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Trouble using Manual J calculator, I could use some help

kenorakq | Posted in Mechanicals on

I have a couple of companies quoting ASHP for my house is Kenora Ontario, Canada (zone 7).

I have not heard back from all yet but would like an accurate manual J done by me or other(s) that have experience using Load-Calc (or another recommended product).

Despite my strongly worded request that the contractors complete a Manual J they are all (3 co’s) using Rule of Thumb calculations: each reacted the same when I told them the size of the existing furnace (80k btu at 75% eff),,, “Sounds about right”. All were alarmed at the Manual J I completed about a year ago at about 30k btu stating to a man that “Doesn’t sound right”.

I expect to be quoted over-sized equipment but could insist on right-sized if I’m confident in the math…One of the companies quoted a PUZ-HA42NHA5 VRF heat pump outdoor unit as well as several other components. I’ve asked them for clarity on the sizing of that unit but suspect its 42k btu given the model number.

I would appreciate being able to ask Questions about that program as I work my way through it…its been a while since I last stumbled through it!

With that in mind I’m struggling through page 2 Windows and doors..

Windows.. I have windows on each of the 4 walls, all different sizes. What I see is an option for only 1 window per side…Do I add up the square footage per side and average them?

More questions to follow…thanks…

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  1. kenorakq | | #1

    After that..

    I've got two doors...a 36" steel insulated with no glass on the west side and a sliding 72" double pane full glass patio door on the east side...
    Do I omit the shading column (page 2 still) and add sq/ft of each door... problem is E and W are combined and the doors are very different?

  2. Expert Member
    Dana Dorsett | | #2

    For Manual-J purposes it's fine to sum the total window area on each side, assuming the window types are similar (same number of panes, same types of e-coatings, etc.). The window area to include is for the whole assembly, not just the glass.

    The biggest load calculation error in a place as cool as Kenora is the air infiltration assumptions, but if the place is much tighter than the typical house the built-in defaults of most tools are going to shoot to the high side.

    The maximum capacity of a PUZ-HA42NHA5 is probably 48,000 BTU/hr @ +5F/-15C (see the specs on this similar unit: ), but will be quite a bit lower at your much cooler ~-20F/-29C outdoor design temperature. If you look at the chart on page 3, it's maximum capacity at -13F/-25C is already below 40,000 BTU/hr and falling fast.

    I suspect the extended capacity tables for that unit don't cover temperatures anywhere near that low. If your load is 30K @ -29C it'll probably cover it, but not with huge margin. It's certainly not a crazy oversize factor to be quoting for a calculated 30K load in your neighborhood.

  3. kenorakq | | #3

    Thanks... I used some worse case scenarios numbers (I think) and came up with 32364 btu... that is pretty close to what I had calculated before so I'm reasonably confident in the heat loss numbers numbers.
    The infiltration is an astounding 12475 btus (heating)... other than doing all the air sealing I can...and I will...does that sound reasonable since that accounts for over 1/3 of the heat load.

  4. Expert Member
    Dana Dorsett | | #4

    The infiltration number might be high, but it's not out of the question, and certainly not insanely high.

    With -22F outdoors, +70F indoors it's a 92F delta-T.

    At 0.018 BTU per cubic foot per degree-F, the 12,475 BTU/hr represents 2,475 BTU/hr/(0.018 x 92F)= 7,533 cubic feet per hour...

    ...or (/60 =) 126 cubic feet per minute.

    That's about right for a reasonably tight 1200-1500 square foot house in a brisk 15-20mph wind, high for a house that size approaching PassiveHouse air tightness.

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