In this case you say a general fact, you open the window when it is hot, it does not matter if it is in the past, present or future. Abrimos la ventana cuando hizo calor is only referring to the past. Best. (Ex. Nosotros abrimos las ventanas cuando hace calor. Ellos abren las ventanas cuando hace calor. Nosotros abrimos las ventanas cuando hizo calor. Ellos abrieron las ventanas cuando hizo calor).

Thank you very much! @Giorgia

Because "abrimos" can be both present and pretérito indefinido. In this sentence it is in the present tense and it means, as Raúl said too, a general fact (we open the windows when it's hot). If you say "Abrimos la ventana porque hizo calor" it means that in a particular past day it was hot and therefore we opened the windows. Grammatically both sentence are correct.