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  1. #1

    Question about sizing

    I'm in the process of shopping for a new Furnace and A/C. Some have done the Manual J, some have not. They have all come back with similar numbers. However I have a few questions.

    1. One measured from the outside of the house. I thought the manual J needed to be done from the inside? He came back with similar numbers but I was just curious.

    2. The trane dealer wants to put in a 2.5 ton condensor and a 3 ton coil. Shouldn't they match?

    3. I have a general concern about the size of the furnace. The one being replaced is a 44 year old Lennox 100k BTU. It was rated as 54 AFUE. So does that mean I was getting 54000 btus of heat? If so most of the contractors want to put in either a 69k or 88k furnace and 90+ AFUE. The old furnace heated the house fairly well. Doesn't it seem like these new units would be oversized?

    Thanks in advance for your advice.

  2. #2
    Join Date
    Apr 2007
    1. Did you see the or did he break down the numbers by room when he measured the outside of the home? If not how will he know what each room requires?

    2. If they are ARI certified it is a you have model numbers and have you looked to see if it was a match on the ARI website or not?

    3. See heat load/loss calculation. if you don't think it's done correctly try this link and do it yourself...HVAC Calc

  3. #3
    Join Date
    Jan 2004
    Nothing wrong with doing a whole house load for an estimate, saves a lot of time if you don't get the job.
    Common to have a larger indoor coil then the OD unit rating.
    The AFUE could be right. Thats the rating for the whole heating season. But, the steady state efficiency(combustion efficiency) of the old unit when it ran was a lot higher. Good chance in the 70's.
    Contractor locator map


    How many times must one fix something before it is fixed?

  4. #4
    Join Date
    May 2000
    Indianapolis, IN, USA
    When I was selling, I did a quick whole house calc if I didn't know the floorplan. Unless it was a real complicated house didn't take me long. Plus I could justify why I was quoting much smaller equipment than those who based on existing!

    Figure actual output of the relic at around 75% of the input. That's not the AFUE or efficiency that affects your gas bill but a good way to gauge at what the relic put out. So let's say 75,000 BTU.

    It's pretty rare that anything in those days was sized right. Gas was dirt cheap, the thought was bigger the better.

    In the 90+ models, the 88K furnace has an output around 81,000 BTU. The 69K around 64K. Just going from what we see around here, the smaller one is most likely what is right if not a bit big itself.

    But this is where the load calc comes in. Only accurate measuring & calcatating gets you the right answer. Anything else is a guess.

    We always use the 031 coil with a 2.5 ton A-S/Trane unit. Only reason not to is if you need a wider coil than 17.5". Shouldn't be the case here though!

  5. #5
    I have models numbers for a trane and a goodman system.

    The Goodman is AM-SSX140301 Condensor (2.5 ton)
    AM0CAPF3642C6 Coil (3.5 Ton)
    AM-GMV950704CXA - Furnace

    The Trane is 4TTX4030B1000A Condensor (2.5 ton)
    TE47636c175b2922AP Coil (3 ton)
    TUH2B080A9V3VAA - Furnace

    Both contractors have said they would do a manual J if I was going to hire them. How do I check the ARI website to see if these coils should go with the condesors. They both have claimed that i'll get a higher seer rating this way.

  6. #6
    Join Date
    Apr 2007

    2. Accept

    3. Air Conditioners and Air Conditioner Coils Single Package and Split Systems (Std. 210/240-2005)

    4. Refine might have to remove some numbers at the end or substitute * for parts of the model number.

  7. #7
    Join Date
    Dec 2007
    the coils are larger to get the full 14 seer rating upsizing is common with these units,Doing the manual j after job is awarded is common the cost of units one size to another is marginal, alot of contractors dont give out calc info because it gets shopped out, so you do the work so someone can get the job with your calcs because home owner gives out your calcs to get cheaper price,just my opinoin go with goodman ,trane is overated .

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