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  1. #1
    Join Date
    Feb 2005
    Posts
    39

    Question

    Hello,
    We were given a copy of a Manual J from our builder. The numbers seem very low as far as equipment size. We have 1525 sq ft upstairs and 1425 in our walkout basement. They said we needed 23,250 btu required and 28,400 btu recommended total. We also had 42,000 recomended on heat. They installed a 2.5 ton Heat pump. I realize they are sized for cooling, but expecting us to make up the 12,ooo btu needed from electric heat is to costly.
    Another concern is 1025 total cfm required for the entire house. Do these numbers sound reasonable or did Mr. builder go with the lowest HVAC estimate.

  2. #2
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    33,929
    The proper method is size to the cooling needs and let the backup make up the heating. Around here, we sometimes have twice the heat needs than cooling. Many of our 2.5 ton pumps have 20kw backup to make up 30,000 BTU diff in bitter cold weather. Our all electric heat pump homes have cheaper heat than those with gas heat. If you oversize in cooling, you end up humid & clammy. That would make you miserable all summer long.

    Without seeing the house and running my own figures, hard to say but I don't think he'd undersize you. He'd get nothing but grief. I'd trust him and not worry about it unless you can't cool in a heat wave.

  3. #3
    Join Date
    Feb 2005
    Posts
    39
    What does 30,100 btu@47f mean on the Manual J?

  4. #4
    Join Date
    Aug 2004
    Location
    middle georgia
    Posts
    239
    Originally posted by help3
    What does 30,100 btu@47f mean on the Manual J?
    This is the output of the equipment he is quoting at 47 degrees outdoor temp. Manufacturers usually will list the output of the equipment at 47f and 17f to assist in selection of the correct equipment.
    Please step AWAY from the condensing unit.

  5. #5
    Join Date
    Feb 2005
    Posts
    39
    At 47 degrees my heat pump will produce only 75 % of the required heat. This is B.S., should they have installed a larger unit? Also, the unit runs to long on AC and we can't get the temp below 78 degrees on 90 degree days.

  6. #6
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,772
    At 47 degrees you won't need 42,000 btu's, you'll only need that much at od design temp, weather thats 0, 5, or -5, degrees for your area I don't know.
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  7. #7
    Join Date
    Mar 2005
    Location
    5,000 ft
    Posts
    2,190
    The good thing is you have a Manual J done, most our competitors may not know what a Manual J is.

    Every climate region is different, however never size your heat pump no more than 25% of your cooling load parameters (ACCA Manual H).

    Can't comment on your load results, w/o seeing all the specs.

    30K btuh @ 47F is the capacity of the heat pump @ 47F outside.

    Steve

  8. #8
    Join Date
    Dec 2000
    Location
    Central PA
    Posts
    244
    I've always used a balance point sheet to size my heat pump equipment. In PA the heat loss is typically double the heat gain but by sizing the equipment between the heating and cooling loads I sacrifice some comfort in the few summer months to help with heating costs in the many winter months.
    SRJ

  9. #9
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,772
    SBuckJ:

    Do you just do that with the 10 seer, or with 12 seer and above also.
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