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  1. #1

    Furnace replacement Need quote help

    I am going to replace my 20 year old lennox and have gotten several quotes. I live in Minnesota, some nights (most of January) can get down to -20 to -30 before wind chill.

    I decided to go with the Carrier infinity system

    Furnace 59MN7
    AC 24ANB6
    Infinity Controll

    Now the load calc (wrightsoft) came out to 63k BTU. Problem is Carrier only make a 60k and 80k. The installer wants to use the 80k. Sound right?

    Please help, I dont want to be oversized.

  2. #2
    Join Date
    Jun 2006
    Location
    Richmond, VA
    Posts
    2,976
    The 60 puts out 60...you will be 3k short

    The 80 puts out 78...you are over by 15k, but not under. if the load calc is accurate with attention to detail...I would put in the 80k

  3. #3
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,540
    Unless your going to make insulation and or leakage improvements to your home, get the 80,000.

    Although I believe that the calc is high by a several thousand BTUs for heating. No use risking it.
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    How many times must one fix something before it is fixed?

  4. #4
    Join Date
    Aug 2001
    Location
    Pavilion, NY
    Posts
    2,185
    I'm on board with the above guys as well. Remember that if your loss is 78k and you are putting in a maximum of 78k then you technically cannot raise the temperature in your home. If you set the stat back to 65 at night and want 70 during the day then it could take hours to raise the temp in your home. While this is ideal from an efficiency standpoint few people would tolerate this. Round up and have some extra capacity.
    ...

  5. #5
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,368
    "Working backward "
    your residence is something like
    2,700 square feet
    R-40 ceiling
    R-13 walls
    R-2.5 ( U-value 0.4) windows 300 sq feet <-- must be KNOW from MFG data
    ACH < 0.55
    Heat loss 62,600.

    You would have to know ACH Accurately to state that 60,000 would be adequate for 95'F difference from outside to inside (70 - -25).
    Otherwise, take that 62,600 as +/- 12%

    ACH 0.35 54,800 BTU/HR
    ACH 0.75 70,500 BTU/HR
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  6. #6
    Join Date
    Feb 2010
    Location
    butler pa
    Posts
    1,073
    please post after your new furnace is installed..

  7. #7
    Quote Originally Posted by kangaroogod View Post
    I'm on board with the above guys as well. Remember that if your loss is 78k and you are putting in a maximum of 78k then you technically cannot raise the temperature in your home. If you set the stat back to 65 at night and want 70 during the day then it could take hours to raise the temp in your home. While this is ideal from an efficiency standpoint few people would tolerate this. Round up and have some extra capacity.

    Thanks all for the information.

    Since this is a modulating furnace, should I use setbacks? From what i have heard setbacks are bad with modulating?

  8. #8
    Join Date
    Aug 2010
    Location
    West Monroe, LA
    Posts
    1,537
    I also agree if load cal is correct and no future plans to add more insualtion, change windows etc... I would Chosse the 80,000 btu furnace.

    What size is current furnace? How has it done? What are the run times of existing furnace? Did it keep you comfortable? What was/is your gas bill? What temp do you like your home to be in the winter? Important things to review. If your home is like most with furnaces that age it is more then licky oversized?

  9. #9
    Join Date
    May 2006
    Location
    Philadelphia, PA
    Posts
    1,397
    With the Infinity Control that furnace handles setback very well.
    Climate Control Solutions for your Home or Office

    Serving Northeast Philadelphia and Surrounding Areas

  10. #10
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    Personally, I'd go 60k... but that's just me. Do you ever plan on adding zoning? If yes, get the my touch toughscreen and go with the 60k. You can run a very, small zone with that furnace before it dumps to other zones.

    IMO< it also depends on whether the ductwork can handle 1250cfm and if it will be too noisy for comfort. The 80k needs about 1250cfm on high, the 60k 950cfm.

    You option as I see it is this:

    80k will allow a larger setback in the coldest weather, but min fire is about 31kBTU. IF you load calculation is correct, it will only run continously below about 20-25F.
    60k will run longer and offer a little more comfort in mild weather and min fire is 24k BTU. This would run almost continously below 30-35F. But will limit how large of a setback you can use. Probably only 2-3F.

    A another option, you could go dual fuel. Most utilities offer usually about a $400 rebate that nearly covers the increased cost and in mild weather it usually a little cheaper to operate even with natural gas. If you went dual fuel, I'd go 80k as the best compromise.


    I don't think you'll be unhappy with either unless your ductwork is marginal for the 80k.

  11. #11
    Well I got it installed yesterday, overall I think they did a good job.

    Positives
    1. This thing is extremely quiet. I have the fan running on continuous low, cant even hear it running.

    2. House seems more evenly heated, I did some tests and each room was within a degree of the thermostat

    3. Happy I went with the 80k, from setback this morning a mild (30F) going from 67 to 70. Using smart recovery set at 8:00am. It ran at 65% from 6:30 to 7:00, then 100% from 7:00 to 7:30 then modulated down to 53% by the time 8:00 came around. So the whole house was heated well. Can wait to see what it does in the dead of winter next year.

    Cons

    1. Price


    Questions

    1. Will the oily smell go away? They said it would take awhile to burn off

    2. What should my static pressure be? I was getting .56 on high fan and .17 on low fan.

  12. #12
    Join Date
    Jan 2013
    Location
    The Cheese State
    Posts
    98
    This is the Ask Our Pro's forum. In order to post a response here, you must have verified qualifications and have been approved by the AOP Committee. You may ask a question by starting a new thread.

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    Last edited by jpsmith1cm; 03-16-2013 at 05:14 PM. Reason: non AOP member

  13. #13
    Join Date
    Mar 2013
    Location
    Central IL
    Posts
    103
    This is the Ask Our Pro's forum. In order to post a response here, you must have verified qualifications and have been approved by the AOP Committee. You may ask a question by starting a new thread.

    You can find the rules for posting and qualifications here.

    Additional infractions may result in loss of posting privileges.
    Last edited by jpsmith1cm; 03-16-2013 at 05:16 PM. Reason: non AOP member

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