before vowels therm-, word-forming element meaning "hot, heat, temperature," used in scientific and technical words, from Greek thermos "hot, warm," therme "heat" (from PIE root *gwher- "to heat, warm").
word-forming element used in making names of devices for stabilizing or regulating (such as thermostat), from Greek statos "standing, stationary," from PIE *ste-to-, suffixed form of root *sta- "to stand, make or be firm." First used in heliostat "an instrument for causing the sun to appear stationary" (1742). Related: -static.
Others are reading
<a href="https://www.etymonline.com/word/thermostat">Etymology of thermostat by etymonline</a>
Harper, D. (n.d.). Etymology of thermostat. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/thermostat