Actual meaning of Warmieter and Kaltmieter in German

I knew that the Kaltmieter is the rent without the other costs (Nebenkosten in German), and Warmieter is the rent with Nebenkosten. I did not know why they call it Kaltmieter and Warmieter.

Until today, the person sit next to me in a computer lab said that it is cold in the room. Cold is “Kalt” in German. Then I realize that the Nebenkosten in some apartments includes the cost of heating from Gas. Nebenkosten can include a lot of cost such as cabel, house maintenance, insurance, central electricity,  heat, etc. “Warm” in German means hot. I guess that Warmieter refers to the cost of the rent and the Nebenkosten, because it includes the cost of heating.