Questions About Furnace Sizing
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  1. #1
    Join Date
    Sep 2007
    Posts
    9

    Questions About Furnace Sizing

    Hi All,

    I am getting a number of quotes for a new furnace, for a home I just moved into last fall. My current furnace is a VERY old 100kBTU, and is extremely over-sized. My house is 1200 sq. ft., plus an un-finished, un-heated basement.

    Most contractors seem to be quoting me furnaces of either 60 kBTU or 70 kBTU, but based on my gas bill and a couple calculations I don't think I need anywhere near that large. I requested a Manual J from them, and one provided it, showing a heat load of 49.3 kBTU for a 0 F day.

    I don't have a lot of data for my house, not having lived here long, so I talked to my best buddy next door who has the same floor plan house, plus a conditioned/finished basement and a ~300 sq. ft. addition on the main floor. Based on his gas consumption, local monthly average temperatures, and baseline summer gas consumption, I am getting a heat loss for his house between 0.30 and 0.36 kBTU/h per 10F temperature difference, assuming his 10+ year old furnace is 80% efficient. For a design -10F day (I am in Michigan) that works out to a heat load between 24 and 29 kBTU.

    Should I just bring up this point with the contractors? It seems like most furnace manufacturers don't make gas furnaces below 60 kBTU, so it doesn't seem like I have a lot of options anyway. Any other suggestions?

  2. #2
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    33,817
    There are definitely furnaces below 60K input. Are you looking at 80 or 95%? Will you be making any improvements to the home that will reduce the heat loss? How's attic insulation? If weak, blowing more in is cheap and can cut the size of the equipment notably.

  3. #3
    Join Date
    Sep 2007
    Posts
    9
    Quote Originally Posted by BaldLoonie View Post
    There are definitely furnaces below 60K input. Are you looking at 80 or 95%? Will you be making any improvements to the home that will reduce the heat loss? How's attic insulation? If weak, blowing more in is cheap and can cut the size of the equipment notably.
    I am looking for 95% efficiency. I just bought the house, so I have a few improvements planned over the next few years, insulation included. The attic insulation at the moment is extremely poor, but there is also very little ducting going to the second floor. For the most part the main floor of the home is the only area that his heated, though once I have improved attic insulation and replaced the rather drafty second-floor windows, that may change. I will also eventually finish the basement, including wall insulation and ducting. The house I modeled that heat calculation off of also has a finished, heated basement.

    EDIT: The only sub 60k furnace I've seen so far is a Lennox 45k 95% dual-stage. I'd originally planned to get a modulating furnace, and would still be interested in one if I could find one in the 45 kBTU range. Does anyone know of any options for multi-stage or modulating gas furnaces in that output range?

  4. #4
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    33,817
    Rheem also has a 45K 2 stage 95%. ICP, Bryant & Carrier have a 40K 2 stage. Goodman's 2 stage 95% goes down to 46K. So there are options, just no modulators.

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