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can't remmy the numbers

Hello fellow wallies and lurkers, I can't remember where I see or read this, there was a ballpark btu numbers given for sqaure ft in home with forced air system. Knowily we all are to do the heat loss in given homes. As I'm a boiler guy may get this forced air replacment in crawlspace. Old furnace have cracked exchanger and the 120,000 btu is way too big for 1670 sq feet home... Thanks for all ur inputs.

Comments

  • Mark Eatherton
    Mark Eatherton Member Posts: 5,853
    Magic numberz...

    10, 20 and 30.

    10 for newer highly insulated real tight construction.

    20 for middle of the road

    30 for old brick, really draft homes.

    All rules of thumb, and can get you into trouble in certain (high glass) areas...

    ME
    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • Brad White
    Brad White Member Posts: 2,398
    I would have said

    20, 30 and 50, at least here in MA.

    I have some older, free-standing uninsulated masonry homes with over 60 BTUH/SF with a 0-degree ODT and 68 degree IDT. Have to define those.

    SIP construction numbers I do not have direct experience so will defer to those that have them. I have heard 10-15.

    One I just completed calculations and a report on an 1886 uninsulated brick and Tudor gambrel which worked out to 47.11 BTUH/SF with infiltration by blower door test (30% of that number was infiltration). By insulating the "soft structure" and then sealing the obvious leaks, we could get this down to 36 and 28 BTUH/SF respectively.

    An uninsulated brick row house might come in at 35-40 if in the middle of the block and about 5 BTUH more PSF if the "end of the loaf". Insulated, we can get these down into the low 20's.

    My own house is 48,000 BTUH and is 1500 SF so works out to 32 PSF. This for an improved 1873 balloon framed house insulated at the attic floor line.

    My Susan's is 46,000 BTUH and 1650 SF so 27.9 PSF for a 1922 gambrel, also improved. (Does not include an uninsulated rear enclosed but heated porch, egads.)

    These numbers for our houses are coming down incrementally as we identify leaks and holes in the bucket, but they are representative.

    On a separate subject (which I will post as a separate thread once I pull together my spreadsheet) is to collect and maintain a representative database of homes all over the Wall geographic area, with BTU/SF, Degree-Day data and other common factors to get a benchmark (BTUH/SF/DD for example) figure for each.

    But as you said, Mark- these are rules of thumb, useless for calculations but derivative of heat loss calculations actually made in their climates. "Just how they work out".

    Respectfully,

    Brad
    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"



    -Ernie White, my Dad
  • jp_2
    jp_2 Member Posts: 1,935
    rule of thumb, no outside temp?

    kind of silly and meaningless don't you think?

    southern ohio or northern minnesota?

    I know one builder that works for 10-15.



    added: by the way, my cabin 60 btu's /sq ft...
  • 5,7, &10 in the deep south.

    Not enough info for a generic formula that covers all climates.

    Noel
  • thanks to all

    Thanks to all for responding... My bad as I should have put down the "0" degrees day for Chicagoland design temp with wood frame home with average windows...
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,853
    Reality versus theoretical...

    Brad, I'm sure you've probably done this, but how often do you go to a project you actually did the prescribed design work on, at design condition, and see your physical plant running at 100 % of capacity.

    Closest I've seen is a 50% duty cycle, except with modcons, in which case it is dead nuts on! (modulating to the load)

    The numbers I'm quoting are based on actual energy consumed at design conditions (70 degree DT here in Denver). Like you, the theoretical usually comes in quite a bit higher.

    With even more respect, respectfully yourn.:-)

    ME
    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • Brad White
    Brad White Member Posts: 2,398
    Hi Mark!

    A whole 'nuther subject but one worth touching on.

    In the days of old when homes were cold and Knights were not invented.... " Ahem... ;)

    As you implied, the older systems ran that way, too far over-sized to ever remain running even in the coldest weather. That to me is not an indicator of heat loss (unless as you say, the boiler keeps running on a design day, maybe shutting off for a minute just to say, "Damn, I am good!".

    My numbers have a certain "look-back" aspect to them. The homes nearest and dearest to me (my own, my Susan's, my sister's and certain select friends) give me the luxury of comparing fuel bills over time. Not perfect in that the operating parameters differ (Susan works at home two days a week, my sister goes to the office 5 days per week, different setpoint temperatures, etc. etc.). Then there is the system efficiency aspect, often a guess but it is "the variable that remains" once I back-out the other factors.

    I also back-out the two lowest months' therms from all heating months to subtract domestic HW usage. (Not a small number with three girls aged 7, 11 and, ok, 47!)

    The infiltration is a guess also, but now that I have a blower door, I am gathering and revisiting a few things...

    Degree-Day data I try to get as "current" for the nearest locale. This is also a guess as it can vary 15% from the nearest certified locale. My own house faces NW and has a sliver of solar gain, generally from early to mid-afternoon. My sister's and Susan's see sun from late AM to late PM...

    So- not perfect but with at least some methodology to it.

    You can see why I want to begin assembling a database from Wallies of their systems, sizes and if known, usage.

    My hat is off to you, Mark. But with the advancing cold and receding hairline, it will not be off for long, hope you understand :)

    Folically yours,

    Brad
    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"



    -Ernie White, my Dad
  • Weezbo
    Weezbo Member Posts: 6,232
    :)

    it is a good thing you do heat loss...guessing at it increases the waste of energy :)
    it is way better to waste the lead in a #2 pencil than a tank of #2 fuel oil :)

    not all systems are "designed "the same a quick hop to HVAC.com wall of shame would convince any thing of that fact.

    in some of the mech rooms on display the very walls floors and Paint cries out'

    ''Hey! you ever heard of an eraser and pencil?''
  • Boilerpro_5
    Boilerpro_5 Member Posts: 407
    Hi Doc

    I'm in your climate and Mark's numbers line up with my experiences. My own 1906 2800 sq ft two story with 700 sq ft of glass and R-19 walls, R-32 ceiling, and aluminum stomr on weatherstripped original windows runs about 17 btu/ sq ft.

    Boilerpro

    To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,853
    We, the folically challenged....

    I went and got a hair cut yesterday. The girl got done, handed me a hand mirror so I could look at the back of my head to makes sure she'd done it to my liking. I GASPED and said "Someone stole the hair off the back of my head!!" She laughed and asked me to look under my pillow...

    Sure enough, there is a pile of hair at the base of my pillow every day. I thought my WIFE was losing the hair, NOT ME!! It's hell to get old...

    Can't see, can't pee, can't keep MY hair on ME :-)

    And I agree, a real time energy spread sheet would be an interesting thing to have. Not sure what the value of it would be, but would still be interesting to see.

    Wyoming would probably look the worst. They use log chains on telephone poles up there for wind speed and direction. If the log chain is sticking straight out, it's considered a breeze. If the links are snapping off the end of the chain, the wind is blowing :-)

    Wait a minute, no one's crazy enough to try and live in Wyoming are they :-)

    ME
    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
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