Manual J Does it sound correct

Discussion in 'HVAC Heating & Cooling' started by weather777, Oct 30, 2008.

  1. weather777

    weather777 New Member

    Messages:
    36
    A Manual J was done to produce these units,but seems to me the units are too small.I am mainly concerned about the cooling capacity.

    Location:East TN Blue Ridge Mountains
    Heating Degree Days: 4400
    Cooling Degree Days: 1000

    House:Upper Level 1400 sq ft
    Lower Level 1400 sq ft

    A very small portion of the lower level is true basement ( the north wall and maybe fraction of the 1400sq ft) and the rest basically 1st floor like at 7 feet above ground.However,since the entry door is in the upper level its referred to as basement There is around 800 sq ft of crawl space under the lower level and will be sealed with a dehumidifier and 4inch pipe air supply.The house is tightly insulated and 2x6 construction.I want the temp in the summer to be able to cool to 65 degrees when 90 degrees outside.

    Results:

    Upper Level system: 2.5 ton
    Lower Level system: 1.5 ton

    Air Source Heat Pumps with Propane back up- 13 SEER A/C with variable speed fans

    Does that lower unit seem large enough to get the temps to desired level---seems low to me.

    Thank you

    PS--Does it matter if the lower level was entered into Manual J as basement vs 1st floor as far as tonnage it showed (1.5 tons)?
    Last edited: Oct 30, 2008
  2. jimbo

    jimbo Plumber

    Messages:
    8,997
    Location:
    San Diego
    I will let the HVAC pros weigh in on the numbers. But I will say you are asking a lot to cool to 65º. Why do you want to do that? Most folks would find 65 to be uncomfortably cold.
  3. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    27,286
    Location:
    Cave Creek, Arizona
    temerature

    Heating and cooling are almost a geometric curve. It takes a lot more energy to cool from 75 down to 65, than it does to cool from 85 down to 75. Why would you want a temperature that low?
  4. weather777

    weather777 New Member

    Messages:
    36
    Due to health related issues.
  5. veesubotee

    veesubotee New Member

    Messages:
    11
    manual J?

    Stating heating and cooling degree days does not give a true value for sizing heating/cooling, since your dealing with averages of temperatures in a period.

    Did your 'manual J' state heat loss and heat gain (sensible and latent)? This is what's needed.

    V
  6. jimbo

    jimbo Plumber

    Messages:
    8,997
    Location:
    San Diego
    Don't mean to pry....but do you need it that cold, or do you need to reduce the humidity and filter the air. The latter two can be accomplished without the very low temp.
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