"I opened the window because it was hot."
Translation:Άνοιξα το παράθυρο γιατί είχε ζέστη.
ήταν ζεστό refers to an object (or similar) that was warm/hot, not the weather. Ζεστός/ή/ό is the adjective and ζέστη (noun) = heat, used to describe the weather. The English phrase is a bit ambiguous if you think that maybe it = the room was hot. Not quite an "object", but I hope you see what I mean. So your translation is a possible correct one, if not the most obvious without context as far as I can tell.