noun
waste heat; remaining heat
Refers to heat that is generated as a byproduct and not used for work, often in industrial, engineering, or environmental contexts.
工場から出る廃熱を再利用する技術が注目されている。
Technology to reuse waste heat from factories is attracting attention.
データセンターの廃熱で地域の暖房をまかなう計画がある。
There is a plan to use waste heat from data centers to provide local heating.
余熱 means 'residual heat' or 'remaining heat' that is still useful (e.g., after cooking), while 廃熱 is heat that is discarded as waste.
Compound of 廃 (hai, 'discard') or 排 (hai, 'exhaust') and 熱 (netsu, 'heat'). The exact historical derivation is uncertain; both kanji forms are used in modern technical writing.