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Thread: Math Quiz

  1. #1

    Post Math Quiz

    Please check my math...

    If my home losses 80000 BTUh, and I live in a region with 3784 Heating Degree Days, then I use 7,265,280,000 BTU per year.
    (BTUh x 3784 x 24)

    Which is 72652.8 Therms per year.
    (BTU / 100000)

    If I pay 113 cents per Therm, then my base cost without factoring AFUE is: $8209.77 per year.
    (Cents per Therm x Therms per year / 1000)

    This figure seems pretty high to me.

  2. #2
    Join Date
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    Quote Originally Posted by unclebingo View Post
    Please check my math...

    If my home losses 80000 BTUh, and I live in a region with 3784 Heating Degree Days, then I use 7,265,280,000 BTU per year.
    (BTUh x 3784 x 24)
    If you lost 80,000 BTUH every day for a year, you would only lose 700,800,000 BTUH (80,000 BTUH x 24 x 365). Are you at one of the poles?

    The figure (80,000) is what you house needs (at design temperature); so you will need to burn more gas to yield that quantity. Sorry.

    Perhaps one of the PROs has a formula for approximating usage based on HDDs.

    Amp
    Last edited by ampulman; 09-13-2009 at 05:33 PM. Reason: formatting

  3. #3
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    I use bin data. HDD's is too misleading.

    When its 64° for 24 hours, you will have had 1 HDD. Your not going to loss 80,000BTUs in 24 hours at 64° outdoor temp.
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  4. #4
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    Quote Originally Posted by unclebingo View Post
    If my home losses 80000 BTUh,
    at what inside/outside temp, averaged over 24 hours?

  5. #5
    I've yet to collect the HDD figures, and for the purpose of building the formula I was using default figures from www.hvacopcost.com for both HDD and BTUH loss.

    When I can gain access to the house I plan to perform a full Manual J to calculate the BTUH loss, and I'll calculate HDD per "beenthere's" post, i.e.

    Base Temp 65f
    Monthly 1% DBf
    Jan - 54 - 31 days - 341 HDD

    I'll only use monthly 1% DB averages so it's going to be an approximation.

    If I understand you guys correctly if (subject to ManJ) the furnace has to make up a 80,000 BTUH loss to maintain 70f inside, which totals 700,800,000 BTUH per year. (80,000 x 24 x 365). And wikipedia's formula to calculate annual kWh is correct, i.e.

    Area x U-value x HDD x 24/1000

    Then do I just multiply the yearly BTUH loss of 700,800,000 (Area x U-value) by the number of HDD's by 24 / 1000 = yearly kWh used

    Then I calc my cost per kWh?

  6. #6
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    The furnace only has to make up 80,000BTUs when the outdoor temp is at design.
    Which probably is only a few hours/days a year.

    When its 54 outside(11 HDD), it may only have to make up 13,538 BTUs an hour.
    But, even that is not a true statement. HDD does not take into account, solar gain. So at night time, it might loose 13,538 BTUs an hour, but during the day with the sun out. It might only loose 8,000BTUs an hour.
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  7. #7
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    heat loss times times degree days divided by (indoor design temp-outdoor design temp) x a correction factor, divided by system efficiency
    The way we build has a greater impact on our comfort, energy consumption and IAQ than any HVAC system we install.

    http://www.ductstrap.com/

  8. #8
    Quote Originally Posted by Carnak View Post
    heat loss times times degree days divided by (indoor design temp-outdoor design temp) x a correction factor, divided by system efficiency
    If I'm using monthly 1% DB data then outdoor design temp varies from month to month. Does this suggest I need to calc each month?

    What is the correction factor?

  9. #9
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    you use the design temperature difference, usually for a house the 97.5%

    the correction factor compensates, depends on your area.
    The way we build has a greater impact on our comfort, energy consumption and IAQ than any HVAC system we install.

    http://www.ductstrap.com/

  10. #10
    So the correction factor is only needed if I adopt a standard 97.5% design temp difference?

    I know that the Manual J spreadsheet I have automatically calc's the HTD for me when I enter the climate data. Does that mean I can omit the correction factor from the formula?

  11. #11
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    no

    The correction factor is sort of like an average of how long the furnace runs per hour over the entire heating system, if the furnace was sized spot on to the design heat loss

    So in a deep freeze it runs steady, maybe in mild winter weather it runs 10 minutes per hour

    It depends on where you live. Same lattitude you could be in the mountains, on the coast , maybe in the prairies, the geography plays a part


    you have to pony up and say where you live
    The way we build has a greater impact on our comfort, energy consumption and IAQ than any HVAC system we install.

    http://www.ductstrap.com/

  12. #12
    Cedar Rapids, Iowa

    But understanding the formula is more important to me than just getting you guys to tell me the answer.... I wanna learn!

  13. #13
    Join Date
    Feb 2007
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    You can get an accurate figure for HDDs and other info at:

    http://www.wunderground.com/history/...omHistory.html

    Type in your zip code, then follow the 'custom' table.

    Amp

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