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  1. #66
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    Feb 2008
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    Dallas & Longview, TX
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    629
    Quote Originally Posted by Joe Mascitti View Post
    I would bet the farm that the contractors have no info on how many soffet vents to put, nor the size of them. They just slap a 4X12 every 4 feet down the soffet line and that's it. There's no way it is matched to the PAV, and therefore in 99% of the cases, the PAV creates a negative pressure effect in the attic and that draws conditioned air thru the attic stair case, can lights, cracks, etc. thru the attic and inside while pulling in hot air from the outside thru drafty doors, windows and fireplace vents.
    I bet that that's why the PAV's get a bad rap. Installed properly, would they match/exceed the benifit of ridge venting? That's what I want to know.

  2. #67
    Join Date
    Feb 2008
    Location
    Tennessee
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    91
    I don't think it matters too much how big the attic is. What happens is with just one PAV there is not enough soffit vent to feed the fan therefore it pulls conditioned air from the house.

    Has anyone heard about PAV's starting house fires? Talking to a guy here working for the local utility company and says they can start a fire pretty easy. What happens is the home owner oils the fan motor (uses plenty of oil I'm sure) then later it mixes with dust and this turns into the fuel for a fire. Then all it takes is a super heated motor and a spark. He says they are usually fine until their oiled.

  3. #68
    Join Date
    May 2007
    Location
    Naples, Fl
    Posts
    889

    Try the FSEC for data and studies on PAV

    .3 Disabling Attic Exhaust Fan

    The third retrofit involved turning off the attic exhaust fan. Average cooling energy consumption (based on an eight-month cooling season) decreased by an additional 36%. The 14.3 kWh/day saved breaks down to 11.2 kWh/day from the exhaust fan motor and 3.1 kWh/day from a reduction in air conditioning energy. Savings over an eight month cooling season are projected to be 3208 kWh or $289. Assuming a service call cost of $50 for turning off the attic fan, the simple payback would be after two months. Peak electric demand (including attic fan power of 0.47 kW) was reduced by 21% from 3.4 kW to 2.7 kW.

    Impacts of Disabling Attic Fan on Ventilation, Humidity, and Building Pressure
    Turning off the attic fan produced dramatic impacts upon the office. The large driving force (pressure differential) causing air conditioned air to be sucked out of the occupied space and hot humid air to be drawn into the occupied space is gone when the fan is off. Table 3 is located in the Conclusions section of this paper and summarizes measured ventilation rates, relative humidity, and building pressures during initial, duct repair, and attic fan off monitoring periods. The natural air change rate (all mechanical equipment off) is not shown in Table 3, but was measured on a couple testing days and averaged 0.17 ach.

    Building pressure decreased from -0.064 in.WC (-15.9 Pa) to -0.0024 in.WC (-0.6 Pa). This resulted in a large drop in the ventilation rate and the indoor relative humidity. The 24 hour average relative humidity levels plummeted from 77% to 61%. Relative humidity levels are shown in Figure 4 during a composite 24 hour day consisting of one business week of data. Each hour represents the preceding hour of gathered data. For example, hour 2 represents data gathered beginning at hour 1 and ending at hour 2. Non-business hours from 12 AM to 7 AM show the humidity drop from an average of 83% when the fan is on, to an average of 65% when the fan was off. During business hours from 11 AM to 5 PM, the relative humidity drops from an average of 68% when the fan is on to an average of 55% when the fan is off. The building ventilation rate decreased from 0.79 air changes per hour (ach) to 0.33 ach. Due to diminished ventilation, the peak carbon dioxide concentration increased from an average 614 ppm to 1054 ppm during weekday hours of 3 to 5 p.m. Figure 5 shows carbon dioxide concentrations during a composite 24 hour day consisting of one business week of data.


    Figure 4 Daily composite of indoor relative humidity before and after attic fan was turned off.

    Figure 5 Daily composite of carbon dioxide levels before and after attic fan was turned off.

    Measured ventilation rates and carbon dioxide concentrations indicate that this office needs additional ventilation after duct repair and with the attic fan off to be in accordance with ASHRAE 62-1989. Typical occupancy during normal business hours is eight adults. According to ASHRAE 62-1989, the desired amount of ventilation air would be 20 cfm (9.4 l/s) per person which totals 160 cfm (76 l/s) for eight people. Infiltration testing by means of tracer gas decay indicated a total of only about 58 cfm (27 l/s) or just over 7 cfm (3.3 l/s) per person. A suggestion was made to the building manager to contact a qualified air conditioning and ventilation contractor to increase the ventilation rate, but no changes were desired to be made by the business during the time that monitoring was conducted. It was not desirable to turn on the attic fan because it increased the relative humidity significantly. A discussion of how the ventilation could have been provided follows.

    http://www.fsec.ucf.edu/en/publicati...sec-pf-406-98/

    --------------------------------------------------------------------------------

  4. #69
    Join Date
    Mar 2008
    Location
    Jackson, NJ
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    176
    So for those of us with Ridge Vents, Eaves, Gables, and a PAV - can we simply not turn it on in order to realize the value of the roof's venting? I mean, if I just leave it off and I doing myself any justice? I really don't want to go banging plywood on the underside of the roof ....

  5. #70
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    May 2007
    Location
    Naples, Fl
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    889
    From my point of view yes leave it off.

  6. #71
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    Feb 2008
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    Dallas & Longview, TX
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    Quote Originally Posted by adrianf View Post
    From my point of view yes leave it off.
    If you have a suspended ceiling with no deck of any type between the conditioned space and the attic then yes.

    "With only duct repair completed, the building continued to be depressurized by the attic exhaust fan 24 hours a day. The fan provided a source of pressure differential across the ceiling, and the suspended panel ceiling in this office was not a good air barrier, resulting in airflow from the conditioned space into the attic."


    What about a study that shows a regular house with a sheet rock ceiling? I know the ceiling must be tight along with the ductwork.

    I'm still leaving my PAV on until I see more data.

  7. #72
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    Feb 2008
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    Dallas & Longview, TX
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    Quote Originally Posted by Jopopsy View Post
    So for those of us with Ridge Vents, Eaves, Gables, and a PAV - can we simply not turn it on in order to realize the value of the roof's venting? I mean, if I just leave it off and I doing myself any justice? I really don't want to go banging plywood on the underside of the roof ....
    If you have ridge vents then you are pulling heated air back through the ridge vents if you run the PAV. I'd turn off the PAV and use the ridge vent to passivly remove the heat.

  8. #73
    Join Date
    Mar 2008
    Location
    Baltimore MD and Ridgebury PA
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    542
    Quote Originally Posted by Daltex View Post
    Much preferred? Where do you source this from? I would suggest that without independent verification that the small amount of heat extraction from a ridge vent verses an air turnover rate much greater with a PAV that the PAV "could" be more efficient because it does remove more heat from the attic. You aren't saying an attic 20* hotter is not an issue are you? Are you saying that a PAV doesn't remove more attic heat?

    Just looking for documentation. Not saying you are wrong.
    Quote Originally Posted by Daltex View Post
    The PAV's need to be chosen like an AC/Heating system. You must calculate the flow rates and have an adequate amount of soffit/gable incoming air to match it. 1000-3000 cfm is pretty passive regarding the entire volume of an attic. What make it problematic is when you don't have adequate intake and you create to much negative pressure.

    Still want documentation on properly designed PAV vs. Ridge Vent.
    As you pointed out in your follow up post, it is possible a PAV could be more efficient that a ridge vent if the system was properly designed and sealed. However, given that builders and homeowners generally pay even less attention to proper attic ventilation (number and size of vents etc) than they do to a properly designed heating/cooling system... and we all know how much attention is generally given to the heating/cooling system... a PAV is, in general, not going to help. Other than solid insulations (such as spray-foam) most insulations (including fiberglass, blown, etc) have almost no R value when air infiltrated and, in general, a PAV is going to cause more air infiltration than a ridge vent. Ultimately, we would have to quit theorizing and actually study a properly designed ridge vent in a properly sealed home and compare it to a properly designed PAV in a properly sealed home... unfortunately, to the best of our knowledge, a properly designed PAV in a properly sealed home is a myth.
    Last edited by platchford; 04-08-2008 at 05:15 PM.

  9. #74
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    Feb 2008
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    Dallas & Longview, TX
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    Agreed. The concern is that the sheet rock isn't going to infiltrate, just the penetrations and ductwork. Spray foam, of course would solve this, but the foam is applied to the decking thus eliminating the venting all together. I guess the conclusion is that unless you have eliminated all points of leakage then go with passive venting. If all is sealed tight and your soffit vents are proper then you should in my opinion (subject to change) go with power vents.

  10. #75
    Join Date
    Aug 2004
    Location
    North Richland Hills, Texas
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    And if you are in an area like mine here in the DFW area of Texas, where water heaters and 80% furnaces are typically located in, or draw their combustion air from, the attic, you should never under any circumstances even contemplate the idea of installing a PAV.
    If more government is the answer, then it's a really stupid question.

  11. #76
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    Feb 2008
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    Dallas & Longview, TX
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    Quote Originally Posted by mark beiser View Post
    And if you are in an area like mine here in the DFW area of Texas, where water heaters and 80% furnaces are typically located in, or draw their combustion air from, the attic, you should never under any circumstances even contemplate the idea of installing a PAV.
    Because it would/could draw from the flue? Didn't think about that one.
    I would think the draft would be way to strong to be disturbed by a PAV unless there was little to no intake venting (soffits) at all? The furnaces wouldn't be running at the same time as the PAV but the water heater would.
    The updraft on a water heater would be much weaker than the furnace also. Is there anyway to test this in the summer such as using a cumbustion analyser or something?

  12. #77
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    Mar 2008
    Location
    Baltimore MD and Ridgebury PA
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    542
    Quote Originally Posted by Daltex View Post
    Agreed. The concern is that the sheet rock isn't going to infiltrate, just the penetrations and ductwork. Spray foam, of course would solve this, but the foam is applied to the decking thus eliminating the venting all together. I guess the conclusion is that unless you have eliminated all points of leakage then go with passive venting. If all is sealed tight and your soffit vents are proper then you should in my opinion (subject to change) go with power vents.
    Not always... mine was applied to the attic floor... hence I still need venting but I am sealed extremely well. In hindsight I wish I had the decking sprayed... no venting required usually then.

  13. #78
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    Aug 2004
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    North Richland Hills, Texas
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    Quote Originally Posted by Daltex View Post
    Because it would/could draw from the flue? Didn't think about that one.
    I would think the draft would be way to strong to be disturbed by a PAV unless there was little to no intake venting (soffits) at all? The furnaces wouldn't be running at the same time as the PAV but the water heater would.
    The updraft on a water heater would be much weaker than the furnace also. Is there anyway to test this in the summer such as using a cumbustion analyser or something?
    It doesn't take much of a negative pressure to back draft a water heater, or atmospherically vented furnace.
    Even if you have lots of soffit vents, what happens if the screens get clogged with dust?

    The minimal potential benefit of a PAV, assuming the attic is very well sealed off from the conditioned space, and wiring penetrations through the top plates of walls are sealed, isn't worth the risk.
    Good passive ventilation, and in climates like mine, a radiant barrier product on the under side of the roof works best, has no operating cost, and requires little to no maintenance.
    If more government is the answer, then it's a really stupid question.

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