cistern vs immersion heater

cistern

noun
  • A reservoir or tank for holding water, especially for catching and holding rainwater for later use. 

  • The vessel surrounding the condenser in a steam engine. 

  • In a flush toilet, the container in which the water used for flushing is held; a toilet tank. 

  • A cisterna. 

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