steam heating vs heating system

steam heating and heating system both are nouns.

Word	Noun	Adjective	Verb	Adverb
steam heating	Yes	No	No	No
heating system	Yes	No	No	No

As nouns, heating system is a hypernym of steam heating; that is, heating system is a word with a broader meaning than steam heating:

steam heating: a heating system in which steam is generated in boilers and piped to radiators
heating system: utility to warm a building

Other hypernyms of steam heating include heat, heating, heating plant.

steam heating (noun)	heating system (noun)
a heating system in which steam is generated in boilers and piped to radiators	utility to warm a building