炎 is a bound form (b.f.) that only appears inside compounds. It carries two distinct meanings: (1) extreme heat, as in 炎热 (yánrè, scorching hot) and 炎夏 (yánxià, hot summer); (2) inflammation, as in 炎症 (yánzhèng, inflammation), 发炎 (fāyán, to become inflamed), and medical terms like 肺炎 (fèiyán, pneumonia) and 盲肠炎 (mángchángyán, appendicitis). You should never use 炎 alone — the context determines which meaning applies only when it is combined with another character.