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  1. #1

    Size question !!

    Lets say I had a 75,000 btu furnace installed in Pittsburgh pa. If a manual J was done and it stated a 60,000 would have been enough how badly oversized is this furnace.

    I have a Rheem RGRK 2 stage furnace 75,00 installed and my house is 1800 sq feet living space. my neighbor is 1900 sq ft and recentlygot a trane xv 90 at 80,000 btu. Does my size sound right. I know a manual J is the only true way but looking for input. 4 contractors came to my house and estimate 70-80,000 depending on brand, bu tone said 60,000.
    thanks

  2. #2
    Join Date
    Nov 2007
    Location
    Durham,N.C.
    Posts
    117
    hi sleep different furnaces offer different btu outputs,some brands dont offer 60,000,some jump from 50 straight to 75 so i would not be concerned about that.One thing you should consider is a humidifier.

  3. #3
    Join Date
    Jun 2004
    Location
    4H: Hot, Humid Houston H.O.
    Posts
    3,304
    Quote Originally Posted by SLEEPSTUDIER View Post
    Lets say I had a 75,000 btu furnace installed in Pittsburgh pa. If a manual J was done and it stated a 60,000 would have been enough how badly oversized is this furnace.

    I have a Rheem RGRK 2 stage furnace 75,00 installed and my house is 1800 sq feet living space. my neighbor is 1900 sq ft and recentlygot a trane xv 90 at 80,000 btu. Does my size sound right. I know a manual J is the only true way but looking for input. 4 contractors came to my house and estimate 70-80,000 depending on brand, bu tone said 60,000.
    thanks
    I am not a pro but let me tell you a homeowner's experience in S.Texas. As you can guess, our winters are generally short and mild, however our AC needs require that a certain size furnace be present for enough airflow in summer. My own house Manual J told me 85,000 btuh would be sufficient for heating at design temperature. However the AC installed is 6.5 tons in two units, so we are looking for 42,500 btuh output furnaces. Within the Trane line, I don't think such small units are even available in a furnace that can support 3.5 tons AC. So oversizing 50% to 128,000 btuh output is pretty much forced on me by the manufacturer. In my case two 80,000 input furnaces at 80% efficiency. That is much more than the 25% oversizing in your example, and there are no ill effects I can see.

    It sure sounds like you are wanting to size by square footage and comparison with your neighbor. Some of this questioning could be answered by having your own Manual J heat model done. I've done it myself with Hvac-Calc from this site, it's only $XX and pretty easy. Expect to spend an hour or two measuring things such as your walls and windows, that's pretty much the hard part.

    I've studied as best I can the reasons for right-sizing, and the strongest reasons apply to single stage AC and not heating. It would be more elegant to have just the right size furnace, but I submit there are other problems that should be worked more urgently. While I don't know the needs of the Pittsburgh area, it sounds like your contractors are largely in agreement, and there is not a lot to worry about.

    Hope this helps -- Pstu

  4. #4
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    33,907
    Usually those that guess, guess big to be safe. That's why so much out there is way oversized. You'll spend most of your time on low which helps iron out being oversized. Still would have been nice if the job was done right. Hope you have a 2 stage stat or you'll be constantly cycling with the timer.

  5. #5
    Join Date
    Sep 2001
    Location
    kalamazoo,mich
    Posts
    2,174
    pstu, some furnaces are available with either a 3 or 4 ton drive. It just depends on the Brand.
    Have you hugged the Earth today?
    Donny Baker rules

  6. #6
    Join Date
    Feb 2003
    Location
    S.W. PA
    Posts
    3,298
    75k is fine if you need 60ish and you have 75 X .93% ? so you have 69000 output (if your lucky) that would be the perfect choice for your house assuming that your numbers are right

  7. #7
    Join Date
    Aug 2004
    Location
    North Richland Hills, Texas
    Posts
    14,914
    Quote Originally Posted by SLEEPSTUDIER View Post
    Lets say I had a 75,000 btu furnace installed in Pittsburgh pa. If a manual J was done and it stated a 60,000 would have been enough how badly oversized is this furnace.

    I have a Rheem RGRK 2 stage furnace 75,00 installed and my house is 1800 sq feet living space. my neighbor is 1900 sq ft and recentlygot a trane xv 90 at 80,000 btu. Does my size sound right. I know a manual J is the only true way but looking for input. 4 contractors came to my house and estimate 70-80,000 depending on brand, bu tone said 60,000.
    thanks
    A 75k btuh 80% furnace has about 60k btuh of heating capacity, a 80k btuh furnace would have about 64k btuh capacity, so if your load calc says you have 60k btuh heat loss at design conditions, that 75 or 80k furnace is right on.
    Being a little over isn't going to cause any problems, so a 75k or 80k 90% furnace would still be just fine.
    If more government is the answer, then it's a really stupid question.

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