heating vs immersion heater

heating

noun
  • A system that raises the temperature of a room or building. Compare heater. 

  • The act of making something hot. 

adj
  • Causing heat. 

immersion heater

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

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