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  1. #1

    Replaceing Bryant Furnace with right size

    Here is my question and I would like some technical opinions.

    I had 3 different Manual J Heat Calc done and since I use basement as liveable space they calculated that space well, just with different variables for outside walls and concrete floor being below grade. 2 were around 63,000 and the other was below 40000. I did myelf and cam in around 62500 so I feel confident that is BTU heat loss I need to plan for. I do have opportunity to tighten up attic with additional insulation to maybe save 4000 BTU.

    Current furnace is a Bryant 398AAZ 90+ input at 80k btu. So I assume output was 72k. Furnace serverd us well, but after 23 years the imfamous carrier secondary heat exchanger failure got me. I am loooking at carrier products again, so I can try to match ducts and AC as close as possiable to minimize labor durign replacement.

    Here is the question. Should I go with 80000 btu at 95% which output would be 76,000 or the 60000 at 95% with output of 57000.
    I guess is it better to be 13,000 high or 6000 lower.

    Note: I live in Michigan and Low is 1 degree, since I moved in I upgraded windows to 2 ply gas filled and added 1/4 insolation over wood siding before I vyna sidded upper level. The furnace has seemed to short cycle and generate lots of condensation since I tightend up house. I have certified tech as good friend so I am going to be his helper on this job so I can get any tax credits and keep bryant warranty for cert tech performing startup.

    Any expert opinioins would be appreciated

  2. #2
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    Where in MI? 99% Design temp in most of the LP of MI is 3-6F. Since your so close to 60k, I'd still go with 60k. Most mid range 2 stage furnaces are 96%, so your closer to 58,000BTU output. Manual J has some safety factor built in. A home that needs 60k BTU to heat it in your climate, would have a gas bill of at least $250 @ an average delivered cost of $0.85/therm.

    I'd go smaller. You'll never notice that last 4000BTU. Worst case, on really cold nights it might fall behind 1-2F for a few hours in the morning, but i doubt it.. When you start using appliances and lights, take showers, and the sun comes out, it will catch up quick.

    I'm curous of the one that came up with 40k BTU. I wonder what he saw different. Basements are tricky to calculate. THe actual R Value of a concrete wall below grade isn't that simple.

  3. #3
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,189
    What he said

  4. #4
    @ Motoguy128 Thanks for feedback. I live neat Flint, MI almost in the middle of state. I was told to get from local utility company and I did. I also found same 1 degree on another HVAC site. Do you have diffrent info on this, 4 degrees for each room calc woudl surely drop BTU heat loss.

  5. #5
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    Flint 99% = 3f Saginaw =4f

    This was from an Acca publication I found online. "Acca manual j residential load calculations outdoor design conditions, eighth edition ". It a sprays publication thar just has tables. This is from 2011 so pretty recent.

    Keep in mind we never even hit winter design conditions in the Midwest the last 2 years for more than just a couple hours. And only a couple times the year before that.


    Sent from my SGPT12 using Tapatalk 2

  6. #6
    Join Date
    Jun 2010
    Location
    Philadelphia PA
    Posts
    2,190
    The real unknown is infiltration! Have a blower door test done to see how "tight your home is" Also here is where you might improve your situation so that "right sizing" is done with more data. Rmember, without the data you are just someone with an opinion!!
    You have got to learn from other people's mistakes! Because God knows you don't live long enough to make them all yourself !!!!!!!!

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