"Empty Heat" derives from Yin deficiency. There are two aspects I would like to clarify straight away. The term "Empty" Heat may give the impression that Empty Heat is not "real" Heat. Although deriving from Yin deficiency, Empty Heat is "real" Heat and it heats up as much as Full Heat.
The second important clarification is that, although Empty Heat derives from Yin deficiency, this may occur by itself for years before giving rise to Empty Heat. Thus, although Empty Heat derives from Yin deficiency, it takes time for it to develop. Therefore one may have Yin deficiency for years without Empty Heat.
Wikimedia Commons. (n.d.). [The word "HOT" written in sand]. Wikimedia Commons. Available here.