As user4092 noted, this is the passive of 熱する. Several of the conjugated forms:
- 熱する -- plain
- 熱します -- polite
- 熱させる -- causative
- 熱される -- passive
- 熱せられる -- classical passive (see Update below)
- 熱させられる -- causative passive
- 熱せる -- potential
- 熱さない -- negative
Those last two are (part of) what makes this a "special" する verb: the する in plain form becomes せる in the potential instead of できる, and さない in the negative instead of しない. See also 愛【あい】する, which follows the same pattern: this conjugates like a regular 五段【ごだん】 or "type 1" verb ending in す, and it doesn't follow the pattern for independent verb する. Compare 返【かえ】す, 漏【も】らす, and other "type 1" verbs ending in す.
Notably, the negative is currently in flux for these "special" する verbs, where popular usage seems to accept both ～さない and ～しない endings to varying degrees. For strict grammarians, ～さない appears to be regarded as "more correct".
For the passive itself, there are two forms that appear in texts: 熱される, the more or less regular form, and 熱せられる, the more "traditional" or old-fashioned form. Although many other "special" する verbs have shifted more fully to using ～される as the passive, at least [https://detail.chiebukuro.yahoo.co.jp/qa/question_detail/q1243719877 one online thread] explicitly mentions that there appears to be some resistance to this shift for this term 熱する.
The key is the derivation: this was originally a single kanji + す, the precursor to modern する. The conjugation stems for classical す:
- す -- terminal, plain form
- し -- gerundive or connecting form
- せ -- irrealis or incomplete, used for negative, passive, causative, etc.
- する -- adnominal, used when modifying a noun. This evolved into the modern verb plain form.
- すれ -- realis, used for subjunctive statements and suppositions
- せよ -- imperative
For this classical す as attached to a single kanji, the passive was せられる (or older せらる). This is a bit of a mouthful, and over time, it shortened to される (older さる).
(A similar kind of shortening led to modern いらっしゃる, from older いらせらる.)