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  1. #1
    Join Date
    Oct 2007
    Location
    Washington, DC Metro Area
    Posts
    25

    Not sure about furnace size

    The space being heated is about 5,000 square feet, 2,500 on the first floor and 2,500 in the basement. In the basement only 1,200 square feet is finished.

    The following specs were taken from HVAC plans.
    Heating Specifications

    Heat Loss 87715 BTUH
    Equip Manufacturer Carrier
    Gas furnace model 58STA135-22
    Gas furnace input 132,000
    Gas Furnace output 107,000
    Gas furnace AFUE 80.0
    Rated CFM 1715
    Air Temp Rise 57

    Cooling Specifications

    Heat Gain 53181 Total BTUH/43683 Sensible
    Equip Manufacturer Carrier
    Condenser Model 24ABA3060A30
    Evaporator Model CNPVP6024ACA
    Net MBH Total 57.0
    Net MBH Sens 44.1
    SEER Rating 13.0
    Rated CFM 1420

    Given the above, will a comfort 92 model 58MXB080-20 do what the basse model 58STA135-22 does in regards to heating this space? This is all the info I have.

  2. #2
    Join Date
    Aug 2007
    Location
    MN
    Posts
    2,677
    At 87K heat loss it may end up a bit shy on extremely cold days, but depending on the "fudge factor" of the load calc program, you may be in good shape.

    Who did the load calc?
    You can't fix stupid

  3. #3
    Join Date
    Oct 2007
    Location
    Washington, DC Metro Area
    Posts
    25
    "Who did the load calc?"

    The HVAC company. It is a large local company. Not sure I should mention their name. I thought the furnace was going to be a 58MXB120. However, I did not get the furnace size in writing. By the way, this is new construction.

    Wha's the rule of thumb, furnace output, in this case 77,000 BTUH?, should be equal to or greater than the heat loss 87000 BTUH?

  4. #4
    Join Date
    Aug 2007
    Location
    MN
    Posts
    2,677
    Well based on the load calc, I would say it is undersized, but to me a 2,500sq ft home for most cases an 80K btu 92%er would be great plenty, if they did the load calc wrong (say they inputed the wrong stuff for the below grade space) the calc would come out higher. If they are a large reputable company, then give it a shot, if it can't keep up in sub zero weather, they should back it up and change it out. They did the calc and feel comfortable with their decision to go smaller so give them the benefit of the doubt, they are the pros. Lower BTU's for the most part will cost less to operate, oversized equip that short cycles cost you more to run, and 95% of the yr the equip would be oversized, at the most you would need a 100K not a 120K
    You can't fix stupid

  5. #5
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,323
    Quote Originally Posted by civic4life2 View Post

    The space being heated is about 5,000 square feet,
    2,500 on the first floor and 2,500 in the basement.
    In the basement only 1,200 square feet is finished.

    The following specs were taken from HVAC plans.
    Heating Specifications

    Heat Loss 87715 BTUH
    Gas Furnace output 107,000 Gas furnace AFUE 80.0
    Rated CFM 1715

    Cooling Specifications

    Heat Gain 53181 Total BTUH/43683 Sensible
    Condenser Model 24ABA3060A30
    Evaporator Model CNPVP6024ACA

    Net MBH Total 57.0 Net MBH Sens 44.1
    SEER Rating 13.0 Rated CFM 1420
    Use one or two heat pumps with gas furnace as dual fuel.

    Gas furnace efficiency is below par for a primary heat source.
    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
    Oct 2007
    Location
    Washington, DC Metro Area
    Posts
    25
    Thanks.

  7. #7
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,328
    For new construction, the calcs sound high.
    Must have a lot of glass.
    Contractor locator map

    How-to-apply-for-Professional

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

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