tankless

/ˈtæŋkləs/
adjective
  1. Not having a tank; especially describing a water heater that heats water on demand rather than storing it in a tank.
    • A tankless system provides hot water continuously, but it can be expensive to install.
    • The camper van had a tankless propane heater for quick showers.
    • They installed a tankless water heater to save space in the utility closet.