オン, あたた.か, あたた.かい, あたた.まる, あたた.める, ぬく, あつ, あつし, なお, はる, ゆ, ゆたか
JLPT2 G3 S12 F838
?# is a depiction of a curved lid fitting its container in holding in the contents. 温 adds 水 water → hold in moisture and keep an object *warm/hot*. 温 transforms the top element of ?into 日, but ?is the style found in derivative characters such as 慍, 褞, 縕, 醞 etc.
[Full Kanji Etymology List]