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does sum of window unitrs equal cooling load?

I assume you are installing your air handler in the attic out of a convienient open space. Is it vented to the outside (most common) or part of the conditioned building envelope? If it's the former like most systems in America are, ask yourself this question. Is the effect of window solar gain greater on calculating my cooling load or sticking my air handler and ductwork in a 135 degree unconditioned space? It's a very easy answer.


  • joe_94
    joe_94 Member Posts: 39
    Does sum of window units equal cooling load?

    When it cools off we will start installing plenum in the attic for a Unico/Spacepak type system.
    The question: on the worst days this summer [ 93-95*] we are comfortable with four 5,000 BTU window units in this 1940 two story colonial house of 2.350 square feet.
    [I think the Alcoa aluminum siding rejects a lot of the sun's heat.]
    So..does it seem then that I need only a two ton unit? or 2 1/2. A ton is 12,000 BTU's, right?
    Restated, Are the window units, totalled, a reasonable measure?
    Joe, in Minneapolis
  • Brad White
    Brad White Member Posts: 2,393
    May be coincidence....

    The chances of EWU's (Equivalent Window Units; an acronym I just made up) equaling BTU's is purely coincidental. It is sort of like baking a cake using the size of the oven to measure ingredients. Or something.

    The only way to be sure is to perform an ASHRAE or Manual J cooling load calculation on the entire house.

    Keep in mind, what you have now may well give you better control. I can assume that the window units serve fours spaces on potentially more than one exposure. You have four (4) cooling zones or if the doors are open to an open plan space, four (4) stages of cooling. You are trading that all for one big zone.

    I will grant you, though: Unico/SpacePak systems remove at least 75 percent more moisture by cooling the air down to the 40-42 degree range versus the more conventional 53 to 55 degree range.

    So, I am not saying, "don't do it". Rather, I am saying, "consider the ramifications and benefits" and size her correctly.

    My $0.02

    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"

    -Ernie White, my Dad
  • Boilerpro_5
    Boilerpro_5 Member Posts: 407
    I have the same situation in my home, Brad....

    I have 1904 2,800 sq ft wood frame home with 700 Sq ft of glass and not a whole lot of shade (not much east, none west) but not many windows to the west... most on east and south. We have been running 2- 6,000 btu/hr window units continuously in this current heat wave (with extreme humdity) cooling almost the complete home and temperatures have varied from about 70F first in the morning to about 75F at the peak running about 62% relative humdity (We have a dehumidifier in the basement keeping it dry) A CLTD cooling load calc that I performed said I need about 28,000 btu/hr cooling. We are well insulated R-19 walls, R-30 ceiling with vapor barrier all around. We are also probably more airtight than the average new American home (I followed the older canadian air tightening standards when we renovated the home).

    It seems cooling load calcs are awfully conservative. I suspect a 1 1/2 ton unit would work really well for us. Accepting a bit of a temp swing through the day at peak loads should allow much smaller equipment. It also appears to be very efficient. Our two windows units have an EER rating of about 10 and our highest summer electirc bill is about $115.00 from Com Ed....some of the highest electric rates in the country.


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