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Thread: Size Matters...

  1. #1
    Join Date
    Feb 2013
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    Size Matters...

    Hi Everyone,

    I have an '88 or '89 Lennox Gas 165,000 Btu furnace that died. I know this one is too large* by today's standards. Everything I've read says you don't want a furnace too large or too small, but they don't say anything about what is the acceptable range of size. Practically speaking, there must be a target range of what is the right size.

    I have estimates from qualified and long-experienced contractors who are quoting a new furnace from 75,000 btus on the low end to 115,000 btus.

    There does appear to be something of a judgement call in load calculations, at least the ones that you get when receiving an estimate. If time and money were infinite (for contractors or consumers), I suppose the exact number for a house could be achieved, but that is not reality.

    *So here are my house stats: 1600 sq. ft total, approx 900 in basement. House was built in the 1940s, 2 story, frame and brick. Single pane windows with storm windows. House is insulated, average+ (but not superior--there's only so much you can do on an existing vintage house). Located in Denver, CO.

    So what are your thoughts on what size is "too small" of a furnace, and we'll be cold, and what is "too big"? Are all of these furnaces "right sized" because a range that wide is still fine?

    And if that wide of range is too wide, what should I ask these contractors, or how should I handle the discussion (particularly with one I'd like to work with), if I'd feel better if the btus should be "zeroed in on" more before install, without getting their fur up like I'm questioning whether they did the calc or rule of thumb right or it should be rechecked or telling them how to do their job? (I don't know how to do their job, just trying to do mine as a homeowner doing research.)

    Appreciate your expertise/advice,
    Mark

  2. #2
    Join Date
    May 2000
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    Most dealers won't undersize, they don't want the call when the furnace doesn't keep up. Those that guess usually guess big to be safe. I'd ask them all if they are guessing or did a load calc. This is your comfort and gas bill at stake.

  3. #3
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    Feb 2013
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    Quote Originally Posted by BaldLoonie View Post
    Most dealers won't undersize, they don't want the call when the furnace doesn't keep up. Those that guess usually guess big to be safe. I'd ask them all if they are guessing or did a load calc. This is your comfort and gas bill at stake.

    Thank you for responding. I'm not sure how to respond though, frankly. I can't imagine anyone would admit to guessing; everyone at least knows the sq. ft, so they could use a rule of thumb, which may not be preferable, but it isn't guessing.

    Further, technicians with substantial and substantive est/install hvac experience probably know what btu furnace to put into a house just be walking through it and observing, and probably be correct without actually doing a load calc, right?

    If I ask to see the load calc data, what am I'm looking for to tell if the high btu guy is more correct than the low btu guy?

    And some of it seems to be a judgement call, with respect to air leakage, etc., so it's not all based on undebatable numbers that go into a formula, right?

    MarkJJ

  4. #4
    Join Date
    May 2008
    Location
    Missouri
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    3,421
    That old furnace has about 100k output, so I'd guess they are using that as a guide to size the new furnace. But to be on the safe side, simply request that they do a manual J figure and see what they come up with. Oversizing a +90% furnace will greatly reduce the efficiency of the new furnace (along with reducing the life).

  5. #5
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    Feb 2008
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    Altmar, New York, United States
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    Quote Originally Posted by markjj View Post
    Thank you for responding. I'm not sure how to respond though, frankly. I can't imagine anyone would admit to guessing; everyone at least knows the sq. ft, so they could use a rule of thumb, which may not be preferable, but it isn't guessing.

    Further, technicians with substantial and substantive est/install hvac experience probably know what btu furnace to put into a house just be walking through it and observing, and probably be correct without actually doing a load calc, right?

    If I ask to see the load calc data, what am I'm looking for to tell if the high btu guy is more correct than the low btu guy?

    And some of it seems to be a judgement call, with respect to air leakage, etc., so it's not all based on undebatable numbers that go into a formula, right?

    MarkJJ
    they may not admit it. they will say,"they have been doing this long enough they just know" or " we can only buy certain sizes anyway". i get a few jobs with prior bids like this.

  6. #6
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    May 2000
    Location
    Indianapolis, IN, USA
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    33,886
    Rule of thumb is guessing. A house with 1500 sq ft and single pane glass may need 25000 btu per 500 sq ft. The same house with great windows may need 20000. Then the same house with good windows and R19 in the attic may need 18000 per. So rules of thumb are worthless. In my vinyl village, there are about 4 floorplans. So, yes, I know what a house needs with measuring every one. But older homes that aren't cookie cutter and have varying upgrades in windows, insulation, etc. should be measured. I guess it depends upon how bad a dealer wants a job. I've sold jobs and was told I got the work because I measured the house and did the math while the others didn't. Got one job because HO was impressed I measured his house in a rainstorm. He said that told him I cared about doing it right.

    That old furnace is 132K output. Probably could do with about 1/2 that! But then I'm guessing, like anyone else who doesn't do a Manual J.

    Try: http://www.buildersheating.com/about-us.html

  7. #7
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    Feb 2013
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    Quote Originally Posted by snupytcb View Post
    they may not admit it. they will say,"they have been doing this long enough they just know" or " we can only buy certain sizes anyway". i get a few jobs with prior bids like this.

    Yes, I got the "I've been doing this for a long time," a lot; in fairness, I only chose contractors who HAVE been doing this 20 yrs or more, and with good reputations. It looks like they use sq footage. I don't know if they do more in the office for a more thorough calc before writing an estimate.

    Only 1 guy measured every room in the house; but then when I asked about whether he used an online load calc program, he said "I have my own calculations and program that I wrote." So no way of telling what exactly he's using. All the others used square footage from what I can tell.

    BTW, CORRECT SQUARE FOOTAGE numbers from my first post: 1600 sq ft ABOVE GROUND; 650 sq ft BASEMENT.

    I'm still confused about what do to about this. I need to choose a contractor within about 24 hrs. How do I ask for load calc data and what am I looking for?

    Thanks.
    Mark

  8. #8
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    Sep 2009
    Location
    Arnold mo
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    3,928
    Don't make the mistake of taking somebody's word based on "I've been doing this for 30 years" nonsense. Countless contractors have been doing things wrong for many years, and it doesn't make it right. I could tell you stories. Get somebody trained in building science.
    An answer without a question is meaningless.
    Information without understanding is useless.
    You can lead a horse to water............
    http://www.mohomeenergyaudits.com

  9. #9
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    Apr 2011
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    West of DFW
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    240
    "I want to see the manual J you used to size the equipment I need and will the duct system support the equipment the load suggests."
    That's how you ask!


    Lets go, burnin daylight.

  10. #10
    Join Date
    Feb 2013
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    15
    Quote Originally Posted by BaldLoonie View Post
    Rule of thumb is guessing. A house with 1500 sq ft and single pane glass may need 25000 btu per 500 sq ft. The same house with great windows may need 20000. Then the same house with good windows and R19 in the attic may need 18000 per. So rules of thumb are worthless. In my vinyl village, there are about 4 floorplans. So, yes, I know what a house needs with measuring every one. But older homes that aren't cookie cutter and have varying upgrades in windows, insulation, etc. should be measured. I guess it depends upon how bad a dealer wants a job. I've sold jobs and was told I got the work because I measured the house and did the math while the others didn't. Got one job because HO was impressed I measured his house in a rainstorm. He said that told him I cared about doing it right.

    That old furnace is 132K output. Probably could do with about 1/2 that! But then I'm guessing, like anyone else who doesn't do a Manual J.

    Try: http://www.buildersheating.com/about-us.html

    Hi BaldLoonie,

    Builders Heating did come out and they didn't measure. In fairness, since it's the first time I've gone through this, I didn't ask for him to do it for the estimate; he also didn't start measuring rooms or ask to do so. Their recommended btu size is next to the high of 115,000.

    It's sort of the reason for the question, because that co. with loads of experience/history is at the high end, others are in the middle, and one at 75,000.

    Are you affiliated with Builders Heating or know people there?

    Mark

  11. #11
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    Feb 2013
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    15
    Blazey10 and everyone,

    Thank you, this is helping.

    So after I've said Blazey10's statement about manual J and duct system support, and they do the msrmts, calcs, and present the report(s), anyone could show one to me, I think, and snowball their way through it that it says so and so and a newbie such as myself wouldn't know the difference, right or not?
    Can anyone email me a filled out example of one of these reports off line, or direct me to where I can see one of these online that has a manual j and duct system support report explanation for complete newbies, so I can educate myself?
    Mark

  12. #12
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    Feb 2013
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    ...Or I don't suppose there is a free online manual J questionairre somewhere that does the calcs that homeowner can fill out?
    Mark

  13. #13
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    Oct 2010
    Location
    Anderson, South Carolina, United States
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    6,338
    Not free, but www.hvaccalc.com they're also some quick load calc apps on android and App Store. Hvac buddy is one among others.

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