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  1. #1

    Manual J Calc. Question/Decision Confirmation

    Hey All,
    Here is a summary of my contractor's Manual J calculation--he had thought 2 ton, which he is sticking by, because we are only conditioning the top floor of a raised ranch, and he feels the extra half ton will allow cool air to drop without short cycling. Do y'all agree? Some basic house info is below. If so, we'll be going with the Trane XB13 (same warranty as XR, but a bit louder--not an issue based on placement) and TAM7 (better dehumidification, quieter, bumps to 14 SEER).

    Gain BTU Loss BTU Gain CMF Loss CFM Base Board Tonnage
    17731 30320 592 574 56 1.5

    Home is in Southern NH
    2x4 walls
    20" blown-in insulation
    All new windows, doors, siding with 1/2" foam insulation
    Raised ranch, 46 x 26
    Basement half underground

    Anything else needed to confirm our decision, please let me know. Thoughts welcome!

    Thanks,

    S

  2. #2
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,614
    Sounds good to me.

  3. #3
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,646
    Quote Originally Posted by thinkxingu View Post
    Hey All,
    Here is a summary of my contractor's Manual J calculation--he had thought 2 ton, which he is sticking by, because we are only conditioning the top floor of a raised ranch, and he feels the extra half ton will allow cool air to drop without short cycling. Do y'all agree? Some basic house info is below. If so, we'll be going with the Trane XB13 (same warranty as XR, but a bit louder--not an issue based on placement) and TAM7 (better dehumidification, quieter, bumps to 14 SEER).

    Gain BTU Loss BTU Gain CMF Loss CFM Base Board Tonnage
    17731 30320 592 574 56 1.5

    Home is in Southern NH
    2x4 walls
    20" blown-in insulation
    All new windows, doors, siding with 1/2" foam insulation
    Raised ranch, 46 x 26
    Basement half underground

    Anything else needed to confirm our decision, please let me know. Thoughts welcome!

    Thanks,

    S
    Looks like it will be an air tight home. Any thoughts on fresh air ventilation and supplemental humidity control both winter and summer?
    A healthy home needs an air change every 4-5 hours when occupied. Some of the these homes only get an air change in 10-24 hours. Fresh air is needed to purge indoor pollutants and renew oxygen. During calm warm weather, most homes stop ventilating naturally. Without mechanical fresh air ventilation, these home are unhealthy during calm weather.
    Regards TB
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

  4. #4
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    How do you know it's air tite. Sounds like it's not new construction and has have insulation retrofitted. Loose fill in the attic that does little to stop air movement and foam board, that may or may not be taped and no mention if the sill plate was sealed. IF air movement in the wall wasn't stopped, the insulation imrpovement of the wall will be far less than expected.

    I'd rather take a look at the heating bills for a couple months with the average daily temperature and what indoor temp they use. Mainly a comparison of previous bills ot current.

  5. #5
    I wouldn't say the house is airtight, it was built in 1971 and has been updated. I'm not sure about the air cycling thing, but we (hopefully) won't be using the system straight through but rather on those weeks when temps become less than comfortable.

    S

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