CDD (cooling degree days) and HDD (heating degree days) are used in estimating the energy used to cool buildings in the summer and heat them in the winter. Fuel consumption is proportional to the number of cooling/heating degree days.
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A base of 65°F is often used when computing heating degree days. If the daily average temperature was recorded as 58°F on a winter day, how many HDD would be used to heat your home?
The difference between 65° and 58° is 7° (65 - 58 = 7).
So, this would be a 7HDD.