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  1. #27
    Join Date
    Jul 2004
    Posts
    253
    Originally posted by whos on call
    Tuccillo
    I am not assuming. Everything in nature always attempts to take the easiest route in order to accomplish it's task.
    In the case of replacing exhausted air, the air will be replaced by air that is closest in proximity, and in this case it will come from the nearest vent opening. Like I said , this is not my opinion, but it is a fact. Your not going to get negative pressure in a room that has equal inlet and outlet pressures, nor is the space under the attic going to try to compensate for exhausted air that is being replaced on a countinuous basis, by inlet air that is readily available close by.
    Sorry sport, the attic will always have negative pressure with the PV running otherwise you could not have a pressure gradient that forces air movement. The pressure gradient is a 3 dimensional field that will vary throughout the attic. While the mass flux entering and leaving the attic may be the same, this only happens with a pressure gradient. You are making assumptions about the geometry of the attic and where air will come from. Without modeling it, you dont know where is coming from. I dont believe you know more than the people who study this stuff for a living. You are are in over your head - I suggest you give it up.

  2. #28
    Join Date
    Jun 2005
    Posts
    37
    Well I was doing some research and the vents I have in the attic are suppose to vent a 104 sq feet of attic space.

    There are only three of these vents. So 3 times 104 is 312 sq feet of attic. The attic itself with the attatched garage is like 1800 sq feet. Look's like me an the builder are going to get into it again!!

  3. #29
    Join Date
    Aug 2003
    Posts
    626
    Smp7015--Venting codes in attics are to prevent ice dams. In the 70's people started turning this into the "get the hot air out" reason for having vents. If you are concerned about cooling your house properly, the most important areas of consideration are; leaky ducts-air handlers if in the attic, holes between the attic and rooms below, and the insulation levels. Those are things that will save much more energy than a slight decrease in attic temps. Not only that but they will save in cold weather also.

  4. #30
    Join Date
    Jun 2005
    Posts
    4

    how did this get nasty?

    tuccillo
    o.k thats all I wanted to hear. I couldn't care less about being wrong about the absence of negative pressure "Sport". I did notice, however subtle you were trying to be, that you admit that you are not going to get makeup air from the living space when you already have enough in the attic. I do not know who you think you are talking to, but I am not the type to ball my ass off when I am wrong about something, however you on the other hand sound like many others I've encountered before. Here's a tissue.

  5. #31
    Join Date
    Jun 2004
    Location
    Dallas
    Posts
    179
    Uktra- I gave you the last laugh on the PAV- they do draw too much from the house.

    BUT- I find it hard to believe that you would say lower attic temps are not a good thing no matter how you get them. PAV, radiant, sprayfoam, whatever.

    To the original poster- I will answer a very confident and qualified:

    NO - you do NOT have enough venting for your attic. PAV is the riskiest method and uktra will probably find out where you live in break it in the night.

    You need more ventilation- period. get it however you can.

    AC units are NOT designed to be run in attics that are rediculously hot. Yes, they can do it, but they are NOT made to do it with any kind of efficiency.

    Tucillo has, BY FAR, the best setup with the sealed attic and spray foam, but I am afraid it is too late for you.

    I am not saying PAV is the way to go- you should at least look into replacing two of those vents with whilry birds and having the third covered up- yes, covered up- it will do more harm than good if you don't. Also make sure you have lots of UNBLOCKED soffit vents. Check them- the builder may have skimped. The are often blocked with insulation.

  6. #32
    Join Date
    Aug 2003
    Posts
    626
    Old Yeller--I never said lowering attic temps is not a good thing. What I have said time after time is that spending money to lower temps in the attic to try to lower cooling bills or increase comfort is NOT the best return on investment due to the fact that hot air doesn't tranfer as much heat to the attic floor through the insulation as the radiant transfer. What is a better return on investment is to seal ducts and the air handler (you can lose 30% of the cooling power with leaky ducts). Sealing the holes between the attic and rooms below (this can be a do it your self project with a high cost/benefit ratio). and last make sure there is enough insulation. These things pay back both summer and winter.

  7. #33
    Join Date
    Jun 2004
    Location
    Dallas
    Posts
    179
    I completely agree with that.

    If I were to seal everything- walls, ducts, etc.. and my AC still works too hard or can't handle the job, I should then look into taking steps too bring attic temps down- that heat WILL find its way to the ceiling through the insulation- no matter how thick. OFten the excessively thicker insulation will actually hold MORE heat in the summer- even if it is beneficial in winter. That is proven.

    I admit you didn't SAY that- you seemed to make the point by discounting any arguments for lower attic temperatures.

    At SOME point ludicrous attic temperatures WILL have an effect on heat load.


  8. #34
    Join Date
    Jul 2004
    Posts
    253

    Re: how did this get nasty?

    Originally posted by whos on call
    tuccillo
    o.k thats all I wanted to hear. I couldn't care less about being wrong about the absence of negative pressure "Sport". I did notice, however subtle you were trying to be, that you admit that you are not going to get makeup air from the living space when you already have enough in the attic. I do not know who you think you are talking to, but I am not the type to ball my ass off when I am wrong about something, however you on the other hand sound like many others I've encountered before. Here's a tissue.
    I am not trying to be subtle. You dont know where the air is coming from and you dont have a clue about the physics of the situation. Since you dont know what you are talking about I suggest you shutup. Is that nasty enough for you, sport?

  9. #35
    Join Date
    Jun 2004
    Location
    Dallas
    Posts
    179
    Hows about you two take this outside to the parking lot...

  10. #36
    Join Date
    Aug 2003
    Posts
    626
    Old yeller--insulation does't hold heat. Insulation slows heat tramsfer by making heat change its means of transfer. hot air transfers heat to the rooms below by two methods through insulation. conduction and convection. conduction is where the hot air meets the top of the insulation surface. convection is where the hot air blows through or around the insulation. Many times there is more heat transfer from convection than conduction. That is why sealing all the holes is important and cost effective if you do it yourself. Just sealing the holes (beneficial in both summer and winter) will save more energy than any pav.

  11. #37
    Join Date
    Sep 2001
    Location
    Vancouver Canada
    Posts
    996
    How about a power venter in the roof and a matched fan supplying outside air into the attic? Makes sense to me. I am not too concerned with saving money on this guys power bill but I am concerned with seeing his A/C unit achieve the proper level of temperature he is looking for in his home. Now I suppose I am going to be told that putting a dedicated fan to introduce outdoor air to the attic will only add warm outdoor air into the house via infiltration. I would think some good gable vents and a couple of decent whirly birds would also do the trick. Cross ventilation is the key. If you put a pile of roof vents in there they are only going to vent as much air as they can pull into the attic at a lower point do to the nature of convection a(hot air rises).
    "Go big or Go Home"

  12. #38
    Join Date
    Aug 2003
    Posts
    626
    Black Adder--what you suggest would lower the amount of conditioned air coming from the rooms below. The point I am trying to make is that the best things to do to lower the load and let his unit reach set point are the three things i suggested before.

    [Edited by uktra on 07-02-2005 at 11:01 AM]

  13. #39
    Join Date
    Apr 2004
    Posts
    7,680
    OK, IM lost....

    How hot is your attic on say an 85 or 90 degree day? So far we dont know that, at least I dont. If it is above 120 degrees, then you got an attic issue. You also need to know what kind of temperautre rise your ducts are experiencing from start to end. You can run your fan only and read return temp and supply air temp at the grilles. This is not always a true measure though. Then you need to run the machine (increases temperature difference and read your supply air at the plenum and the register as well as the return plenum and return air grille. Add the two differences. You can actually figure the lost btuhs if you know the actual CFM.

    I dont have any holes in my ceiling and my ducts are sealed and insulated so an attic fan will simply not pull from the house, especially if you have ample opportunity for it to come in either through soffits or gables. I'm afraid the air would take the path of least resistnace. It would be a good idea not to have the fan close to the gable ends or the work is usless. The greater the temperature differecne, the faster and greater the heat transfer. So if you ceiling is hot you will absorb heat into the room (hot goes to cold). If you can keep your ceiling cooler by not exposing it to such extremes the heat transfer amount and rate are much less. This also goes for the ductwork. It needs to be well sealed and insulated to slow the transfer of heat..

    As far as pressure differences go, yes ther has to be a difference or air will not move. However, it does not need to be great provided you have plenty of opening. A leaky return is a perfect example. The atmospheric pressure around the return is much higher than the return's negative pressure.

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