The tag geothermal-heat seems a bit tautologous. I suppose the idea is 'geothermal energy for heating'. But it's not a super-popular tag (only 15 uses), and some questions (e.g. yesterday's Is there a correlation between hot springs and earthquakes?) are about geothermal but not about heating. I added the tag to that question, but it doesn't seem quite right; a more general geothermal tag would be better.
No idea what the process is for changing a tag name, but thought I'd start here. Cheers!