb 2008
    Location
    Tennessee
    Posts
    91
    Quote Originally Posted by Brian GC View Post
    More people here seem to have had positive results from PAVs than not. Those who object to them seem to base there conclusions mainly on theory. Brian
    Theory???

    I guess people choose to believe what they want. If you go up in your attic and it feels cooler and you believe your PAV is saving you money then I guess you can be happy.

    I started this thread to try to get to the truth and from all that I have read here tells me that a PAV is not going to do what I want it to do, save me money on my cooling bill every month. When someone post a thread with a link to a study someone has done on the subject and the findings are the same as most people here say then I'm a believer in the majority. I don't see a theory. sorry

  5. #83
    Join Date
    Feb 2008
    Location
    Dallas & Longview, TX
    Posts
    629
    Quote Originally Posted by printmanjackson View Post
    Theory???

    ...... When someone post a thread with a link to a study someone has done on the subject and the findings are the same as most people here say then I'm a believer in the majority. I don't see a theory. sorry
    I also posted a follow-up to the study that said the study involved a virtualy open ceiling. It was a drop ceiling with no containment for the conditioned air. This is not to say there may not be another study that is also proving that the PAV is bad.

    I think you said you had ridge vents and I have already stated that the use of both the ridge vents and PAV is not productive. So agreed that in your case the PAV is a negative thing.

    I'm still not sold that the PAV in a proper setup isn't good. Just my 2 cents.

  6. #84
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,088
    I would also amend my postings here to state that if the ceiling between the attic and the house interior is well sealed, the counterproductive aspects of a PAV are reduced/minimized.

    The question then becomes whether running a PAV offsets the reduction in heat load to the a/c. It could be a wash...in theory!

    For Brian...those of us who've offered our cautions regarding PAV's have done so stating reasons based on observation and understanding of heat transfer, psychrometrics, pressure differential, etc. It is true the use of such principles could be misunderstood and/or misapplied, and I will respect the point made that not all houses are the same. Mark and I are perhaps a bit influenced by the common housing stock in our area, which typically do not favor PAV installations due to construction quality. For other regions of the nation, a PAV might not be as detrimental, in others, far worse. Perhaps the gist of this thread is whether PAV's are a blanket solution for overheated attics, to which I would answer no.

    Reducing initial heat gain to an attic is the most intelligent approach. Exhausting superheated air from an attic that could've been mitigated initially by either radiant barrier or an insulated roofdeck is far smarter. Years ago we did not have RB widely available for homeowner use. PAV's were offered as a solution. Now we have RB...if you're building a new home you're crazy not to use the roof deck with the RB already applied to the deck underside. It WORKS. The spray-on retrofit is not as effective but it is far better than leaving the decking untreated.

    In conclusion:
    • Install an appropriate radiant barrier for the roof deck
    • Install sufficient soffit and ridge venting
    • Seal all penetrations through the ceiling and into the attic
    • Increase attic floor insulation to recommended level for the region, if needed
    • Repair any leaking ducts that are in attic
    • Enjoy a passive, low maintenance system that will work "off grid" to help keep your energy costs down and your home more comfortable, year round
    • If your heart is still set on a PAV, do all of the above and replace that electric PAV with a solar powered job

  7. #85
    Join Date
    Jun 2001
    Location
    DFW
    Posts
    683
    To start off with, follow Shophounds advice - all of it is exactly right.

    For all of you that think an attic fan is a good idea, consider this.
    Cooling an attic with an attic fan is about the same as trying to cool an oven with a muffin fan - while the oven is on. It just isn't going to happen.

    Consider an attic with 500 sq ft of south facing roof area. At 1 pm, that section of roof is gaining about 20Kw of solar heat or a about 70,000 btu/hour.

    It is not practical to pull enough air through the attic to cool the roof which is being directly heated by the sun. Most of the heat is radiated back towards the open sky. Some is radiated towards the attic insulation, or anything else that is in the attic, until they reach the same temperature as the decking.

  8. #86
    Join Date
    Feb 2008
    Location
    Dallas & Longview, TX
    Posts
    629
    Quote Originally Posted by paul42 View Post
    To start off with, follow Shophounds advice - all of it is exactly right.

    For all of you that think an attic fan is a good idea, consider this.
    Cooling an attic with an attic fan is about the same as trying to cool an oven with a muffin fan - while the oven is on. It just isn't going to happen.

    Consider an attic with 500 sq ft of south facing roof area. At 1 pm, that section of roof is gaining about 20Kw of solar heat or a about 70,000 btu/hour.

    It is not practical to pull enough air through the attic to cool the roof which is being directly heated by the sun. Most of the heat is radiated back towards the open sky. Some is radiated towards the attic insulation, or anything else that is in the attic, until they reach the same temperature as the decking.
    What? The point of attic ventilation is to decrease the temp of the attic air below the temp of the decking. The power assisted fan will most definitely remove more of the heat. The concern is whether the vacuum created will pull conditioned air from the house or exhaust from a gas appliance in the attic. See what cools faster- opening the oven you mentioned a crack after it reaches a certain temp. and wait 10 minutes. Then power it back to the same temp and this time crack it open and hold a fan pulling air out of the top for 10 minutes. Think the passive cooling is faster than the power version?

    I agree that Shophound's advice is good.

  9. #87
    Join Date
    Mar 2008
    Location
    Baltimore MD and Ridgebury PA
    Posts
    542
    Quote Originally Posted by The Doctor View Post
    Depends on your house. I had a two-story rowhouse in 21228, and the PAV made a significant temperature difference on the top floor. The AH is in the bsmt, so very few holes are available for air infiltration from 2nd floor to attic.
    We lived without a PAV for a couple years there, and noticed a difference in temp immediately after install (the very next day, hot summer day).
    But I suppose if you had a different style house such as a ranch it might not be as cost-effective, or even make a sensible temp difference.
    The Doctor, I don't think Jopopsy realized your comment was specifically directed at me... zip code is a big clue for me. Anyway, my home is a two-store single family home in 21227 (funny I lived in a two-story "townhouse" in 21228 before I bought this home) that we gutted after purchase. The attic 'floor' is insulated with spray foam. While the PAV may not suck conditioned air out of my house (although it does to some extent at the attic pull-down stairs) it runs just about all summer long drawing energy so is it really saving me any money?

  10. #88
    Join Date
    Apr 2008
    Posts
    10

    PAV worked for us

    When we bought our 1942 house in 1993 in the Wash. DC area, it had no attic fan, just two gable vents. The previous owners had ceiling fans in the bedrooms on the 2nd floor, but we couldn't stand them and all they did was move the air around anyway. The plant was single zone with air handler in the basement. So I boosted the attic insulation and put a PAV at one of the gable vents. This virtually eliminated the temperature differential in summer on the 2nd floor.

    Recently we replaced our original slate roof with composite (sad but slate was just toooo expensive) and we had a ridge vent put in. Unfortunately, the roof design precludes adding soffit vents, so the air has to come in from the gable vents. I haven't yet been able to figure out if the ridge vents have eliminated the benefit from the PAV (for reasons described in another thread) but I hope to be able to figure that out this summer.

  11. #89
    Join Date
    Apr 2008
    Location
    Spokane
    Posts
    16
    My experience with attic fans is good: When I first bought my house (single level slab on grade house with 30 soffit vents) we had a window mounted 10K BTU air conditioner, the July temps were near 100 every day for weeks. We were using the air conditioner for 14 - 16 hours a day. I then installed a 1350 cfm attic fan with a t-stat, gable-mounted, and the air conditioner use dropped immediately to 3 - 5 hours a day. The attic temp dropped about 50 degrees. Great results.

  12. #90
    Join Date
    Feb 2008
    Location
    Tennessee
    Posts
    91
    Quote Originally Posted by Znobyrd View Post
    My experience with attic fans is good: When I first bought my house (single level slab on grade house with 30 soffit vents) we had a window mounted 10K BTU air conditioner, the July temps were near 100 every day for weeks. We were using the air conditioner for 14 - 16 hours a day. I then installed a 1350 cfm attic fan with a t-stat, gable-mounted, and the air conditioner use dropped immediately to 3 - 5 hours a day. The attic temp dropped about 50 degrees. Great results.
    maybe a gable mounted PAV works better because it pulls across instead of up?

  13. #91
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,070
    Quote:
    Originally Posted by paul42
    To start off with, follow Shophounds advice - all of it is exactly right.

    For all of you that think an attic fan is a good idea, consider this.
    Cooling an attic with an attic fan is about the same as trying to cool an oven with a muffin fan - while the oven is on. It just isn't going to happen.

    Consider an attic with 500 sq ft of south facing roof area. At 1 pm, that section of roof is gaining about 20Kw of solar heat or a about 70,000 btu/hour.

    It is not practical to pull enough air through the attic to cool the roof which is being directly heated by the sun. Most of the heat is radiated back towards the open sky. Some is radiated towards the attic insulation, or anything else that is in the attic, until they reach the same temperature as the decking.

    What? The point of attic ventilation is to decrease the temp of the attic air below the temp of the

    Do the calcs! Removing 70,000 btus/hour to maintain <110^F requires 4,000 cfm of air flow! The fan power is more than the a/c and the attic is still 110^F. I may want to continue selling attic fans because I make money doing it. OK- now I understand. Regards TB
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

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