Isn't it ironic that 淡 uses 炎, duplication of 火, as its phonetic component? Does the phonetic component for 淡    bemuse anyone else? Why not develop or embrace some other phonetic component that isn't semantically antonymous to 淡? Doubtless, water ( 氵) is 淡, but 火, let alone 炎 (duplication of 火), isn't 淡.
The phonetic component can not be used to explain the meaning of characters. It is used for the guidance of pronunciation. There are a lot of characters with 炎(tan) as the phonetic component such as 氮，谈、痰、毯, etc. I think none of these is related to fire.