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  1. #1
    Join Date
    Nov 2008
    Posts
    2

    Default Furnace sizing question

    I have a question about furnace size.

    I am about to get the underside of my roof insulated with 6 inches of open cell foam. This is supposed to cut my heating bills by about 40%. (I have a Cape built in 1952 and Capes are notoriously leaky.) My question is whether this insulation change will be significant enough to warrant a smaller furnace.The house is about 1800 sq. ft. and has virtually no insulation in the walls (ok, there was originally 2 inches of rock wool insulation but that settles after 50 years, so call it no insulation!) I live in Rochester, NY which is pretty cold. It's 32 degrees now! The existing furnace is an 80,000 BTU single stage gas furnace and is 12 years old.

    Will the added insulation make enough of a difference that a smaller furnace will be appropriate? If so, how much smaller would be right for this size house? What is the consequence if one has a 'too large' furnace? And how much too large does a furnace have to be to cause a problem or significant inefficiency? (I.e., is something like 20,000 extra BTU no big deal?) I am very lucky that I will actually have to pay only half the cost of the furnace and installation, so initial cost is less of an issue than it would normally be.

    Also, one furnace guy told me that the choice between a 90% efficient single stage furnace or a 90% efficient 2 stage furnaces is mainly a comfort issue, not an issue of operating cost savings...Is that true?

  2. #2
    Join Date
    Aug 2007
    Location
    The Great White North
    Posts
    4,045

    Default Re: Furnace sizing question

    Quote Originally Posted by Mary S View Post
    I have a question about furnace size.

    I am about to get the underside of my roof insulated with 6 inches of open cell foam. This is supposed to cut my heating bills by about 40%. (I have a Cape built in 1952 and Capes are notoriously leaky.) My question is whether this insulation change will be significant enough to warrant a smaller furnace.The house is about 1800 sq. ft. and has virtually no insulation in the walls (ok, there was originally 2 inches of rock wool insulation but that settles after 50 years, so call it no insulation!) I live in Rochester, NY
    I'm trying to see if I read this correctly ..... you will have the foam installed behind the sloped ceiling ?
    If so you will have approx. R24 upstairs in the ceiling but very low R value in the rest of the home ..... I highly doubt you'll see a 40% reduction in your heating bills .......it will help the comfort level upstairs.
    Doubtful this would have any signifigant impact to warrant a reduction in furnace size.


    The existing furnace is an 80,000 BTU single stage gas furnace and is 12 years old.

    Will the added insulation make enough of a difference that a smaller furnace will be appropriate? If so, how much smaller would be right for this size house? What is the consequence if one has a 'too large' furnace? And how much too large does a furnace have to be to cause a problem or significant inefficiency? (I.e., is something like 20,000 extra BTU no big deal?) I am very lucky that I will actually have to pay only half the cost of the furnace and installation, so initial cost is less of an issue than it would normally be.
    There's no way anyone over the internet can say what size of furnace would be correct .... you need to have a Manual J heat load evaluation done .
    Chances are you may end up with the same sized unit if it was sized correctly .... depends on the load calculation.

    Going to a larger size serves no purpose ... in fact has negative impacts. A larger unit will heat the space too quickly and short cycle often instead of running the optimum duration and frequency..... short cycling will wear the furnace out quicker.
    A larger furnace will also put out a higher volume of air and likely your duct work wouldn't be correctly sized for this creating velocity noise.

    Also, one furnace guy told me that the choice between a 90% efficient single stage furnace or a 90% efficient 2 stage furnaces is mainly a comfort issue, not an issue of operating cost savings...Is that true?
    This is true .... they are both 90% units .... the 2 stage has better control for comfort.
    "" an ounce of perception -- a pound of obscure "
    - Rush

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