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  1. #27
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    5,031

    Lower Heat-Gain & Down Size A/C Equipment

    Quote Originally Posted by will_t_greens View Post
    Update:
    An energy audit was performed last week by an HVAC/plumbing company that offers the service. After testing infiltration, duct leakage, and completing a Manual J, they provided some results.
    They showed approximately 7,000 BTU of loss due to duct leakage, but about half of that leakage was in the supply ducts that leak into a conditioned space in the basement.
    They believed the return leakage was drawing some hot air from the attic space past the top plates and drywall since no caps seal the return air vents in the walls. My level of infiltration rated my house as "Loose".
    My AC is also undersized since the Manual J showed a load of 38,000 BTUs. With a theoretical rating of 36,000 BTU minus 7,000 from duct leakage, my system is doing no more than 29,000, placing me at a deficit. Infrared imaging also showed substantial heat coming through an east facing bedroom that is always much hotter than other rooms.
    The suggestions were for me to close the leaks, do what I can to stop the heat gain in the one hot bedroom, and at least replace the unit with a newer, more efficient 3 ton coil/condenser, but was told my duct was adequate for 4 tons but would would need a higher CFM furnace. I told him I was already supplementing with a 9,000 BTU floor unit in the hottest bedroom, but he said he would be hesitant to go all the way to 4 tons.
    This is great, now that you know where the problem areas are, fix them.

    If it will help, you could to increase the insulation & save on energy costs.
    You don't need a higher CFM furnace, the idea is to downsize the A/C equipment as much as possible.
    Listen to beenthere! - Darrell

  2. #28
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,376
    Quote Originally Posted by beenthere View Post
    Improve your homes envolope first.
    Then duct system leaks.

    Then you won't need a larger more expensive to operate A/C.
    I second that.

    Getting your existing duct system with your existing a/c as tight as possible will net you a tonnage capacity increase with NO need to upsize the equipment, or even change out what you have now. Tightening up your building envelope will meet with the same result.

    Many builders fail to grasp how loosely they build a house. Yours is eight years old. Mine is almost fifty. This summer I could keep my house interior comfortably at 75 degrees, and the unit would occasionally cycle, with the outdoor temperature at 105. I had my attic insulation level increased, and spray-on radiant barrier installed. I sealed up ceiling penetrations and other areas of infiltration in the house. My system is 3.5 tons. If I can see this level of performance on a fifty year old house that still has the original single pane windows in place, I'm certain you can take steps to improve the performance of an eight year old house.

    Good move with the energy audit...wish more people would get that done and know more about where their energy dollars are going.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  3. #29
    Quote Originally Posted by shophound View Post
    I second that.

    Getting your existing duct system with your existing a/c as tight as possible will net you a tonnage capacity increase with NO need to upsize the equipment, or even change out what you have now. Tightening up your building envelope will meet with the same result.

    Many builders fail to grasp how loosely they build a house. Yours is eight years old. Mine is almost fifty. This summer I could keep my house interior comfortably at 75 degrees, and the unit would occasionally cycle, with the outdoor temperature at 105. I had my attic insulation level increased, and spray-on radiant barrier installed. I sealed up ceiling penetrations and other areas of infiltration in the house. My system is 3.5 tons. If I can see this level of performance on a fifty year old house that still has the original single pane windows in place, I'm certain you can take steps to improve the performance of an eight year old house.

    Good move with the energy audit...wish more people would get that done and know more about where their energy dollars are going.
    I just had an insulation consultant/installer out and he frowned on a radiant barrier in the attic, stating that if I require additional attic cooling (via a radiant barrier) to lower my heat load in the house, then my insulation levels are inadequate. His suggestion was to reduce the heat transfer through the insulation layer. He said there are studies that indicate the benefits of radiant material but other information showing that once heat has contacted the roofing material, most of the heat is changed from radiant to conductive and transferred through moisture in the air and heat build up in the framing. His plan is to add 8 inches of dense-pack cellulose on top of the existing fiberglass blow-in. He suggested that nothing can be done for the finished 500-600 sq. ft. of vaulted ceiling, but some of the heat gain could be countered by adding cellulose for added insulation and reduced infiltration in the remaining 900 sq. ft. of flat ceiling. His plan is also to add 8 inches of cellulose with adhesive on the vertical transition walls that exist between the flat and vaulted spaces as they are "notorious" for heat gain. He also stated that the additional insulation would result in at least a 15% reduction in cooling requirements based on his past experiences. The cost for the added insulation is far less than an upgraded, larger AC system, but the money is only well spent if his claims are true. Any input?

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