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  1. #1
    Join Date
    Jul 2012
    Posts
    32

    Had Estimates.. What size?

    Hello All,

    Central Ohio, mid 70's house, a little over 2700 sf.

    Had 4 contractors come out. One sized based on the size of old system, 2 did short form manual j's based on the plans, only one actual measured the windows (but estimated insulation and air leakage).

    Want to go with 2 stage variable furnace and single stage ac.

    Everyone suggested 80k 95% furnace (most complete manual J recommended 100k at first based on a 78k heat loss, but once I mentioned he agreed). The two short forms are at 3 ton AC while the other is at 3.5 ton.

    I am most comfortable at the moment with the one who actually measured windows, his knowledge, and price (works with utilities to make sure I get all discounts, others didn't even mention), but am afraid once I make some improvements on the house I would only need 3 ton. He assures that I wouldn't be able to get down to 3, but how sure can he be estimating insulation and air leakage? My only concern is that he was quick to oversize furnace.

    Without seeing my house, what do you think about recommendations for my climate and my concerns? Am I overthinking the half ton? Do I need to keep getting quotes until I am 100% certain with one?

    Thanks for the help!!

  2. #2
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,151
    With a 78K heat loss I'd want the 80K 95%. But there are some who will say that is cutting it too close. They don't want to risk a customer not understanding why the house isn't toasty in freak bitter cold weather. So with that number, the 100K isn't out of line. I'm not sure I'd fault him for suggesting the bigger furnace. Same with cooling. Some are nervous cutting it close. They don't want to hear the house is warm in a summer like we just had - warmest since the dust bowl days. How cold do you keep it? Industry standard is to size to 75 inside. If you like it colder, better go up to the 3.5 ton. Otherwise, better humidity control cutting it close to the 3 ton mark.

  3. #3
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
    Posts
    4,265
    Unless you live north of the mason-dixon line 80k might even be oversized for 2700sqft. Furnaces are quick to be oversized because higher BTU furnaces aren't much more expense than small furnaces. Contractors are scared to death of undersizing furnaces, but in the real world have have yet to see a single undersized gas furnace. Most furnaces I run into are 2-3 times the size needed.

  4. #4
    Join Date
    Jul 2012
    Posts
    32
    I actually do live juuust north of it. It is just been very hard to figure out who and what brands to trust. Search just on here and you see plenty of people with differing opinions on every brand out there.

  5. #5
    Join Date
    Jul 2008
    Location
    Rochester NY
    Posts
    4,728
    Quote Originally Posted by 54regcab View Post
    Unless you live north of the mason-dixon line 80k might even be oversized for 2700sqft. Furnaces are quick to be oversized because higher BTU furnaces aren't much more expense than small furnaces. Contractors are scared to death of undersizing furnaces, but in the real world have have yet to see a single undersized gas furnace. Most furnaces I run into are 2-3 times the size needed.
    +1

    60k. 2.5 ton or 3 ton 2 stage. Never seen manual j not oversize. I bet 60k will shut off at worst case outdoor conditions, meaning still oversized.

    How would he have any idea "what you wouldn't be able to get down to..."? He ever crossed that line? Ever even come close to seeing it?

    If you have 2.5 ton, how many minutes a year will it be undersized? Can he predict that? And can he describe the catastrophic life threatening sailing of the edge of the flat earth that will occur? He's making WAGuesses, and attempting to sound certain of his crystal ball. Sounds like you went into this better educated then everyone who came to your house.
    Do you have any idea about hourly run times at worst case for heating and cooling with current equipment? You can reconcile load to that.

    Try doing your own modeling:


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    Password: gizmo

    Good luck,
    Pj


    (from discussion Linkedin )

    An early video walk through which may be a good place to start:


    Last edited by tedkidd; 10-16-2012 at 11:03 PM.
    Which makes more sense to you?
    CONSERVATION - turning your thermostat back and being uncomfortable. Maybe saving 5-10%
    ENERGY EFFICIENCY - leaving your thermostat where everyone is comfortable. Saving 30-70%

    DO THE NUMBERS! Step on a HOMESCALE.
    What is comfort? Well, it AIN'T just TEMPERATURE!

    Energy Obese? An audit is the next step - go to BPI.org, or RESNET, and find an auditor near you.

  6. #6
    Join Date
    Jul 2012
    Posts
    32
    tedkidd... I really appreciate your feedback. I have read a lot of your posts and really am impressed with your knowledge.

    Here is my issue, I am by no means a pro, but I feel like I know a decent amount from reading some of the knowledge on here. I keep expecting one of you to walk through the door, but it hasn't happened yet and I'm sure won't ever. Do I trust the pros here and pick the size I want with whoever I think will do the best install, even though the sizes I suggest will make them freak out? How do I deal with that inevitable conversation?

  7. #7
    Join Date
    Jul 2008
    Location
    Rochester NY
    Posts
    4,728
    Wow, thanks for the nice words. Most find my writing style abrasive.

    We are starting to see homeowner knowledge eclipse the "free quote" sales "designer" a lot here. They tend to waiver, fear is used against them, some cave and some don't. It's all good, 10 years ago consumers would have put pressure to go up a size, now they're on the other side. This is a fantastic thing.

    Tracking results will fix this problem. We are on the cusp. I digress, sorry.

    The universe seems to deliver for you. I ran across this just now:

    To determine heating capacity required for existing building one may simply extrapolate design load by graphing metered use to outdoor temperature for a series (preferably a minimum of 12) of billing periods. See HeatingHelp dot com > search Therm_lag (that's me) and scroll to "Graphical Load Estimating Method." The guide is attached to this reply. I use Etracker software (freeware) developed by Kissock at U Dayton.
    http://homeenergypros.lbl.gov/group/...g_com_gr_forum

    Where this stuff seems to go wrong is people fixate on one piece of information and conclude design based upon that. That's a Gantt chart failure. They don't step back and see if design disregards/disconnects from other critical issues.

    I see getting this right as using triangulation. Not relying on any one piece of information as gospel, but looking at the general direction various pieces point you in, drawing a circle at that intersection, then finding equipment that falls somewhere within that circle.

    You are fully capable of using Phil's software, and if you have a blower door number and your annual consumption that tool is fantastic. A great point of triangulation

    Another option - Shortcircuit hired me. Everyone was telling him to put in 100,000/4ton, he put in 60,000/2 ton. Now, a year later, he is extremely happy. He says 60,000 is oversized, but it's a mod and drops all the way to 20, so nbd. We knew 2 ton was aggressive but were concerned his duct couldn't handle 3 (he wasn't interested in duct replacement so we designed to the duct). 2 ton is not enough to recover at the record temps we had this summer, so he abandoned antiquated setback behavior.

    Setback as prescriptive, undiagnosed, energy saving with no M&V is blatant malpractice. I say; if it saves, prove it. Also, if it saves for you concluding it saves for everyone is an absurd assumption.
    Every house is different. What works in one can not be assumed to work in another. We need M&V.

    I track, nobody else seems to, and yet people like to tell ME "stupid thermostat tricks" that save energy!? Anyway, his bill is 1/2 his neighbors,
    what more can he save?! There are a lot of side benefits to abandoning setback. The house is so well dehumidified by the continuously operating equipment his wife complains it's too cold if the thermostat is below 76 - in Humid Baltimore.

    BTW, Randy is an engineer at the NSA.

    So, get this right and everything comes together in an amazingly elegant way. Comfort, energy bills, and long term since the equipment runs continuously instead of cycling, I suspect repairs will go way down also. I'm confident you are on the right path.
    Last edited by tedkidd; 10-17-2012 at 12:26 PM.
    Which makes more sense to you?
    CONSERVATION - turning your thermostat back and being uncomfortable. Maybe saving 5-10%
    ENERGY EFFICIENCY - leaving your thermostat where everyone is comfortable. Saving 30-70%

    DO THE NUMBERS! Step on a HOMESCALE.
    What is comfort? Well, it AIN'T just TEMPERATURE!

    Energy Obese? An audit is the next step - go to BPI.org, or RESNET, and find an auditor near you.

  8. #8
    Join Date
    Jul 2012
    Posts
    32
    I again really appreciate your help.

    You are definitely more aggressive than even most of the contractors on here, but given the results you have seen it seems to be the right call.

    The big question, how do I communicate this to contractors I am dealing with? I believe most if not all would be against going that much lower than a load calc in fear I will be screaming at them when it is a little colder or warmer than I have it set at. And what if I am wrong for one reason or another because of factors with my home?

  9. #9
    Join Date
    Jul 2008
    Location
    Rochester NY
    Posts
    4,728
    You combat them with information. Squeeze the WAGuessing out of the equation. Oversized is seen by most of the world as "safe" still. Look, these guys are used to taking too much responsibility for things that are beyond their control. Eventually they learn that when the customer makes the specification, responsibility for "perfection" no longer falls on their shoulders. But it's a long path between here and there.

    Run the load calc.

    Do you have a blower door cfm50 #? That number alone is a WAG for most load calcs, which is why people always seem to guess high.

    Do you have any airflow test numbers on your duct (design max airflow)? Duct layout? External Static Pressure with current equipment?
    Which makes more sense to you?
    CONSERVATION - turning your thermostat back and being uncomfortable. Maybe saving 5-10%
    ENERGY EFFICIENCY - leaving your thermostat where everyone is comfortable. Saving 30-70%

    DO THE NUMBERS! Step on a HOMESCALE.
    What is comfort? Well, it AIN'T just TEMPERATURE!

    Energy Obese? An audit is the next step - go to BPI.org, or RESNET, and find an auditor near you.

  10. #10
    Join Date
    Jul 2012
    Posts
    32
    I do not have any of those at the moment. I actually just got done scheduling an energy audit through my electric company for next week. That will give me some of that data and is required for a rebate through them. I am not sure how thorough they will be, but it at least includes the blower door test. Hopefully after that I would be much more informed of what I need to do.

  11. #11
    Join Date
    Jul 2012
    Posts
    32
    Also, is anyone else's thinking any different? I would love a few more opinions.

  12. #12
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,286
    Quote Originally Posted by jmon22 View Post
    Hello All,

    Central Ohio, mid 70's house, a little over 2700 sf.

    Had 4 contractors come out. One sized based on the size of old system, 2 did short form manual j's based on the plans, only one actual measured the windows (but estimated insulation and air leakage).

    The two short forms are at 3 ton AC while the other is at 3.5 ton.

    I am most comfortable at the moment with the one who actually measured windows, his knowledge, and price
    (works with utilities to make sure I get all discounts, others didn't even mention), but am afraid once I make some improvements on the house I would only need 3 ton.

    He assures that I wouldn't be able to get down to 3, but how sure can he be estimating insulation and air leakage?
    My only concern is that he was quick to oversize furnace.

    Without seeing my house, what do you think about recommendations for my climate and my concerns? Am I overthinking the half ton?
    Do I need to keep getting quotes until I am 100% certain with one?

    Thanks for the help!!
    SO, be My Eyes.
    ORIGINAL Windows ?
    How many windows/ direction /type?
    N __
    S___
    E___
    W__
    about 6 windows = 90 Sq Ft. each side
    Total ~ 340 Square feet ?

    Infiltration _Loose _ is going to give a High Estimate.

    R-11 Walls
    R-30 Ceiling
    2 story ? _ 30' x 48' ? 8 foot ceiling height
    No basement?

    Above info is needed to independently verify "What Size?" at ~ +/-15%.

    Manual J Short form is rather easy to calculate a "block load" ( overall house losses versus losses Per Room).
    80k furnace is more than enough but you are not likely to be able to get down to a 60,000 BTU/HR one.
    However, I would be suprised if heat load might be ~67,000 ( ~11,000 less than you 78k)
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  13. #13
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,286
    Quote Originally Posted by tedkidd View Post
    Wow, thanks for the nice words. Most find my writing style abrasive.

    The universe seems to deliver for you. I ran across this just now:



    You are fully capable of using Phil's software, and if you have a blower door number and your annual consumption that tool is fantastic.

    [/B]Every house is different. What works in one can not be assumed to work in another. We need M&V. [B]
    When abrasive, just use ... 600 grit instead of 120.
    Any abrasions will heal quickly.
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

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