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  1. #1

    Oversizing furnace

    I'm getting quotes to install a new furnace and one brand that I like does not have a unit that meets the 95% efficiency to take advantage of the tax credit. The contractor has suggested to install a larger unit (90,000 btu's rather than the 75,000 btu unit) to get higher efficiency and meet the IRS rules for the tax credit. I questioned him about this since I'ver heard to not oversize. He did, to his credit, note that it is not good to oversize but with multi-stage units it is not as critical with single stage units. His rational is as follows: since the unit is a multi-stage furnace, it would start at the low setting (70% of max output) and run until the house heats up. In other words, the two units would operate the same the majority of the time. When the demand is higher it would start at max output and heat the house more quickly than the lower efficiency unit. This makes sense to me but I'm still not totally convinced this is the correct thing to do. I save some money with the tax credit (it more than offsets the cost of the higher efficincy furnance) and I save a few dollars on my heating bill. Any comments????

  2. #2
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,613
    As someone with an oversized 2 stage furnace, I don't recommend it. You are really getting an expensive single stage furnace. Or if it is on a single stage stat, it will constantly cycle between high, low & off.

    I'd either: get a brand whose 95% is right for the house

    or get this brand in the right size and give up the $150.

    To me, not worth $150 to oversize, 2 stage or not.

  3. #3
    Join Date
    Jun 2005
    Location
    New England
    Posts
    906
    a furnace oversized for the ductwork can have a shorter life due to too much heat staying over the furnace with no place to go.
    BTW - why do you say it's oversized? Are you assuming your old furnace is right-sized? If it's an older furnace you don't really know the efficiency and can't say that.
    do a heat calc!
    /j

  4. #4
    My current furnace is 75,000 Btu and both contractors have measured the house and have determined that a 75,000 Btu furnace is the correct size.

    I have a quote for a Goodman that meets the high efficiency criteria and is a 75,000 Btu but the repair ratings (Consumer Reports 2/2005) are scaring me. From what I've read the installation is the main issue on having a good long-lasting furnace so maybe I'm over-reacting. Also, the repair ratings are 2-1/2years old and products change (for the better and worse).

  5. #5
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,613
    Which Goodman? Some don't allow for a 2 stage stat, they just guess based on past cycles and will not run long periods on low like those with 2 stage stats.

  6. #6
    The quote is for a Goodman GMH9070. The contractor would replace my thermostat with a new one. Not sure what you mean by 'stat'.

  7. #7
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,613
    The GMH cannot use a 2 stage thermostat (aka stat). It uses a built in timer that will bring on high in 1 to 15 minutes of running on low. This is the wrong furnace for oversizing. It will not run long periods on low like a "normal" 2 stage furnace with 2 stage thermostat will. If you really want this furnace, why not get the right size and get the 70,000 BTU unit? Or for REAL comfort, step up to the GMV with 2 stage thermostat so the furnace fires based on the needs of the house.

  8. #8
    The Goodman is sized properly (70,000 Btu and 95% efficiency). I guess that was not clear in the messages.

    It was the other brand, Rheem, that was suggested to be oversized. He did not mention a model of the Rheem.

  9. #9
    Quote Originally Posted by Dash Riprock2 View Post
    The Goodman is sized properly (70,000 Btu and 95% efficiency). I guess that was not clear in the messages.

    It was the other brand, Rheem, that was suggested to be oversized. He did not mention a model of the Rheem.
    Perhaps the experts here can confirm, but I think you can oversize a rheem because of it's fully modulating system. It will probably never run near the highest btu, but the oversize gives you a higher ton blower to move more air. Plus, it gives you room for future expansion to your home. I think..... Let's see if I get corrected.

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