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Qh = U x HTDh x Area
Where:
Qh = Heat flow out of surface, Btuh
U = U-Value, or heat transfer coefficient from Table 2 or 4A Manual J 8, Btuh/ft²·°F
HTDh = Winter temperature difference (inside temperature - outside temperature), °F
Area = Net area of surface, square feet (ft²)
Ref: Manual J 8 - Page 6-1, 7-1
Note: Heating HTM values, defined by the equation HTMh = U x HTDh, are determined from the winter temperature difference, material type, and insulation.