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  1. #1
    Join Date
    Jun 2008
    Posts
    100

    Clues about how to get central AC sized right

    I want dehumidifying mostly out of a central AC. I have a little 2-story house, with 660 sq ft on the top floor. The bottom floor doesn't get hot, it's half finished basement and half garage. I am running dehumidifiers down there though so that heats things up.
    Right now I'm leaving the windows open at night, that and the ventilated roof keeps it fairly comfortable. But I would like to stop leaving windows open, and keep the upstairs dehumidified, which would heat it up a lot.
    How could I get it sized right? I'm worried about getting it oversized and an automated kind of estimate from contractors. I read that Manual J is often oversized!
    Laura

  2. #2
    Join Date
    Jul 2008
    Location
    Kansas
    Posts
    61
    Quote Originally Posted by plarian View Post
    I read that Manual J is often oversized!
    That sounds like a scape-goat from contractors not wanting to do the work to perform the proper sizing methods.

    If your house has not had central AC before you NEED a manual J done. There may also be duct work considerations, manual D, even if you have central heat. Call around, research some contractors and find one that is willing to do the work to give you the system you desire.

    Here's some good reading on finding a reputable contractor:
    ACCA Link

    Contractor rating

  3. #3
    Join Date
    Jun 2008
    Posts
    100
    Quote Originally Posted by Talz View Post
    That sounds like a scape-goat from contractors not wanting to do the work to perform the proper sizing methods.

    If your house has not had central AC before you NEED a manual J done. There may also be duct work considerations, manual D, even if you have central heat. Call around, research some contractors and find one that is willing to do the work to give you the system you desire.

    Here's some good reading on finding a reputable contractor:
    ACCA Link

    Contractor rating
    Thanks for the good advice. I will ask them for a Manual J.
    Is it true that if I get AC with an option to run very slow, I don't need to worry a lot about getting it oversized - because I can just run it very slow to dehumidify? Dehumidifying is the big deal here, since I'm sick a lot from allergies
    Of course I'll try not to get it oversized, but if that is true, I won't be too worried about it and possibly get something undersized.
    I've found good people in the past by talking to people with related businesses. Who don't actually do the work I'm looking for, but who know about it. For example for HVAC one might ask a plumber. I haven't had good luck with references from neighbors and friends.
    Laura

  4. #4
    Join Date
    Jun 2008
    Posts
    100
    Quote Originally Posted by Talz View Post
    That sounds like a scape-goat from contractors not wanting to do the work to perform the proper sizing methods.
    No, that was actually some research I read online, they had evaluated air conditioning for some houses that had had Manual J's done to size them, and the AC was generally oversized, by 25%, if I remember right. One study is at http://oikos.com/esb/50/manualj.html.
    I saw other comments that the contractor should do an "aggressive" Manual J calculation, carefully including various factors that reduce the AC load. For example I do almost all my cooking with a microwave, and it doesn't heat up my place much. Also that they often add in "safety factors" on top of what Manual J recommends.
    It sounds like it would be a good idea for me to do a Manual J myself, because I would be aggressive about it and not add fudge factors.
    Laura

  5. #5
    Join Date
    Jun 2008
    Posts
    80
    After they do a Manual J ask what the heat gain is. Check the proposed unit to see that it matches very close to what the unit's cooling capacity is (look it up if you have to). Also ask for a Manual D to be sure your duct work is properly sized.

    It is critical they size your A/C right in a humid environment: too small and you get descent dehumidification, but not enough cooling (like my house ); too big you get the opposite. I think the HVAC-Calc 4.0 manual does a great job of explaining why it is a tricky balance:

    "A/C is a little more difficult than sizing furnaces. First let’s look at the job of
    an air conditioner. An air conditioner actually has two jobs, job one is to lower the inside temperature (i.e. remove heat) and job two is to lower the inside humidity (i.e. remove moisture).

    The tricky part comes in because thermostats control air conditioners and they are devices
    that know nothing about humidity. When an air conditioner is running, the warm moist inside
    air is blown over a cold air conditioning coil called an evaporator. This cools the air and in so
    doing, humidity in the air condenses to water, and is routed to a drain. This works fine when
    the air conditioner is running, but, if the air conditioner can cool the air too quickly, it does not
    run long enough to remove the humidity.

    If a home is cooled by an air conditioner that is too large, the occupants tend to keep turning it
    cooler and cooler because they are not comfortable. The house is cool and damp. So the
    goal is to match the cooling load of the house (heat gain) quite closely to total cooling capacity
    of the air conditioner. No safety factor should be added to the cooling load when choosing the
    size of air conditioner."

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