Why national standard heating temperature set at 18 ℃ plus or minus 2 ℃?

Through the understanding, heating heating temperature standards for 18 ℃ & plusmn; 2 ℃, why national standards set at 18 ℃ heating temperature? According to the national standards for heating regulations 'standard of indoor air quality', national standards for the heating temperature of 18 ℃ & plusmn; 2 ℃, that is, on the basis of 18 ℃, 2 ℃ to fluctuate, which is between 16 ℃ and 20 ℃, achieves heating standard. 18℃± 2 ℃ has been in use for decades, more than 90% of the cities in China according to the execution. In the 1980 s of the civil building energy efficiency design standards and specifications for design of heating ventilating and air conditioning in the file, under the indoor temperature of 18 ℃ & plusmn; 2℃。 Now most cities regulations across the country are under 18 ℃ & plusmn; 2 ℃ standards, only a few individual cities such as Beijing has reached 20 ℃ & plusmn; 2℃。