oil burner vs water boiler

oil burner

noun
  • A heating device which burns fuel oil; an oil furnace. 

  • A machine that uses oil as its fuel. 

  • A device whose operation causes apparent combustion of lubricating oil. 

water boiler

noun
  • An electric device which heats water up to boiling temperature. 

How often have the words oil burner and water boiler occurred in a corpus of books? (source: Google Ngram Viewer )