CALCULATION OF HEATING DEGREE DAYS
Heating Degree Days (HDD) for a particular climate is obtained by subtracting each day's mean outdoor dry bulb temperature from the balance point temperature; this result is the number of HDDs for that day. For example, if the maximum and minimum outdoor dry bulb temperatures of a place were 80°F and 20°F respectively, and the balance point temperature were 65°F, then HDD of the place for that particular day would have been 65-[(80+20)/2] = 15. If the mean outdoor dry bulb temperature is equal to or higher than the balance point temperature, then the HDD would be equal
to 0.
Degree Days and Annual Heating loss
A preliminary estimate of annual heating load, using degree day method, can be obtained by the following formula:
H = PHL x 24 x HDD /▲T
Where
• H = Annual heating load in Btu
• PHL = peak heating load (heat loss) in Btu/hr
• HDD = heating degree days
• ▲T = temperature difference, °F
CALCULATION OF COOLING DEGREE DAYS
Cooling Degree Days (CDD) for a particular climate is obtained by subtracting each day's mean outdoor dry bulb temperature from the balance point temperature; this result is the number of CDDs for that day. For example, if the maximum and minimum outdoor dry bulb temperatures of a place were 90°F and 60°F respectively, and the balance point temperature were 65°F, then CDD of the place for that particular day would have been [(90+60)/2]-65 = 10. If the mean outdoor dry bulb temperature is equal to or lower than the balance point temperature, then the CDD would be equal to 0.
Annual cooling load
A preliminary estimate of annual heating load, using degree day method, can be obtained by the following formula:
C = PCL x 24 x CDD / ▲T
Where
• C = Annual cooling load in Btu
• PCL = peak cooling load (heat gain) in Btu/hr
• CDD = cooling degree days
• ▲T = temperature difference, °F
