immersion heater vs warmer

immersion heater

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

warmer

noun
  • Something that warms, such as a heater or a soup. 

  • An introductory activity, for example in a lesson, to stimulate interest in a topic. 

  • A piece of clothing for warmth, such as a bodywarmer or leg warmer. 

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