immersion heater vs oil burner

immersion heater

noun
  • A device for supplying domestic hot water in which a thermostatically-controlled heating element is immersed within a tank of water 

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. 

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