Feeding with heat can be also indirectly: warm objects emits thermal
infrared - a few micrometers wavelength.
We want to use this energy in much lower temperatures - for example to
power MP3 players:
http://sciencemag.org/cgi/reprint/320/5883/1585.pdf

Photosynthesis for these wavelength could be more difficult than for
visible light, but if it's possible thermophiles should have found
it... (?)