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Calculating cost to heat house using R-values

  1. Nov 12, 2012 #1
    1. The problem statement, all variables and given/known data

    Consider a flat roofed rectangular house that is 10.0 feet high with 39.0 feet long sidewalls and 20.0 feet long front and back walls.

    We are given that:
    Wall: (R Value =0.20 plaster)+(R Value=10.00 insulation)+(R Value=0.60 plywood)+(R Value=0.50 cedar shingles).
    Roof: Same as walls except R Value=31.00 insulation.
    All R values are in units of ft2*oF*hr/Btu

    If average winter indoor and outdoor temperatures are 69.0oF and 18.0oF respectively and the furnace produces 105 Btu/gal of oil at $4/gal, what is the monthly heating cost in dollars? (Use 30 days/month and Do not enter units)

    2. Relevant equations

    P=(1/R)*A*/\T

    3. The attempt at a solution

    P=(1/Rfront wall, back wall)/\T*(Afront wall+Aback wall)+(1/Rroof)*/\T*Aroof
    =3894.4 Btu/hr
    multiply by 24 hours, then by 30 days=2803968 Btu
    Divide by 10^5 Btu/gal
    multiply by $4/gal
    =112.16
     
  2. jcsd
  3. Nov 12, 2012 #2

    haruspex

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    I think you've counted only one wall in each direction instead of a pair.
     
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