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Manual J questions , Reality check pls

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Homesteader
Homesteader Member Posts: 9
edited February 2022 in THE MAIN WALL
Several years ago I downloaded the free Manual J worksheet and bought the handbook. My goal was to get an idea of the actual heating and cooling requirements of my house. Unfortunately for me , none of the HVAC people I’ve run into (pro’s and semi-pro) are very familiar with the spreadsheet so I’m left with some unanswered questions:

Q1.) which of the calculated totals should be used when shopping for heating or cooling equipment-
   Line 14 …Sub total
   Line 20 Tot Sensible loss/gain
   Line 21 Total Latent Gain

Q2.) In my case , I replaced a 4-ton AC unit with a 36000 btu, 2-stage, 16 seer, amana ASXC160361 and air handler.  The Manual-J info I had after filling in my sheet was:

Tot heating/cooling sq/ft…2000
indoor design heating…70
indoor design cooling….75
indoor design RelHum….50
sub totals heat/cool…..32478 / 13767
tot sensible loss/gain heat/cool … 41431 / 19210
tot Latent gain cooling …..3056
—-
My thinking was that although the calculation said a 24000 btu AC unit should suffice, a two stage 36000 unit would also work because it could operate in low-stage if that’s all the output required.  After 3 cooling seasons, it seems to work fine but I’ve always wondered if I oversized  and sacrificed efficiency. 

Was this the right choice?? Based on the data would a two ton unit have been sufficient?

Q3.) I looked at my gas boiler and it is rated at 113,000btu output. Since moving in I’ve altered somewhat the btu requirements: added R25 to the attic and replaced all aluminum storms w double pane windows. However, it seems that a 113,000 btu boiler is way overkill. If I were to replace the 45 yr old boiler with a modern compact wall hung unit could I trust the Manual-J heating load calculation of 42k btu ?


Thanks very much for any feedback. 

Comments

  • Hot_water_fan
    Hot_water_fan Member Posts: 1,897
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    If I were to replace the 45 yr old boiler with a modern compact wall hung unit could I trust the Manual-J heating load calculation of 42k btu ?


    You won't find a mod-con (wall hung or otherwise) that small, so it's not super important. You'll find 80kBtu units that'll turn down to 8kbtu, which will cover you just fine.

    You can double check using this:
    https://www.greenbuildingadvisor.com/article/out-with-the-old-in-with-the-new, and find out BTUs/Heating degree day, which is based on your actual house's performance.
    Homesteader
  • hot_rod
    hot_rod Member Posts: 22,327
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    On my shop I have a Lochinvar Knight 55,000, it turns down to 8,300BTIU/hr. About ideal for your job.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 15,710
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    Looks to me that you could have gone with a 2 or 2 1/2 ton ac system

    Don't know where you are located. I am in MA and the cooling load is usually about 1/2 the heating load which matches what you have.

    You can also download the "Slant Finn" app and do a heat loss electronically to double check your #s

    To answer your question use the total sensible heat loss for heating

    For cooling use the total sensible heat gain + the latent heat gain for cooling
    Homesteader
  • Hot_water_fan
    Hot_water_fan Member Posts: 1,897
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    @hot_rod do you know if is it a business or engineering decision that the lowest modulation is about 8,000 for mod cons? 
  • hot_rod
    hot_rod Member Posts: 22,327
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    @hot_rod do you know if is it a business or engineering decision that the lowest modulation is about 8,000 for mod cons? 
    My understanding it that it is tough to keep a clean and stable flame at lower turndown rates
    Maybe a boiler rep could chime in?
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • nosirra1Arrison
    nosirra1Arrison Member Posts: 57
    edited February 2022
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    hot_rod said:

    On my shop I have a Lochinvar Knight 55,000, it turns down to 8,300BTIU/hr. About ideal for your job.

    Interesting note: The Lochinvar Knight 80,000 has almost the same low end output (8,500 vs 8,300) due to differing turndown ratios.
    Homesteader
  • Homesteader
    Homesteader Member Posts: 9
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    Thanks for the helpful suggestions. 
    I’ll check out the slant fin app. 
    The Lochinvar units seem expensive at $4k , are they especially good or desirable?

    @Hot_water_fan: that link you supplied is very helpful because it clearly explains how I can check my heating load independent of the manual-j calculation by using the gas’s bill and weather info.  It quite ingenious.  The only problem is that besides a gas boiler , I also run a wood stove but I’m going to run the numbers, it’s a good technique to know.  
  • hot_rod
    hot_rod Member Posts: 22,327
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    A Lochinvar 110 Nobel is another option with a few less control features. I see them online for a lot less than 4K? In fact not much more than a copper tube or cast equivalent.
    Finding any boiler these days can be a challenge.

    Shop the companies with banner ads to the right, or below :)
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    Homesteader