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  1. #1
    Join Date
    Aug 2009
    Posts
    28

    Sizing unit: Manual J good enough or is a Blower door test needed?

    I am shopping for a new a/c unit and have had a few estimates. Most just look at my old unit and tell me what a new one would cost to replace it.

    After reading for a bit on here I guess that is not the right way to size a new unit.

    So, I called around and one company is going to do a heat loss calculation first for free.

    Another company I called which seems extremely knowledgeable said they will do a manual J calculation for free but they strongly recommended to do a blower door test as well and they use infrared sensors to detect hot and cold areas, check the static pressure, etc... that service runs $150

    This last company is ok with doing just a manual J assessment but said that would really just be a "guess" at what I need, and to do it right I could tell they were kind of pushing the blower door test.

    So, is a manual J good enough?

    Thx.

  2. #2
    Join Date
    Aug 2003
    Location
    Lincoln, Nebraska
    Posts
    1,051
    A Just a manual J is much better than not doing one and just going off of the old sizing. The problem with that is they would need to guess at the infiltration for your house. I would not go with any company that dosn't do a manual J.

    Doing a blower door test will tell them the actual infiltration so they can put that in the manual J for accuracy.

    The other advantage of the blower door test is that they will be able to show you where your house is leaking and many times with some minor repairs you will be able to downsize your equipment and have much better energy bills.

    The static pressure test will let them know how your duct system will perform. This is important as the newer high efficiency equipment is not near as forgiving as the older stuff was. You would be supprised how many duct systems in this country are undersized.

    If I were you I would have the tests done if affordable. You may be ammazed at the results.

    By the way, you may want to remove your pricing as it is against site rules.
    Its a good Life!

  3. #3
    Join Date
    Jul 2008
    Location
    Rochester NY
    Posts
    4,630
    Quote Originally Posted by m kilgore View Post
    ...many times with some minor repairs you will be able to downsize your equipment and have much better energy bills.

    You would be supprised how many duct systems in this country are undersized.
    Downsizing has a cumulative effect. Your duct work doesn't need to be as big, so if it's undersized, downsizing can help improve not only operation, but longevity, efficiency, noise, dust, and temperature evenness.

    You can throw darts blindfolded or with the blindfold off. Your choice.
    Which makes more sense to you?
    CONSERVATION - turning your thermostat back and being uncomfortable. Maybe saving 5-10%
    ENERGY EFFICIENCY - leaving your thermostat where everyone is comfortable. Saving 30-70%

    DO THE NUMBERS! Step on a HOMESCALE.
    What is comfort? Well, it AIN'T just TEMPERATURE!

    Energy Obese? An audit is the next step - go to BPI.org, or RESNET, and find an auditor near you.

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