hot water heater vs immersion heater

hot water heater

noun
  • An apparatus, usually fuel-burning, electric or solar-powered, in which water is heated to be used directly as domestic hot water or process hot water (but not for supplying space heating, calorifiers etc, for which a boiler or furnace is used). 

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 hot water heater and immersion heater occurred in a corpus of books? (source: Google Ngram Viewer )