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Manual D/J questions

KeithH | Posted in Mechanicals on

Hi all,

Awhile back, I had a Manual D/J performed. I reviewed it the best I could at the time but now that I’m looking into a system change-out and getting ready to lean on the D/J, I find some wall configs and windows are not correct and some of the basement numbers don’t add up.

So if anyone out there is a D/J wizard, I have some specific questions:
1. Is there any way to back of the napkin the reduced load for the walls/windows that were entered with standard efficiencies instead of enhanced efficiencies?
2. Why would the basement room loads not be the sum of their wall and window component? Is loss to the slab handled elsewhere? I’m seeing higher btus/sf loads for the basement than some upstairs space. It’s hard to believe that the slab loss is greater than exterior wall loss.
3. With highly insulated walls, is there any point to the wall loads? Since the Manual D/J is generally considered to be conservative in sizing, could you just size for glazing?
4. I certainly can calculate U factors and look up glazing SHGCs but I don’t know how to incorporate these values with aspects and orientation. Are these calculations common property somewhere that a smart person can find to perform their own calculations?

If it matters, the software that was used was WrightSoft.


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  1. GBA Editor
    Martin Holladay | | #1

    The answers to your questions all involve math. Either the information has to be re-entered into the WrightSoft software, or you have to start over with a pencil and paper.

    Here are links to a blog series that tells you how to calculate these values:

    How to Perform a Heat-Loss Calculation — Part 1

    How to Perform a Heat-Loss Calculation — Part 2

    Calculating Cooling Loads

    Note that window orientation is irrelevant for heating load calculations, because the design heating load is calculated for night conditions. (That's when the heating load is highest.) Window orientation only matters for cooling load calculations.

  2. KeithH | | #2

    Ha. I knew you'd get me for failure to adequately google so I read those last night after posting this.

    Did you ever write part 4? Couldn't find it.

    I am also concerned with cooling loads.

    Thanks for awesome library of knowledge!

  3. GBA Editor
    Martin Holladay | | #3

    Here is a link to Part 4: When Do I Need to Perform a Load Calculation?

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