Hi, everyone

A question was confused me for a long time, so have to the email to ask you, which is why CDD50 is used in Ashrae 90, CDD50 is defined as following:

cooling degree-day base 50?F (CDD50): for any one day,when the mean temperature is more than 50?F, there are as many degree-days as degrees Fahrenheit temperature difference between the mean temperature for the day and 50?F. Annual cooling degree-days (CDDs) are the sum of the degree-days over a calendar year.

However, I am very confused why 50?F is used as the threshlod between cooling and heating? That means when the outdoor air temperature is more then 50F, the cooling starts to work?In chinese standard, 26? is as the base of CDD.

Yongqing