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  1. #14
    Join Date
    Jun 2004
    Location
    4H: Hot, Humid Houston H.O.
    Posts
    3,304
    Quote Originally Posted by cokoliso View Post
    I do not see any insulation on the foundation on the outside of my house. In fact, there are sidewalks against the foundation all the way around the house.

    Inside (in the garage), I see about a half inch "joint" between the floor slab and the footer for the wall. It appears that the footers for the walls were poured and then the slab was poured with this expansion joint material against the inside of the footer.

    My ducts are under the slab, by the way. I don't know whether that's good or bad, but there is no way to do much about it. Can ducts under the slab leak air?
    If you used Hvac-Calc Manual J7 and it told you 2 tons, you have done a very good thing but I would not accept that 2.0 ton sizing until verified by some other analysis like ACCA Manual S. I'm a homeowner in S.Texas and have used Hvac-Calc, in my opinion its BTUH rating is trustworthy but it fails to account for derating the AC at your summer design conditions.

    Your pro who did a calc and estimated 3.0 tons may actually be agreeing with you, I would show him my Hvac-calc numbers and ask him to discuss it with you. It would not hurt to post the actual sensible and latent BTUH numbers you got from the Hvac-Calc model. Pros on this board have been very helpful in interpreting these numbers, so you can understand what they really mean.

    One other issue to worry about, is matching up the ductwork with the AC airflow required. That will be less of a problem if you go with the smaller AC. Your existing duct system has the capacity only for a certain amount of airflow, I hope it is enough for your actual BTUH needs. Asking a pro is the only responsible way to go, but I would also tell him I want to receive a measurement of ESP (external static pressure, I think you can call it back-pressure) in writing, to see how close you are to the limits of your air handler. This is only my opinion but cannot see anything wrong with getting this information.

    Hope this helps -- Pstu

  2. #15
    Join Date
    Nov 2007
    Location
    Rio Rancho, NM
    Posts
    14
    Using HVAC Calc's standard design conditions for my location, sensible heat gain comes out to be 19,075 and latent is 460 BTUH. I understand that air flow is adjusted to achieve the heat rise for the furnace, and to 400-450 CFM per ton for cooling. So the ducts need to handle whichever would be higher.

    I wonder if the old blower shows the CFM rating on a label.

  3. #16
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,350
    Quote Originally Posted by cokoliso View Post
    My ducts are under the slab, by the way. I don't know whether that's good or bad, but there is no way to do much about it. Can ducts under the slab leak air?
    Ducts in the slab are OK...if you never need to alter them, if they never get water in them, if they aren't full of construction debris. I do not imagine they leak much air if they are surrounded by concrete and dirt...depends on what material the duct is made with and how well it's put together. One thing for certain, in summer you have NO heat gain from your ducts, which for the typical attic duct run scenarios, actually robs cooling capacity from the system, maybe a ton or more sensible capacity.

    If the ducts in slab are not well insulated, you may be losing a little heat to the ground and slab when the furnace runs. Probably not a significant amount, but it's a fact regardless.

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