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  1. #1
    Join Date
    Jul 2005
    Posts
    4
    Hello-
    I hope this is not too off topic. I'm having a new Trane hvac unit installed to replace my current system. I've also always considered a whole house fan; seems that people like them and allows the hvac to run less by cooling the attic. I'm curious though, in an upflow or in-attic blower/furnace installation, it would seem 'easy' to install a simple automated damper in the main supply trunk line which would allow the hvac blower in Fan mode to run right out into the attic, thereby pulling cool outside air through the house, into the return, and out into the attic. Am I nuts? why don't people do this? (or do they?) Since wwf's do pretty much what hvac systems are already designed for, it seems logical....

    I did a search on whole house fans, but not much came up.

    (Disclaimer - clearly I am not a professional!! just looking for professional advice)

    Thanks-
    ed

  2. #2
    Join Date
    Sep 2002
    Location
    I don't know
    Posts
    2,903
    Air conditioners are also designed to remove humidity. How can you maintain a reasonable humidity level if you're pulling moist air into the house overnight when it's cooler outside?

    It takes at least a day of cooling to dehumidify a home properly - if the dewpoint is like 68 or 70 degrees, it's difficult to pull the air (passing over coil) temperature down by 15 - 20 degrees.

    Warning – this is not coming from a pro, so take it with a grain of salt.

  3. #3
    Join Date
    Apr 2003
    Location
    Northern Virginia, Fairfax County
    Posts
    641
    Another home owner here, non-pro. Maybe this would void the HVAC warranty???

    Aside from that, check the CFMs that whole house fans pull and compare that to the CFMs of the HVAC blower, there is no comparison.
    Al

  4. #4
    Join Date
    Mar 2005
    Posts
    997
    I should have thought of that! It could be done very easy! You will not be able to seal the damper off 100% and condensation would be a problem! Id go with 100% outside air through the return and have your damper's their. Then just open a window are two.

  5. #5
    Join Date
    Jun 2004
    Location
    4H: Hot, Humid Houston H.O.
    Posts
    3,304

    Why do we always omit location?

    I'm a homeowner too, in S.Texas. Where do you live? Your climate ought to make a major difference on whether you need to worry about humidity control.

    Whole house fans used to be fairly common in Texas but are getting out of fashion. And I can see why. Under the best of circumstances, you have created a pretty big hole between your house and attic, which needs to be sealed (as much as you reasonably can) when running the AC. And studies by FSEC (Florida Solar Energy Center, a very good info source!) tell us that a certain amount of moisture is absorbed into carpet, curtains, etc. which must be removed by the AC during the day.

    Are you in a climate with enough humidity to even worry about?

    Hope this helps -- P.Student

  6. #6
    Join Date
    Mar 2005
    Posts
    997
    em The humidity would be a problem like the rest have said! But I would think you would be using this in spring and fall when RH is not as much a concern (unless your on the cost)

  7. #7
    Join Date
    Jun 2004
    Location
    4H: Hot, Humid Houston H.O.
    Posts
    3,304

    Say what?

    >>you would be using this in spring and fall when RH is not as much a concern

    Do you really think RH is not a problem in spring and fall? Because in my climate that is when the problem is greatest. In the summer the AC works a lot at humidity removal and the results are OK. In the winter the air contains so little moisture, that when warmed to room temperature the RH tends to be low. It's in the mild months, especially when rainy, that AC is not called for cooling and RH gets really up there.

    Where are you from anyway? Maybe your statement makes sense given a certain climate.

    Regards -- P.Student

  8. #8
    Join Date
    Mar 2005
    Posts
    997
    VA not a big problem during that time of year! Might have a few days with high RH but not to long to cause a problem.

  9. #9
    Join Date
    Jul 2005
    Posts
    4
    Thanks for the feedback. I am in central california; humidity is fairly low. This does give me some things to think about though. I appreciate the sanity check!
    -ed

  10. #10
    Join Date
    Jul 2004
    Posts
    322
    In theory this might work on say a cool evening in the spring and fall. You could run the blower by putting the fan setting to ON, open a big damper on the supply in the attic and open some windows. This would create a partial vacuum in the house and pull air through the windows, thus cooling down the house. But I don't know if it would cool the attic much. A properly vented attic usually has slight negative pressure, due to a combination of the stack effect and Bernoulli effect which cause air to vent out the top through roof vents, and draw cooler air through the soffit vents. Pumping air into the attic would tend to create a high pressure zone in the attic and might not accomplish much.

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