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  1. #1
    Join Date
    Jan 2004
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    We are about to replace our heat pump system.
    I've had a bunch of contractors out, most of them sales reps but a couple with technical/repair backgrounds, and not a single one has brought up the subject of "Manual J". In one case, when I asked about getting one, the guy did not know what I was talking about. They all seem to assume that if a 2 ton system is in here now, that's what the replacement needs to be.

    I've downloaded a bunch of spreadsheets to try to do calculations myself.
    Some versions are easier to use than others, but they all seem to show a 22- 24KBtu heat loss. I'm getting different figures for the heat gain but the best estimate seems to be about a 12KBtu morning (AM) heat gain and ~24K Btu PM heat gain. That's with a fair amount of detail on interior appliances, window orientation, etc. My experience is that, yes, the heat gain in this house does increase greatly in sunny afternnoons.

    Are there any particular "gotchas" that a homeowner should beware of in these do-it-yourself calculations? Or are there so many that I really need to try to find a contractor to do it? Some of the data points ("dry bulb" this and that) in some of the sheets require more technical knowledge than I have.

  2. #2
    Join Date
    Apr 2006
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    there is nothing to stop you from learning the details on the calculation(s), see and the link to "New: Manual J-8 Abridged Edition" (there is a member and non-member price, difference ~$20)

  3. #3
    Join Date
    Jun 2004
    4H: Hot, Humid Houston H.O.
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    Pros and cons

    In case you had not noticed, your numbers do appear to agree with the sizing you have in place (although I would prefer to see a pro use Manual S to verify this is true). To the best of my knowledge there are not many big pitfalls doing your own Manual J. I have done it with HVAC-CALC and find that even with weird assumptions the total load varies less than I expected.

    With an existing installation, your job is easier. At least you have experience and know whether the old AC was maintaining temperature (and humidity). While I am a homeowner who greatly admires using Manual J, Manual D and all the best ACCA practices, in the case of a replacement I submit it is reasonable to expect to use the same size as before. This is largely assuming there is no big change in latent vs. sensible capacity with the new AC, which I understand is unlikely.

    Anything you can do with finding and fixing air leaks, will improve your system performance. This is very important with ducts in attics, less so with ducts inside the conditioned space.

    If there is any big pitfall, it would be with up-sizing when you replace. In other words, be very skeptical of suggestions that a 2.5 ton system is a "safe" replacement. One thing homeowners overlook, and techs prefer not to point out, is that any bigger system requires more airflow. Without upsizing your duct system, chances are large that you will be overloading the existing ducts. But if the tech *does* point out a need for duct upgrade, he will lose the "lowest bidder" game that most buyers and sellers play. When you are upsizing airflow, beware. When no upsize is needed, you at least should be as good as you were before.

    My own experience was getting a VS (variable speed) air handler which effectively upsized airflow from 1110 CFM to 1400. The result was a loud airflow noise as a flex duct was carrying 860 feet/min air speed -- Manual D says the max should be 700. I reduced the problem by going to 350 cfm/ton airflow which is about 1225 CFM. So I know this pitfall first from experience, and only second from book learning.

    It certainly is regrettable that some techs don't know about Manual J, but IMO it may not be critical in your instance. Just don't upsize blindly.

    Best of luck -- Pstu

    [Edited by pstu on 05-01-2006 at 10:57 AM]

  4. #4
    Join Date
    Feb 2003
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    I have such a spreadsheet, first done in mid1980s --
    look at ALL the details

    how does the calculated btu/h compair with what size your present unit is?
    are you comfortable with present?

    now, you should know what to improve to reduce the losses --

    did you inspect the ducts & plennums for holes? & good insul?
    are penetrations & boxes & trim sealed? (infiltration)
    harvest rainwater,make SHADE,R75/50/30= roof/wall/floor, use HVAC mastic,caulk all wall seams!

  5. #5
    Join Date
    Jan 2010
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    Quote Originally Posted by tom21769 View Post

    I've downloaded a bunch of spreadsheets to try to do calculations myself.
    Some versions are easier to use than others, but they all seem to show a 22- 24KBtu heat loss. ...
    I would be grateful for links to any such spreadsheets freely available online.

  6. #6
    Join Date
    May 2006
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