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Thread: AC size

  1. #1
    Join Date
    Jul 2005
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    We moved in to a 2,800 square feet (sq footage for 1st and 2nd level does not include basement or garage) house in Northern Maryland about a year ago. The house is cooled and heated with a single zone system and the outside AC unit is rated at 41,000 BTUH. From the day we moved in the AC system seemed to have difficulty keeping the house cool. This being a brand new house, we though that this may have been due to the absence of blinds and drapes which. Over last summer and into last fall we installed blinds and drapes on all the windows, with the exception of the large palladium window over the front door in the open 2 story entrance foyer (there are not other open 2 story rooms in the house).

    So far this summer the house is not been keeping cool. In fact on hot days (temp above 90 degrees F and RH above 80%) the AC runs none stop from dawn to midnight. We usually set the thermostat at 74 or 75 degrees F and the temperature in the house rises 4 to 5 degrees F during the day. The rooms in the South west corner of the house which includes the living room and dining room on the 1st floor and two bedrooms on the second floor are uncomfortable to be in from mid afternoon till mid evening.

    I purchased a software program that is ACCA Manual J complaint and modeled my house using this software. I have ran two scenario one with the normal number of people in the house and another with a house full of 20 people (i.e., we have a party). The 1st scenario rated the total Heat Gain at ~48,000 BTUH and second at ~62,000 BTUH resulting in a deficit of 7,000 BTUH and 21,000 BTUH, respectively. However, through the reading I have done I know that there is more to an AC system then just the outside unit (i.e., Coil, Air flow over the coil, ducting, etc.), and that an oversized outside unit is just as bad as an under sized unit.

    With all this said I am looking for some advice on the following:

    1) Is it normal to have temperature change of 4 to 5 degree F in the course of a typical (i.e., not record braking heat) hot summer day?
    2) Should the AC system be able to maintain temperature or even drop temperature during the course of the day?
    3) Should the unit be running continually from dust to midnight on a normal hot summer day?
    4) Is the Manual J calculation accurately representing the correct heat gain?

    Sorry for such a long posting, but I wanted to give as much information as possible so to get answers to my questions. I look forward to responses on my issues.

  2. #2
    Join Date
    Aug 2002
    Office and warehouse in both Crystal River & New Port Richey ,FL
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    1.No,should be able to maintain the design temperature,however it could have been designed to maintain 78F.

    2.Maintain setpoint,go below a little,IF the outdoor temp. is below design.

    3.May run a lot until an hour or two after sunset.

    4.Man. J is as accurate as the "input" data,if anything it will oversize slightly.

  3. #3
    Join Date
    May 2002
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    According to Manual J(7), a swing of 3 F is considered normal. In other words, you can expect the temperature to rise 3 F above the set point late in the afternoon and possibly at other times during the day.

    What happens to the inside RH when your system runs for several hours?

    When you entered your data into the Manual J program, how did you determine the infiltration rate?

    Did you fudge on any temperatures?

    Did you measure just the area of the windows' opening and not the entire cased or frame area?

    Are your windows just clear glass? Manual J(7) won't directly handle the newer windows with low SHGCs, and so you have to do manual adjustments. Using j7's "low e" option alone substantially overstates the cooling load of most newer windows.

    Did you use inside floor/ceiling area rather than the outside (foundation) area?

  4. #4
    Join Date
    Jan 2004
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    Is this new constrution.

    If so, you should have told your contractor.

    If not, did you have it checked to see if the coils are dirty.

    You could have a dirty air filter, and being seeing the effects of it.
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    How many times must one fix something before it is fixed?

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