The Degree-Day Formula
The degree-day formula was devised some 40-odd years ago by the American Gas Association and other groups and has since been revised (see pages 627-28 of the 1970 ASHRAE Guide) to reflect
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Table 4-7 Reduction in Standard Degree Days for All Areas Except the Pacific Coast
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Courtesy National Oil Fuel Institute |
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Table 4-8 Reduction in Degree-Day (DD) Base When Calculated Heat Loss (Btu per Degree Temperature Difference) is Less Than 1000
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Table 4-9 Using Corrected Heat Loss Formula for Determining Comparative Fuel Requirements
Assume 500 Btu/h per degree temperature difference (35,000 Btu/h at 70°F difference); degree days (dd) with 65°F base, 5542; average indoor temperature 73°F.
Correction factor from Table 4-7 is 4.60%.
From Table 4-5, revised degree-day base is 62°F.
Reduction in base is (65 — 62) 3°F.
Multiply 0.0460 X 3 = 0.1380 and deduct from 1.0000 = 0.8620. Multiply 5542 (dd at 65° base) by 0.8620 = 4557.
Example:
No. 2 Oil:
35 X 0.00304 X 4557 = 485 gallons per year Natural Gas:
35 X 0.00429 X 4557 = 684 therms per year
Courtesy National Oil Fuel Institute
Internal heat gains and the levels of insulation. The correction factors involved in the revision are given in Tables 4-10 and 4-11. These correction factors should also be applied to the corrected heat loss formula (see the previous section).
The degree-day formula is based on the assumption that heat for the interior of a house or building will be obtained from sources other than the heating system (e. g., sunlight and body heat of the occupants) until the outside temperature declines to 65°F. At this point the heating system begins to operate. The consumption of fuel will be directly proportionate to the difference
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Table 4-10 Unit Fuel Consumption per Degree Day per 1000 Btu Design Heat Loss
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Table 4-11 Correction Factors for Outdoor Design Temperatures*
Outside Design Temp., °F
-20 -10 0 +10 +20
Correction factor 0.778 0.875 1.000 1.167 1.400
*To be applied to Table 4-1 when design temperature is higher or lower than 0‘F. Courtesy National Oil Fuel Institute
Between the 65°F base temperature and the mean outdoor temperature. In other words, three times as much fuel will be used when the mean outdoor temperature is 35°F than when it is 55°F. The mean outdoor temperature can be determined by taking the sum of the highest and lowest outside temperatures during a 24- hour period, beginning at midnight, and dividing it by 2. Each degree in temperature below 54°F is regarded as 1 degree day.
The degree-day formula is applied by dividing the heat loss figure by 1000 and multiplying the result by the figure for the unit fuel consumption per degree day per 1000 Btu design heat loss, which, in turn, is multiplied by the total number of degree days in the heating season (calculated on a 65°F base) and then by the correction factors given in Tables 4-10 and 4-11. The application of this formula is illustrated in Table 4-12.
Table 4-12 Application of Degree-Day Formula
35,000 Btu/h loss; outside design temperature 0; no correction factor needed; degree days, 5542.
No. 2 oil, 140,000 Btu per therm, 80% efficiency:
35 X 0.00304 X 5542 = 590 gallons per year Natural gas, 100,000 Btu per therm, 80% efficiency:
35 X 0.00429 X 5542 = 832 therms per year Electricity, required Btu/year
832 (therms) X 100,000 (Bru/therm) X 0.80 (efficiency)
66,560,000 = 66,560,000 = 19,502 kWh
KWh = heat loss 3413
X annual degree days X constant (usually 18.5) divided by difference between indoor and outdoor design temperature. Example:
35,0 Btuh
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3413 1030 X 5542 X 18.5 |
= 1030
= 15,086 kWh
70 — 0
Courtesy National Oil Fuel Institute
Posted in Audel HVAC Fundamentals Volume 1 Heating Systems, Furnaces, and Boilers